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From the Archives of the AFIP

Benign Musculoskeletal Lipomatous Lesions¹

¹ From the Departments of Radiologic Pathology (M.D.M., J.F.C., T.L.P.) and Orthopedic Pathology (F.H.G.), Armed Forces Institute of Pathology, 6825 16th St NW, Bldg 54, Rm M-133A, Washington, DC 20306; Department of Radiology, Medical University of South Carolina, Charleston (T.L.P.); Departments of Radiology and Nuclear Medicine, Uniformed Services University of the Health Sciences, Bethesda, Md (M.D.M., D.J.F.); Department of Radiology, University of Maryland School of Medicine, Baltimore (M.D.M.); Department of Radiology, National Naval Medical Center, Bethesda, Md (D.J.F.); and Department of Radiology, Mayo Clinic, Jacksonville, Fla (M.J.K.). Received June 1, 2004; revision requested June 7 and received June 24; accepted June 24. All authors have no financial relationships to disclose. Address correspondence to M.D.M. (e-mail: murphey@afip.osd.mil).

	Abstract

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Soft-tissue Lipomatous Lesions Bone, Joint, and Tendon... Conclusions References

 
Benign lipomatous lesions involving soft tissue are common musculoskeletal masses that are classified into nine distinct diagnoses: lipoma, lipomatosis, lipomatosis of nerve, lipoblastoma or lipoblastomatosis, angiolipoma, myolipoma of soft tissue, chondroid lipoma, spindle cell lipoma and pleomorphic lipoma, and hibernoma. Soft-tissue lipoma accounts for almost 50% of all soft-tissue tumors. Radiologic evaluation is diagnostic in up to 71% of cases. These lesions are identical to subcutaneous fat on computed tomographic (CT) and magnetic resonance (MR) images and may contain thin septa. Lipomatosis represents a diffuse overgrowth of mature fat affecting either subcutaneous tissue, muscle or nerve, and imaging is needed to evaluate lesion extent. Lipoblastoma is a tumor of immature fat occurring in young children, and imaging features may reveal a mixture of fat and nonadipose tissue. Angiolipoma, myolipoma, and chondroid lipoma are rare lipomatous lesions that are infrequently imaged. Spindle cell and pleomorphic lipoma appear as a subcutaneous lipomatous mass in the posterior neck or shoulder, with frequent nonadipose components. Hibernoma appears as a lipomatous mass with serpentine vascular elements. Benign lipomatous lesions affecting bone, joint, or tendon sheath include intraosseous lipoma, parosteal lipoma, liposclerosing myxofibrous tumor, discrete lipoma of joint or tendon sheath, and lipoma arborescens. Intraosseous and parosteal lipoma have a pathognomonic CT or MR appearance, with fat in the marrow space or on the bone surface, respectively. Liposclerosing myxofibrous tumor is a rare intermixed histologic lesion commonly located in the medullary canal of the intertrochanteric femur. Benign lipomatous lesions may occur focally in a joint or tendon sheath or with diffuse villonodular proliferation in the synovium (lipoma arborescens) and are diagnosed based on location and identification of fat. Understanding the spectrum of appearances of the various benign musculoskeletal lipomatous lesions improves radiologic assessment and is vital for optimal patient management.

Index Terms: Bone neoplasms, 40.363 • Lipoma and lipomatosis, 40.363 • Soft tissues, neoplasms, 40.363

	LEARNING OBJECTIVES FOR TEST 6

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Soft-tissue Lipomatous Lesions Bone, Joint, and Tendon... Conclusions References

 
After reading this article and taking the test, the reader will be able to:

• List the radiologic spectrum of benign musculoskeletal lipomatous lesions.
  
• Recognize the pathologic basis of the radiologic features of benign musculoskeletal lipomatous lesions.
  
• Identify the radiologic manifestations that may allow differentiation of the various benign musculoskeletal lipomatous lesions and the implications for diagnosis and treatment.
  

	Introduction

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Soft-tissue Lipomatous Lesions Bone, Joint, and Tendon... Conclusions References

 
Benign musculoskeletal lipomatous lesions are common in both soft tissue and bone. Benign lipomatous lesions involving the soft tissues have been recently categorized by the World Health Organization’s Committee for the Classification of Soft Tissue Tumors in 2002 (1) into nine entities, including lipoma, lipomatosis, lipomatosis of nerve, lipoblastoma/lipoblastomatosis, angiolipoma, myolipoma of soft tissue, chondroid lipoma, spindle cell/pleomorphic lipoma, and hibernoma. Benign lipomatous lesions affecting bone include intraosseous lipoma, parosteal lipoma, and liposclerosing myxofibrous tumor (LSMFT). Benign lipomatous lesions may also affect joints and tendon sheaths either focally or more commonly diffusely (lipoma arborescens).

Imaging features of benign lipomatous lesions are often pathognomonic. Radiologic evaluation frequently reveals tissue with either diffuse or focal areas that are similar or identical to subcutaneous fat. These intrinsic features and lesion extent are best depicted with either computed tomography (CT) or magnetic resonance (MR) imaging. In this article, the clinical characteristics, pathologic features, spectrum of radiologic appearances, and treatment for the various types of benign musculoskeletal lipomatous lesions are discussed and illustrated.

	Soft-tissue Lipomatous Lesions

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Soft-tissue Lipomatous Lesions Bone, Joint, and Tendon... Conclusions References

 
Lipoma
The lipoma represents the most common soft-tissue neoplasm, accounting for almost 50% of all soft-tissue tumors in the largest series to date by Myhre-Jensen (2) of 1331 lesions. The prevalence of soft-tissue lipoma has been estimated at 2.1 per 100 people (2–5). Lipomas are much more frequent than liposarcoma by an estimated ratio of 100:1 (2–5). It is unclear if a soft-tissue lipoma represents a benign neoplasm, a local hyperplasia of fat cells, or a combination of both processes. Soft-tissue lipomas can be categorized as superficial or deep. Superficial lipomas are subcutaneous and are extraordinarily common lesions, accounting for 16%–50% (2,3) of all soft-tissue tumors in several large series (Fig 1). These lesions most frequently affect the upper back, neck, proximal extremities (particularly the shoulder), and abdomen. Lipomas most commonly occur in the 5th to 7th decades of life, with 80% of lesions found in patients 27–85 years of age (2,3). There is no clear gender predilection. Superficial lipomas are smaller than 5 cm in 80% of cases, with only 1% of lesions greater than 10 cm in size (3).

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 1a.  Subcutaneous lipoma superior to the clavicle in a 32-year-old woman. (a, b) Coronal T1-weighted (repetition time msec/echo time msec= 583/30) (a) and T2-weighted (2580/80) (b) MR images show a mass that is isointense relative to subcutaneous fat (*). It is surrounded by a low-signal-intensity capsule (arrows), which allows it to be distinguished from adjacent adipose tissue. (c) Photograph of the sectioned gross specimen reveals diffuse fat throughout the lesion (*) and the surrounding capsule (arrowheads).

View larger version (125K): [in this window] [in a new window] [Download PPT slide]   Figure 1b.  Subcutaneous lipoma superior to the clavicle in a 32-year-old woman. (a, b) Coronal T1-weighted (repetition time msec/echo time msec= 583/30) (a) and T2-weighted (2580/80) (b) MR images show a mass that is isointense relative to subcutaneous fat (*). It is surrounded by a low-signal-intensity capsule (arrows), which allows it to be distinguished from adjacent adipose tissue. (c) Photograph of the sectioned gross specimen reveals diffuse fat throughout the lesion (*) and the surrounding capsule (arrowheads).

View larger version (130K): [in this window] [in a new window] [Download PPT slide]   Figure 1c.  Subcutaneous lipoma superior to the clavicle in a 32-year-old woman. (a, b) Coronal T1-weighted (repetition time msec/echo time msec= 583/30) (a) and T2-weighted (2580/80) (b) MR images show a mass that is isointense relative to subcutaneous fat (*). It is surrounded by a low-signal-intensity capsule (arrows), which allows it to be distinguished from adjacent adipose tissue. (c) Photograph of the sectioned gross specimen reveals diffuse fat throughout the lesion (*) and the surrounding capsule (arrowheads).

View larger version (128K): [in this window] [in a new window] [Download PPT slide]   Figure 2a.  Deep infiltrating intramuscular lipoma of the calf in a 17-year-old boy with a palpable soft-tissue mass. (a) Axial T1-weighted (600/12) MR image shows a mass isointense relative to subcutaneous fat that infiltrates the soleus muscle (arrows) and extends across the intermuscular planes to more mildly involve other calf muscles (arrowheads). (b) Photograph of the axially sectioned gross specimen reveals infiltrating lipoma (black *) extensively interdigitating with the soleus muscle (white *).

View larger version (93K): [in this window] [in a new window] [Download PPT slide]   Figure 2b.  Deep infiltrating intramuscular lipoma of the calf in a 17-year-old boy with a palpable soft-tissue mass. (a) Axial T1-weighted (600/12) MR image shows a mass isointense relative to subcutaneous fat that infiltrates the soleus muscle (arrows) and extends across the intermuscular planes to more mildly involve other calf muscles (arrowheads). (b) Photograph of the axially sectioned gross specimen reveals infiltrating lipoma (black *) extensively interdigitating with the soleus muscle (white *).

Lipomas are estimated to be multiple in 5%–15% of patients (2,3,10–12). Patients with multiple lesions tend to be male (6.6:1) (2,3,10–12). Multiple lipomas predominate in the back, shoulder, and upper arms; may be symmetric; show a predilection for the extensor surface; and are most common in the 5th to 6th decades of life. Multiple lipomas are familial in approximately 30% of cases, with inheritance reported to be both autosomal dominant and recessive but likely polygenic (13–15). Patients with familial multiple lipomas also tend to be male, with lesions located superficially in the forearms, trunk, thighs, and arms. There are no known metabolic abnormalities associated with familial multiple lipomas, and patients are generally affected in the 3rd to 4th decades of life. The number of lesions varies from only a few to several hundred (6). Clinical symptoms are usually limited to disfigurement, although there has been a report of peripheral neuropathy with familial multiple lipomas. The asymmetric distribution and focal encapsulation of these lesions aid in distinguishing them from lipomatosis. Several entities associated with multiple lipomas include Bannayan-Zonana syndrome, Cowden syndromes, Fröhlich syndrome, and Proteus syndrome.

A lipoma is composed of mature adipocytes and uniform nuclei that are identical to those seen in normal adult (white) fat. At gross examination, soft-tissue lipomas are well-circumscribed lesions with a greasy consistency and yellow to orange color. A thin capsule delineates the lipoma from the surrounding tissue. The adipose cells are often slightly larger than normal adipocytes. Although the vascular supply network to lipomas is rich, it is inconspicuous because of compression by the distended lipocytes (4,6). Deep lesions may show microscopic infiltration of surrounding musculature. Additional mesenchymal elements are occasionally apparent. The most frequent nonlipomatous component is fibrous connective tissue that often predominates in septa. Prominent nonseptal fibrous components may also occur, and such lesions may be referred to as fibrolipomas. Other mesenchymal elements that may be seen include bone and cartilage.

Cytogenetic abnormalities have been found in 50%–80% of lipomas (1,6,16). Three major subgroups of genetic aberrations have been identified, including 12q13–15, 6p21–23, and deletions from q13 (1,6,16). The 12q13–15 genetic aberration is most frequent, accounting for two-thirds of lipomas with abnormal karyotypes (1,6,16). The HMGIC gene located in 12q15 is specifically affected and is a member of the high-mobility group proteins family, with overexpression implicated as playing an important role in lipoma origin (1,6,16). The lesions in patients with familial multiple lipomas show similar genetic abnormalities as well as translocation on chromosome 3 (lipoma preferred partner gene) (13–15).

At radiography, small lipomas often appear normal. Larger lipomas may reveal typical radiolucency (Fig 3). Underlying osseous abnormalities are rare. Mineralization is unusual but has been reported, both chondroid and osteoid, in up to 11% of cases (11,12,17).

View larger version (129K): [in this window] [in a new window] [Download PPT slide]   Figure 3a.  Deep intramuscular lipoma of the thigh in a 64-year-old woman with a slowly enlarging soft-tissue mass. (a) Radiograph of the proximal thigh shows a radiolucent soft-tissue mass (arrows). (b) CT scan reveals the low-attenuation lipomatous mass (*) with thin septa (arrowheads) in the anterior compartment of the thigh. (c, d) Axial T1-weighted (500/17) (c) and T2-weighted (2100/90) (d) MR images reveal a mass that is isointense relative to subcutaneous fat with both pulse sequences (*) and that contains thin delicate septa (arrows), which remain predominantly low signal intensity on the long TR image (d). (e) Photograph of the axially sectioned gross specimen demonstrates the lipomatous mass (*) with thin septa (arrowheads).

View larger version (81K): [in this window] [in a new window] [Download PPT slide]   Figure 3b.  Deep intramuscular lipoma of the thigh in a 64-year-old woman with a slowly enlarging soft-tissue mass. (a) Radiograph of the proximal thigh shows a radiolucent soft-tissue mass (arrows). (b) CT scan reveals the low-attenuation lipomatous mass (*) with thin septa (arrowheads) in the anterior compartment of the thigh. (c, d) Axial T1-weighted (500/17) (c) and T2-weighted (2100/90) (d) MR images reveal a mass that is isointense relative to subcutaneous fat with both pulse sequences (*) and that contains thin delicate septa (arrows), which remain predominantly low signal intensity on the long TR image (d). (e) Photograph of the axially sectioned gross specimen demonstrates the lipomatous mass (*) with thin septa (arrowheads).

View larger version (93K): [in this window] [in a new window] [Download PPT slide]   Figure 3c.  Deep intramuscular lipoma of the thigh in a 64-year-old woman with a slowly enlarging soft-tissue mass. (a) Radiograph of the proximal thigh shows a radiolucent soft-tissue mass (arrows). (b) CT scan reveals the low-attenuation lipomatous mass (*) with thin septa (arrowheads) in the anterior compartment of the thigh. (c, d) Axial T1-weighted (500/17) (c) and T2-weighted (2100/90) (d) MR images reveal a mass that is isointense relative to subcutaneous fat with both pulse sequences (*) and that contains thin delicate septa (arrows), which remain predominantly low signal intensity on the long TR image (d). (e) Photograph of the axially sectioned gross specimen demonstrates the lipomatous mass (*) with thin septa (arrowheads).

View larger version (78K): [in this window] [in a new window] [Download PPT slide]   Figure 3d.  Deep intramuscular lipoma of the thigh in a 64-year-old woman with a slowly enlarging soft-tissue mass. (a) Radiograph of the proximal thigh shows a radiolucent soft-tissue mass (arrows). (b) CT scan reveals the low-attenuation lipomatous mass (*) with thin septa (arrowheads) in the anterior compartment of the thigh. (c, d) Axial T1-weighted (500/17) (c) and T2-weighted (2100/90) (d) MR images reveal a mass that is isointense relative to subcutaneous fat with both pulse sequences (*) and that contains thin delicate septa (arrows), which remain predominantly low signal intensity on the long TR image (d). (e) Photograph of the axially sectioned gross specimen demonstrates the lipomatous mass (*) with thin septa (arrowheads).

View larger version (102K): [in this window] [in a new window] [Download PPT slide]   Figure 3e.  Deep intramuscular lipoma of the thigh in a 64-year-old woman with a slowly enlarging soft-tissue mass. (a) Radiograph of the proximal thigh shows a radiolucent soft-tissue mass (arrows). (b) CT scan reveals the low-attenuation lipomatous mass (*) with thin septa (arrowheads) in the anterior compartment of the thigh. (c, d) Axial T1-weighted (500/17) (c) and T2-weighted (2100/90) (d) MR images reveal a mass that is isointense relative to subcutaneous fat with both pulse sequences (*) and that contains thin delicate septa (arrows), which remain predominantly low signal intensity on the long TR image (d). (e) Photograph of the axially sectioned gross specimen demonstrates the lipomatous mass (*) with thin septa (arrowheads).

CT and MR imaging of soft-tissue lipoma reveal a mass of homogeneous adipose tissue in 11%–22% of cases (Figs 1, 2) (12,17,19–21). Hounsfield unit measurements of soft-tissue lipoma are usually between –65 and –120, although the value varies by specific body location and direct comparison with the attenuation of surrounding normal fat is often helpful (17,20). MR images of these lesions reveals tissue that is isointense relative to subcutaneous fat, regardless of pulse sequence. We believe that the percentage of lipomas with this CT or MR imaging appearance in routine clinical practice likely approaches 50% (which is much higher than the stated 11%–22%), because the more complicated cases are referred to academic tertiary case centers. These cases in which the entire lesion is composed of only adipose tissue allow a confident diagnosis of lipoma at CT or MR imaging, because well-differentiated liposarcomas do not demonstrate this homogeneous appearance. No contrast material enhancement is seen at CT or MR imaging in this group of lipomas, except of the fibrous capsule (17,20).

Lipomas frequently (37%–49% of cases) demonstrate intrinsic thin septa (<2 mm) on CT or MR images (Fig 3) (12,17,19–22). The detection of only thin delicate septa, particularly when few in number, also allows confident diagnosis of soft-tissue lipoma (23–28). Well-differentiated liposarcoma have this feature only rarely (4%–9% of cases) and generally not in deep-seated lesions (17,20). In addition, Ohguri et al (20) showed that the septal enhancement pattern on contrast material–enhanced MR images may be helpful in distinguishing lipoma from well-differentiated liposarcoma. In their study, 58% of lipomas with septa showed no enhancement and 37% revealed moderate enhancement (20). Well-differentiated liposarcomas showed moderate (25%) to marked (75%) enhancement of the septa in all cases (20). These researchers did not report differences in enhancement between lesions with thin (<2 mm) versus thick (>2 mm) septa (20). We believe that a soft-tissue lipomatous lesion with only thin septa that do not enhance at MR imaging can be confidently diagnosed as a lipoma.

Soft-tissue lipomas may have a more complex appearance with significant nonadipose elements. On CT and MR images, these foci may include thick septa (>2 mm) as well as nodular or globular regions of nonadipose tissue (reported in 28%–31% of cases) (Fig 4) (12,17,19–21). Such lipomas cannot be distinguished from well-differentiated liposarcoma with imaging alone. We suggest biopsy directed at these nonadipose regions, particularly if nodular or globular, both to diagnose well-differentiated liposarcoma and also to exclude the possibility of dedifferentiation (29). In lipomas, these nonadipose regions correspond to previously described mesenchymal elements or areas of fat necrosis.

View larger version (149K): [in this window] [in a new window] [Download PPT slide]   Figure 4a.  Deep intramuscular lipoma with thick septa in a 60-year-old woman with a slowly enlarging mass in the thigh. (a, b) Coronal T1-weighted (600/30) (a) and T2-weighted (2000/90) (b) MR images show a heterogeneous mass (*) that is predominantly isointense relative to subcutaneous fat. However, prominent thick septations with some nodularity are also present (arrowheads), some of which reveal high signal intensity on the long TR image (b). These features do not allow distinction from well-differentiated liposarcoma. (c) Photograph of the longitudinally sectioned gross specimen demonstrates the lipomatous tissue (*) with thick septa and focal nodularity (arrows).

View larger version (143K): [in this window] [in a new window] [Download PPT slide]   Figure 4b.  Deep intramuscular lipoma with thick septa in a 60-year-old woman with a slowly enlarging mass in the thigh. (a, b) Coronal T1-weighted (600/30) (a) and T2-weighted (2000/90) (b) MR images show a heterogeneous mass (*) that is predominantly isointense relative to subcutaneous fat. However, prominent thick septations with some nodularity are also present (arrowheads), some of which reveal high signal intensity on the long TR image (b). These features do not allow distinction from well-differentiated liposarcoma. (c) Photograph of the longitudinally sectioned gross specimen demonstrates the lipomatous tissue (*) with thick septa and focal nodularity (arrows).

View larger version (95K): [in this window] [in a new window] [Download PPT slide]   Figure 4c.  Deep intramuscular lipoma with thick septa in a 60-year-old woman with a slowly enlarging mass in the thigh. (a, b) Coronal T1-weighted (600/30) (a) and T2-weighted (2000/90) (b) MR images show a heterogeneous mass (*) that is predominantly isointense relative to subcutaneous fat. However, prominent thick septations with some nodularity are also present (arrowheads), some of which reveal high signal intensity on the long TR image (b). These features do not allow distinction from well-differentiated liposarcoma. (c) Photograph of the longitudinally sectioned gross specimen demonstrates the lipomatous tissue (*) with thick septa and focal nodularity (arrows).

View larger version (127K): [in this window] [in a new window] [Download PPT slide]   Figure 5a.  Unencapsulated subcutaneous lipoma in a 40-year old man. (a) Axial T1-weighted MR image (545/16.5) shows a mass (*) with signal intensity identical to that of subcutaneous fat and that blends imperceptibly with surrounding adipose tissue. No low-signal-intensity capsule is seen separating the lipoma from the subcutaneous fat. Note the anterior marker in place for identifying this as the site of a palpable soft-tissue mass. (b) Photomicrograph (original magnification, x175; hematoxylin-eosin [H-E] stain) reveals typical large adipocytes that composed the entire lesion.

View larger version (111K): [in this window] [in a new window] [Download PPT slide]   Figure 5b.  Unencapsulated subcutaneous lipoma in a 40-year old man. (a) Axial T1-weighted MR image (545/16.5) shows a mass (*) with signal intensity identical to that of subcutaneous fat and that blends imperceptibly with surrounding adipose tissue. No low-signal-intensity capsule is seen separating the lipoma from the subcutaneous fat. Note the anterior marker in place for identifying this as the site of a palpable soft-tissue mass. (b) Photomicrograph (original magnification, x175; hematoxylin-eosin [H-E] stain) reveals typical large adipocytes that composed the entire lesion.

Lipomatosis
Lipomatosis represents a diffuse overgrowth of mature adipose tissue. Multiple types of lipomatosis typically differentiated by anatomic location and varying clinical manifestations have been described. These include multiple symmetric lipomatosis, infiltrating congenital lipomatosis of the face, encephalocraniocutaneous lipomatosis, shoulder girdle lipomatosis, adipose dolorosa, pelvic lipomatosis, and mediastinal lipomatosis. We limit our discussion to those lesions primarily involving the muscular and subcutaneous tissue. Extensive involvement of both muscle and subcutaneous tissue typically differentiates a large intermuscular or intramuscular lipoma from lipomatosis. Osseous overgrowth is not as common as is seen with lipomatosis of nerves (with macrodystrophia lipomatosa), and no nerve involvement by the adipose tissue is present in lipomatosis.

Patients with lipomatosis are affected at a much younger age than those with solitary soft-tissue lipomas, usually by the age of 2 years (11). There are rare reports of presentation in adulthood. Clinical features of lipomatosis are a direct result of the massive diffuse accumulation of fat in the affected anatomic locations. Pathologic analysis reveals aggregates of adult type fat that are poorly circumscribed and infiltrative. Imaging of lipomatosis demonstrates the infiltrative pattern of fat overgrowth that affects both the subcutaneous and deep soft tissues.

Brodie originally described multiple symmetric lipomatosis in 1846 (34). Madelung was the first to extensively describe this entity in a series of patients in 1888, and the disease is frequently referred to as Madelung disease (35). Launois and Bensaude reported 65 cases in 1889, further describing this condition, which has also been referred to as Launois-Bensaude syndrome (36). Currently, more than 270 cases have been reported. Multiple symmetric lipomatosis almost exclusively affects middle-aged men and shows a high association with alcoholism (60%–90% of cases) (34,37–45). There is an increased prevalence among patients of Mediterranean descent, and an autosomal dominant inheritance has been suggested. The cause of this disease is not known, although numerous metabolic abnormalities (hyperuricemia, gout, hyperlipidemia, and diabetes) have been inconsistently associated with the disease. Point mutations for mitochondria DNA (codon 8344), which are important in alcohol metabolism, have been detected in 28% of patients (42). Alcohol abuse may then lead to promotion of fat production. The painless, progressive fat deposition occurs insidiously, primarily affecting the neck and upper trunk, arms, cheeks, and axilla. This deposition in the neck frequently causes a donut-shaped collar of fat (lipoma annulare colli). Sensorimotor neuropathies are common (59%–84% of patients), with 50% showing central nervous system involvement (46). Imaging, particularly CT and MR, reveals these infiltrative fatty masses typically in a subcutaneous distribution but more frequently between the deep muscles (sternocleidomastoid, trapezius, and paraspinal) (11,42,44,45,47) (Fig 6). Rare extension into the mediastinum may cause tracheal narrowing, which can be depicted with CT. Treatment of multiple symmetric lipomatosis is often performed for cosmesis. Conservative surgical resection and liposuction have proved effective, although these therapies may be unnecessary as abstinence from alcohol or correction of metabolic disturbances may arrest further progression.

View larger version (140K): [in this window] [in a new window] [Download PPT slide]   Figure 6a.  Multiple symmetric lipomatosis in a 45-year-old man. (a) CT scan shows diffuse infiltration of fat between the lumbar paraspinal muscles (arrows), as well as infiltration of the left anterior abdominal wall musculature (*). (b) Photomicrograph (original magnification, x150; H-E stain) reveals lipomatous tissue (L) infiltrating muscle bundles (M).

View larger version (157K): [in this window] [in a new window] [Download PPT slide]   Figure 6b.  Multiple symmetric lipomatosis in a 45-year-old man. (a) CT scan shows diffuse infiltration of fat between the lumbar paraspinal muscles (arrows), as well as infiltration of the left anterior abdominal wall musculature (*). (b) Photomicrograph (original magnification, x150; H-E stain) reveals lipomatous tissue (L) infiltrating muscle bundles (M).

Encephalocraniocutaneous lipomatosis was first described by Haberland and Perou in 1970 (51). The infiltrative lipomatous lesions commonly affect the temporofrontal area unilaterally, cerebral tissues, and leptomeningeal tissues, as well as the skull, eye, and heart. Cutaneous lipomas are usually confined to the scalp, with involvement of the lower limb and back being more unusual. Additional abnormalities include marked mental retardation with associated cerebral malformations and calcification and early onset of seizures (51–54).

Shoulder girdle lipomatosis was initially described by Enzi et al (55) in 1992 in a report of six cases, all women between the ages of 38 and 72 years. These patients had gradually progressive unilateral enlargement of the shoulder and upper arm caused by lipomatous infiltration between the muscles of the extremity and thoracic wall (Fig 7). MR imaging and pathologic analysis revealed tissue identical to adult mature fat (56). Neuromyopathy, a clinical manifestation also found in multiple symmetric lipomatosis, was present in most patients (55), and compression of the upper airway may also occur.

View larger version (101K): [in this window] [in a new window] [Download PPT slide]   Figure 7a.  Shoulder girdle lipomatosis in a 40-year-old man. (a) Clinical photograph shows diffuse infiltration and deformity of the shoulder and upper extremity. (b) CT scan reveals diffuse lipomatous infiltration largely affecting the subcutaneous tissues as the cause of the clinical disfigurement.

View larger version (132K): [in this window] [in a new window] [Download PPT slide]   Figure 7b.  Shoulder girdle lipomatosis in a 40-year-old man. (a) Clinical photograph shows diffuse infiltration and deformity of the shoulder and upper extremity. (b) CT scan reveals diffuse lipomatous infiltration largely affecting the subcutaneous tissues as the cause of the clinical disfigurement.

Lipomatosis of Nerve
Lipomatosis of nerve was initially described in 1953 by Mason (60) and has been referred to in the past by various terms, including fibrolipomatous hamartoma of nerve, perineural lipoma, fatty infiltration of the nerve, intraneural lipoma, and neural fibrolipoma (60–66). In 2002, the WHO adopted the designation of lipomatosis of nerve (1). There is no known cause or hereditary predisposition for this lesion, although hypertrophy of mature fat and fibroblasts in the epineurium has been postulated.

Patients typically present before the age of 30 years and most commonly at birth or early childhood (60–66) (Figs 8, 9). The upper extremity is affected in 78%–96% of cases, particularly the median nerve (85% of cases) (60–66). The most frequent clinical manifestation is a slowly growing mass at the wrist, hand, or forearm. The ulnar nerve is the second most commonly affected site. The lower extremity is involved in 4%–22% of patients, with other reported cases affecting the radial nerve, brachial plexus, and cranial nerves (11). Pain and neurologic symptoms including carpal tunnel syndrome may be associated with lipomatosis of nerve. Macrodactyly is seen in 27%–67% of cases and has been referred to as macrodystrophia lipomatosa (11,64,66). We prefer to use the term lipomatosis of the nerve with or without macrodactyly. Additional causes of macrodactyly include angiomatosis, neurofibromatosis type 1, Klippel-Trénaunay-Weber syndrome, and Proteus syndrome (66). There is a male predilection in cases not associated with macrodactyly, whereas a female predominance has been noted in cases with digit overgrowth. The lesion also has a propensity to involve the second and third rays of the hand or foot.

View larger version (135K): [in this window] [in a new window] [Download PPT slide]   Figure 8a.  Lipomatosis of the median nerve in a patient with macrodactyly. (a, b) Clinical photograph (a) and anteroposterior radiograph (b) show soft-tissue and bone overgrowth (arrow in b) of the second and third digits with osseous bowing. (c) Axial T1-weighted (500/20) MR image reveals diffuse overgrowth of fat in several digits (arrows). (d, e) Photographs of the sagittally sectioned gross specimen (d) and coronally sectioned whole-mount specimen (H-E stain) (e) of involved digits demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*).

View larger version (119K): [in this window] [in a new window] [Download PPT slide]   Figure 8b.  Lipomatosis of the median nerve in a patient with macrodactyly. (a, b) Clinical photograph (a) and anteroposterior radiograph (b) show soft-tissue and bone overgrowth (arrow in b) of the second and third digits with osseous bowing. (c) Axial T1-weighted (500/20) MR image reveals diffuse overgrowth of fat in several digits (arrows). (d, e) Photographs of the sagittally sectioned gross specimen (d) and coronally sectioned whole-mount specimen (H-E stain) (e) of involved digits demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*).

View larger version (85K): [in this window] [in a new window] [Download PPT slide]   Figure 8c.  Lipomatosis of the median nerve in a patient with macrodactyly. (a, b) Clinical photograph (a) and anteroposterior radiograph (b) show soft-tissue and bone overgrowth (arrow in b) of the second and third digits with osseous bowing. (c) Axial T1-weighted (500/20) MR image reveals diffuse overgrowth of fat in several digits (arrows). (d, e) Photographs of the sagittally sectioned gross specimen (d) and coronally sectioned whole-mount specimen (H-E stain) (e) of involved digits demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*).

View larger version (127K): [in this window] [in a new window] [Download PPT slide]   Figure 8d.  Lipomatosis of the median nerve in a patient with macrodactyly. (a, b) Clinical photograph (a) and anteroposterior radiograph (b) show soft-tissue and bone overgrowth (arrow in b) of the second and third digits with osseous bowing. (c) Axial T1-weighted (500/20) MR image reveals diffuse overgrowth of fat in several digits (arrows). (d, e) Photographs of the sagittally sectioned gross specimen (d) and coronally sectioned whole-mount specimen (H-E stain) (e) of involved digits demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*).

View larger version (97K): [in this window] [in a new window] [Download PPT slide]   Figure 8e.  Lipomatosis of the median nerve in a patient with macrodactyly. (a, b) Clinical photograph (a) and anteroposterior radiograph (b) show soft-tissue and bone overgrowth (arrow in b) of the second and third digits with osseous bowing. (c) Axial T1-weighted (500/20) MR image reveals diffuse overgrowth of fat in several digits (arrows). (d, e) Photographs of the sagittally sectioned gross specimen (d) and coronally sectioned whole-mount specimen (H-E stain) (e) of involved digits demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*).

View larger version (95K): [in this window] [in a new window] [Download PPT slide]   Figure 9a.  Lipomatosis of the median nerve in a patient without macrodactyly. (a) Axial T1-weighted (783/15) MR image of the wrist shows marked thickening of the median nerve with adipose tissue surrounding the nerve fascicles (arrowheads). (b) Longitudinal sonogram of the wrist also reveals cablelike thickening of the median nerve fascicles (arrowheads) with intervening hyperechoic fat. (c) Intraoperative photograph of the wrist dissection demonstrates a diffusely thickened, yellow median nerve (arrow) resulting from the lipomatosis of the nerve. (d) Photomicrograph (original magnification, x200; H-E stain) shows diffuse lipomatous infiltration (L) of the surrounding nerve fascicles (N).

View larger version (136K): [in this window] [in a new window] [Download PPT slide]   Figure 9b.  Lipomatosis of the median nerve in a patient without macrodactyly. (a) Axial T1-weighted (783/15) MR image of the wrist shows marked thickening of the median nerve with adipose tissue surrounding the nerve fascicles (arrowheads). (b) Longitudinal sonogram of the wrist also reveals cablelike thickening of the median nerve fascicles (arrowheads) with intervening hyperechoic fat. (c) Intraoperative photograph of the wrist dissection demonstrates a diffusely thickened, yellow median nerve (arrow) resulting from the lipomatosis of the nerve. (d) Photomicrograph (original magnification, x200; H-E stain) shows diffuse lipomatous infiltration (L) of the surrounding nerve fascicles (N).

View larger version (104K): [in this window] [in a new window] [Download PPT slide]   Figure 9c.  Lipomatosis of the median nerve in a patient without macrodactyly. (a) Axial T1-weighted (783/15) MR image of the wrist shows marked thickening of the median nerve with adipose tissue surrounding the nerve fascicles (arrowheads). (b) Longitudinal sonogram of the wrist also reveals cablelike thickening of the median nerve fascicles (arrowheads) with intervening hyperechoic fat. (c) Intraoperative photograph of the wrist dissection demonstrates a diffusely thickened, yellow median nerve (arrow) resulting from the lipomatosis of the nerve. (d) Photomicrograph (original magnification, x200; H-E stain) shows diffuse lipomatous infiltration (L) of the surrounding nerve fascicles (N).

View larger version (129K): [in this window] [in a new window] [Download PPT slide]   Figure 9d.  Lipomatosis of the median nerve in a patient without macrodactyly. (a) Axial T1-weighted (783/15) MR image of the wrist shows marked thickening of the median nerve with adipose tissue surrounding the nerve fascicles (arrowheads). (b) Longitudinal sonogram of the wrist also reveals cablelike thickening of the median nerve fascicles (arrowheads) with intervening hyperechoic fat. (c) Intraoperative photograph of the wrist dissection demonstrates a diffusely thickened, yellow median nerve (arrow) resulting from the lipomatosis of the nerve. (d) Photomicrograph (original magnification, x200; H-E stain) shows diffuse lipomatous infiltration (L) of the surrounding nerve fascicles (N).

Radiographs of patients without associated macrodactyly often appear normal, although a soft-tissue mass may be seen. Macrodactyly causes soft-tissue and osseous overgrowth, both longitudinally and axially (66) (Figs 8, 9). Bone overgrowth is typically more prominent volarly and distally, often resulting in osseous bowing. The osseous overgrowth usually does not progress after puberty. The bone deformity may lead to premature osteoarthritis. Soft-tissue overgrowth often appears as increased radiolucent tissue that corresponds to fat.

The imaging appearance, particularly with sonography and MR imaging, of advanced lipomatosis of nerve is usually pathognomonic and reflects the underlying disease (62,63,65,66) (Figs 8, 9). Sonography reveals alternating hyperechoic (fat) and hypoechoic (nerve fascicles) bands in a diffusely enlarged nerve, thus creating a cablelike appearance. The MR imaging appearance is similar, with longitudinally oriented cylindrical areas of low to intermediate signal intensity (nerve fascicles) surrounded by adipose tissue in a diffusely thickened nerve. Increased fat content in the digits is also apparent in patients with macrodactyly on MR images (Fig 8).

There is no effective treatment for lipomatosis of nerve, as complete resection is usually contraindicated owing to the severe sensory and motor deficits that result. Carpal tunnel release may relieve some symptoms because of decompression in patients with median nerve involvement. Macrodactyly may be treated with amputation because of the extensive associated deformity.

Lipoblastoma and Lipoblastomatosis
Lipoblastoma is a rare benign mesenchymal tumor of embryonal white fat that occurs in infancy and early childhood. In 1926, Jaffe (67) used the term lipoblastoma to describe an atypical lipomatous tumor of the groin. In 1958, Vellios and co-workers coined the term lipoblastomatosis to describe an infiltrating lesion of the anterior chest wall (68). Chung and Enzinger (69) suggested using "benign lipoblastoma" for the circumscribed lesion, and "benign lipoblastomatosis" for the diffuse, infiltrating lesion. The circumscribed lipoblastoma is more common (approximately 70% of cases) and is located in the superficial soft tissues. The diffuse type lipoblastomatosis (about 30% of cases) has an infiltrative growth pattern that affects the subcutaneous tissue and underlying muscle.

In 1973, Chung and Enzinger (69) published the largest series to date, in which 88% of patients were less than 3 years of age, median age of onset was 1 year old, and the oldest patient was 7 years old. Lipoblastomas discovered after the age of 10 years are rare. Lesions have been reported to occur approximately two to three times more frequently in males (69).

Lipoblastomas most commonly manifest as asymptomatic, painless, progressively growing masses in the superficial or subcutaneous soft tissue of the extremities (19,70–81). Less common locations include the trunk, neck, retroperitoneum, mediastinum, and perineum (Fig 10). Symptoms associated with these lesions are directly related to the location and size of the mass. Upper respiratory symptoms, fever, and intermittent airway obstruction have been described in patients with pleural-based, mediastinal, pulmonary, and lower neck lipoblastomas (101–120). Emesis, diarrhea, anorexia, and abdominal pain have been described in patients with mesenteric or retroperitoneal lipoblastomas (79).

View larger version (161K): [in this window] [in a new window] [Download PPT slide]   Figure 10a.  Lipoblastoma in a 4-month-old boy with an anterior neck mass. (a) Axial contrast-enhanced CT scan shows a mass extending from the left side of the neck and deviating the trachea to the right. The periphery of the mass is composed of small foci of fat (arrowhead) separated by thin septa and areas of low attenuation representing myxoid tissue (curved arrow). A larger focus of soft-tissue attenuation (straight arrow) is present centrally. (b) Photograph of the axially sectioned gross specimen shows a solid central element (arrow). Note lack of the typical yellow color of fat in the surrounding lobules (*) secondary to an admixture of adipose and myxoid tissue in this lipoblastoma.

View larger version (122K): [in this window] [in a new window] [Download PPT slide]   Figure 10b.  Lipoblastoma in a 4-month-old boy with an anterior neck mass. (a) Axial contrast-enhanced CT scan shows a mass extending from the left side of the neck and deviating the trachea to the right. The periphery of the mass is composed of small foci of fat (arrowhead) separated by thin septa and areas of low attenuation representing myxoid tissue (curved arrow). A larger focus of soft-tissue attenuation (straight arrow) is present centrally. (b) Photograph of the axially sectioned gross specimen shows a solid central element (arrow). Note lack of the typical yellow color of fat in the surrounding lobules (*) secondary to an admixture of adipose and myxoid tissue in this lipoblastoma.

At gross pathologic examination, the tumors are light yellow, yellowish gray, white, tan, pink, or red. These tumors are frequently less than 5 cm, but larger lesions, including a 17-cm omental lipoblastoma, have been reported (1,4,83). The circumscribed lipoblastoma is partially or fully encapsulated, whereas lipoblastomatosis is infiltrative and lacks a capsule. At histologic analysis, these lesions are composed of monovacuolated and multivacuolated lipoblasts, spindled to stellate mesenchymal cells, a plexiform capillary network, myxoid stroma, and mature adipocytes organized into lobules separated by fibrous septa. In our experience, the myxoid component is often most prominent in very young patients. A "signet-ring" appearance is seen when lipoblasts are relatively small and round with a single vacuole-like fat droplet. If the lipoblasts have finely vacuolated eosinophilic cytoplasm, they may closely resemble hibernoma cells. Ultrastructural observations (ie, with electron microscopy) suggest that lipoblastomas contain a spectrum of differentiating cells ranging from prelipoblasts (spindle cells) to mature adipocytes. Diffuse lipoblastomas have a less pronounced lobular pattern and can contain skeletal muscle fibers related to the infiltrative growth pattern. Lipoblastomas have chromosomal aberrations with pseudodiploid karyotypes. The characteristic cytogenetic abnormalities are deletions and rearrangement of 8q11–13 seen in the vast majority of patients (81). Lipoblastomas lack the t (12,16) translocation seen in myxoid liposarcomas (1,83).

The imaging appearance of lipoblastoma and lipoblastomatosis reflects the underlying pathologic characteristics and varies depending on the extent of fat versus myxoid stroma. Radiographs may reveal a soft-tissue mass with radiolucency. Lipoblastoma is well defined, often with a lobular appearance and internal septations, at sonography, CT, and MR imaging. Sonography, CT, and MR imaging demonstrate fat in lipoblastoma as echogenic regions, areas of low attenuation, or areas of signal intensity identical to that of subcutaneous adipose tissue with all pulse sequences, respectively (11,12,19). In many patients, particularly older children, fat is the predominant feature of these lesions, and they appear identical to a lipoma, with the diagnosis suggested because of the patients’ young age (11,12,19) (Fig 10). However, in very young patients (infants), the myxoid components may predominate with only small elements of fat (11,12,19). These myxoid areas are hypoechoic at sonography, low attenuation at CT, and at MR imaging are low signal intensity with T1-weighted sequences and high signal intensity with T2-weighted sequences, reflecting their high water content (Fig 10). These areas also enhance with contrast material, owing to the rich capillary network. Lipoblastomas with this imaging appearance are indistinguishable from a myxoid liposarcoma. However, the age of the patient is vital in allowing accurate diagnosis. Liposarcomas are extraordinarily rare in patients less than 10 years of age (two cases of 2500 in the Armed Forces Institute of Pathology series, both presenting after the age of 2 years) (84–86). Thus, a lesion containing fat in a young child (less than 2 years old), even with prominent or predominant nonlipomatous components, is almost invariably a lipoblastoma. Lipoblastomatosis reveals similar intrinsic imaging features but also demonstrates infiltrative growth involving both subcutaneous tissue and muscle without a surrounding capsule.

The treatment of lipoblastoma and lipoblastomatosis is wide surgical resection. Recurrence develops in 9%–25% of cases and is largely associated with the infiltrative lipoblastomatosis owing to incomplete resection (68,69,72). These lesions have no metastatic potential.

Angiolipoma
Angiolipoma was originally described by Howard and Helwig in 1960 (87). It represents a benign subcutaneous lesion most commonly affecting young male patients in the 2nd to 3rd decades of life. The most frequent site of involvement is the forearm followed by the trunk and upper arm. Multiple lesions are seen in approximately 70% of cases (83,88–93). The genetic pattern is unclear, although some cases demonstrate autosomal dominant inheritance. The familial prevalence is estimated at 5% (6,88).

Angiolipomas manifest as small (< 2 cm), slowly growing, subcutaneous mass or masses that are painful to palpation. The pain often diminishes over time and is not intensified by thermal changes. Trauma has been invoked as a possible contributing cause.

At gross examination, these lesions are always encapsulated, subcutaneous yellow nodules with reddish areas corresponding to vessels. The vessels are small caliber capillaries and often contain fibrin thrombi with surrounding mature fat (Fig 11). Lesions previously described as being deep infiltrating angiolipomas have now been recognized by the WHO as being intramuscular hemangiomas, a nomenclature we have employed for many years (1,94). In our opinion, subcutaneous lesions may also represent ablated capillary hemangiomas.

View larger version (78K): [in this window] [in a new window] [Download PPT slide]   Figure 11a.  Angiolipoma in the subcutaneous thigh of a 21-year-old man. (a, b) Coronal T1-weighted (500/14) (a) and axial proton-density fat-suppressed (2000/30) (b) MR images show a subcutaneous mass that is largely isointense relative to fat (arrows). Several small nodular foci are also seen that have a more serpentine appearance (arrowheads), suggestive of vessels on the fat-suppressed MR image (b). (c) Photomicrograph (original magnification, x150; H-E stain) reveals a lipomatous mass (L) with more cellular areas peripherally (arrows). The more cellular peripheral area, shown in a higher power insert (original magnification, x400; H-E stain), contains multiple fibrin microthrombi in small-caliber vessels (arrowheads).

View larger version (95K): [in this window] [in a new window] [Download PPT slide]   Figure 11b.  Angiolipoma in the subcutaneous thigh of a 21-year-old man. (a, b) Coronal T1-weighted (500/14) (a) and axial proton-density fat-suppressed (2000/30) (b) MR images show a subcutaneous mass that is largely isointense relative to fat (arrows). Several small nodular foci are also seen that have a more serpentine appearance (arrowheads), suggestive of vessels on the fat-suppressed MR image (b). (c) Photomicrograph (original magnification, x150; H-E stain) reveals a lipomatous mass (L) with more cellular areas peripherally (arrows). The more cellular peripheral area, shown in a higher power insert (original magnification, x400; H-E stain), contains multiple fibrin microthrombi in small-caliber vessels (arrowheads).

View larger version (135K): [in this window] [in a new window] [Download PPT slide]   Figure 11c.  Angiolipoma in the subcutaneous thigh of a 21-year-old man. (a, b) Coronal T1-weighted (500/14) (a) and axial proton-density fat-suppressed (2000/30) (b) MR images show a subcutaneous mass that is largely isointense relative to fat (arrows). Several small nodular foci are also seen that have a more serpentine appearance (arrowheads), suggestive of vessels on the fat-suppressed MR image (b). (c) Photomicrograph (original magnification, x150; H-E stain) reveals a lipomatous mass (L) with more cellular areas peripherally (arrows). The more cellular peripheral area, shown in a higher power insert (original magnification, x400; H-E stain), contains multiple fibrin microthrombi in small-caliber vessels (arrowheads).

Surgical excision is curative (6). There is no tendency for either local recurrence or malignant transformation.

Myolipoma of Soft Tissue
Myolipoma of soft tissue is an extremely rare benign lipomatous lesion. This lesion was originally described as a distinct entity in 1991 in a report of nine cases by Meis and Enzinger (95). Myolipomas affect adults most frequently in the 5th and 6th decades of life with a female predilection (2:1 ratio) (95–97). The lesions are most commonly located in the abdominal cavity, retroperitoneum, and inguinal areas (95–97). Rare cases in the subcutaneous tissues of the trunk and extremities have been reported.

Clinically, patients present with a soft-tissue mass or less commonly the lesions are incidentally discovered. The majority of lesions are large at initial presentation, ranging from 10 to 25 cm in size with a median size of 17 cm (95–97). Subcutaneous lesions are much smaller because of their earlier detection.

At gross pathologic examination, these large tumors are usually completely or at least partially encapsulated with a yellow to white appearance. Histologic analysis demonstrates a variable admixture of smooth muscle and mature adult adipose tissue. The smooth muscle component often predominates, with a typical ratio of 2:1 (muscle to fat) (6). The smooth muscle component is usually regularly interspersed with the adipose tissue, creating a "sievelike" appearance (6). These smooth muscle components are strongly positive with immunohistochemical stains for smooth muscle actin and desmin.

There is only scant literature describing the radiologic appearance of myolipoma (97). The intrinsic features at sonography, CT, and MR imaging reflect the intermixed pathologic characteristics, with both prominent mature lipomatous components and poorly defined nonadipose areas representing smooth muscle. The nonlipomatous component reveals nonspecific solid intrinsic features with soft-tissue attenuation on CT scans and intermediate signal intensity on T1-weighted MR images and intermediate to high signal intensity on T2-weighted images. Coarse calcification has been reported in large lesions. These features suggest a well-differentiated liposarcoma, and the imaging appearance does not allow distinction between these lesions.

Despite their large size, myolipomas are cured by surgical resection (6). To the best of our knowledge, there are no reports of local recurrence, metastatic disease, or malignant transformation.

Chondroid Lipoma
Chondroid lipoma is a rare benign fatty neoplasm that has only recently been described. This tumor was first identified as a distinct entity by Meis and Enzinger in 1993 (98), although Chan and colleagues likely described the first case in 1986 (99). Patients with chondroid lipoma usually present with a slowly growing, painless subcutaneous or deep soft-tissue mass (98,99). There is a significant female predilection with a 4:1 ratio, and the age at presentation ranges from 14 to 70 years (98,99). The proximal extremities and limb girdles are the most common sites, although the trunk and head and neck may also be affected.

At gross pathologic examination, chondroid lipoma is an encapsulated, often multilobular, yellow to white mass with a size range of 1–11 cm (mean, 4 cm) (100). The characteristic microscopic appearance is nests and cords of lipoblasts in an admixed bed of prominent myxoid or hyalinized chondroid matrix and a variable amount of mature adult fat (Fig 12). These features may closely resemble myxoid liposarcoma or extraskeletal myxoid chondrosarcoma and thus lead to a pseudosarcomatous misdiagnosis. We believe lipomas with chondroosseous metaplasia have also mistakenly been included in this category in the past. Cytogenetic aberrations with t(11;16) translocations have been reported with chondroid lipoma as well as hibernomas, a characteristic that suggests a histogenetic link between these lesions.

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 12a.  Chondroid lipoma in the shoulder of a 40 year-old-woman with scapular pain. (a) Anteroposterior radiograph shows chondro-osseous mineralization overlying the scapula (arrows). (b) Axial T1-weighted (500/17) MR image reveals lipomatous components peripherally (arrows) and low-attenuation chondroid areas centrally (*). T2-weighted MR image (not shown) revealed diffuse high signal intensity. (c) Photograph of the axially sectioned gross specimen shows a peripheral lipomatous component (arrows) with central lobules of chondroid tissue (*). (d) Photomicrograph (original magnification, x200; H-E stain) demonstrates adult fat cells (*) and lipoblasts (arrowheads) with intermixed chondroid tissue (C).

View larger version (141K): [in this window] [in a new window] [Download PPT slide]   Figure 12b.  Chondroid lipoma in the shoulder of a 40 year-old-woman with scapular pain. (a) Anteroposterior radiograph shows chondro-osseous mineralization overlying the scapula (arrows). (b) Axial T1-weighted (500/17) MR image reveals lipomatous components peripherally (arrows) and low-attenuation chondroid areas centrally (*). T2-weighted MR image (not shown) revealed diffuse high signal intensity. (c) Photograph of the axially sectioned gross specimen shows a peripheral lipomatous component (arrows) with central lobules of chondroid tissue (*). (d) Photomicrograph (original magnification, x200; H-E stain) demonstrates adult fat cells (*) and lipoblasts (arrowheads) with intermixed chondroid tissue (C).

View larger version (174K): [in this window] [in a new window] [Download PPT slide]   Figure 12c.  Chondroid lipoma in the shoulder of a 40 year-old-woman with scapular pain. (a) Anteroposterior radiograph shows chondro-osseous mineralization overlying the scapula (arrows). (b) Axial T1-weighted (500/17) MR image reveals lipomatous components peripherally (arrows) and low-attenuation chondroid areas centrally (*). T2-weighted MR image (not shown) revealed diffuse high signal intensity. (c) Photograph of the axially sectioned gross specimen shows a peripheral lipomatous component (arrows) with central lobules of chondroid tissue (*). (d) Photomicrograph (original magnification, x200; H-E stain) demonstrates adult fat cells (*) and lipoblasts (arrowheads) with intermixed chondroid tissue (C).

View larger version (168K): [in this window] [in a new window] [Download PPT slide]   Figure 12d.  Chondroid lipoma in the shoulder of a 40 year-old-woman with scapular pain. (a) Anteroposterior radiograph shows chondro-osseous mineralization overlying the scapula (arrows). (b) Axial T1-weighted (500/17) MR image reveals lipomatous components peripherally (arrows) and low-attenuation chondroid areas centrally (*). T2-weighted MR image (not shown) revealed diffuse high signal intensity. (c) Photograph of the axially sectioned gross specimen shows a peripheral lipomatous component (arrows) with central lobules of chondroid tissue (*). (d) Photomicrograph (original magnification, x200; H-E stain) demonstrates adult fat cells (*) and lipoblasts (arrowheads) with intermixed chondroid tissue (C).

Despite its pseudosarcomatous histologic appearance, resection of chondroid lipoma is curative. The lesion does not recur, metastasize, or lead to malignant transformation.

Spindle Cell Lipoma and Pleomorphic Lipoma
Spindle cell lipoma and pleomorphic lipoma are lipomatous lesions with similar demographics and location along a histologic neoplastic spectrum. Spindle cell lipoma was originally described by Enzinger and Harvey in 1975 (102). Pleomorphic lipoma was first described by Shmookler and Enzinger in 1981 (103). These lesions most commonly affect patients between the ages of 45 and 65 years of age with a striking predilection in men (approximately 90% of patients) (102,104–108). These lesions most commonly affect subcutaneous tissues of the posterior neck (Fig 13), shoulder, and back, accounting for approximately 70% of cases (102,104–108). Other less frequent locations include the oral cavity, larynx, bronchus, breast, orbit, and extremity. Deep lesions are rare. The vast majority of lesions are solitary, although recently Fanburg-Smith et al reported 18 cases of multiple spindle cell lipomas, seven of which were familial (109).

View larger version (107K): [in this window] [in a new window] [Download PPT slide]   Figure 13a.  Spindle cell lipoma in the posterior neck in a 55-year-old man with a long history of a slowly growing mass. (a) Axial contrast-enhanced CT scan shows a complex heterogeneous soft-tissue mass with a small lipomatous component medially (arrowhead) and a markedly enhancing area laterally (*). (b) Photograph of the axially sectioned gross specimen reveals identical features with a yellow adipose component (F) and a red hemorrhagic area (H). (c) Photomicrograph (original magnification, x175; H-E stain) through the more vascular area demonstrates pseudoangiomatoid spaces (A), adult fat cells (arrows), and a single collagenized area (arrowheads).

View larger version (129K): [in this window] [in a new window] [Download PPT slide]   Figure 13b.  Spindle cell lipoma in the posterior neck in a 55-year-old man with a long history of a slowly growing mass. (a) Axial contrast-enhanced CT scan shows a complex heterogeneous soft-tissue mass with a small lipomatous component medially (arrowhead) and a markedly enhancing area laterally (*). (b) Photograph of the axially sectioned gross specimen reveals identical features with a yellow adipose component (F) and a red hemorrhagic area (H). (c) Photomicrograph (original magnification, x175; H-E stain) through the more vascular area demonstrates pseudoangiomatoid spaces (A), adult fat cells (arrows), and a single collagenized area (arrowheads).

View larger version (149K): [in this window] [in a new window] [Download PPT slide]   Figure 13c.  Spindle cell lipoma in the posterior neck in a 55-year-old man with a long history of a slowly growing mass. (a) Axial contrast-enhanced CT scan shows a complex heterogeneous soft-tissue mass with a small lipomatous component medially (arrowhead) and a markedly enhancing area laterally (*). (b) Photograph of the axially sectioned gross specimen reveals identical features with a yellow adipose component (F) and a red hemorrhagic area (H). (c) Photomicrograph (original magnification, x175; H-E stain) through the more vascular area demonstrates pseudoangiomatoid spaces (A), adult fat cells (arrows), and a single collagenized area (arrowheads).

At gross pathologic examination, lesions are a well-circumscribed mass yellow to grayish-white in color and are generally firmer than a lipoma. At histologic analysis, spindle cell lipoma appears with bland spindled cells arranged in parallel to the fat cells, with ropelike collagen bundles (1,4,6). In the majority of cases, the volume of fat and spindle cell components are approximately equal, although either may predominate. Additional histologic components include mast cells, lymphocytes, myxoid tissue, and vascular (hemangiopericytoma-like) elements (pseudoangiomatoid variant) (Fig 13). The myxoid and vascular components are prominent in a minority of cases. At the other end of the histologic spectrum is the pleomorphic lipoma, which is characterized by scattered bizarre multinucleated giant cells with radially arranged nuclei (floretlike pattern). Cytogenetic analysis of these lesions has revealed abnormalities in 70%–80%, with monosomy or partial loss of chromosome 13 or 16 (1,6).

Recently, Bancroft et al (104) have described the CT and MR imaging appearances of these lesions in nine patients that also reflects the pathologic spectrum. Fat was identified in 89% of the lesions, ranging from 25% to 75% of the tumor volume (Fig 13) (104). The nonadipose components had nonspecific features of soft-tissue attenuation on CT scans and were similar in signal intensity to that of muscle on T1-weighted MR images and equal to or greater than that of fat on T2-weighted MR images (104). The nonadipose component enhanced following administration of intravenous contrast material in all cases, reflecting the pseudoangiomatous component (Fig 13) (104). These intrinsic features of a complex fatty mass may initially suggest the possibility of well-differentiated liposarcoma, although the subcutaneous location of the lesion in the shoulder or posterior neck and its foci of marked enhancement are much more suggestive of spindle cell lipoma or pleomorphic lipoma.

Treatment is complete surgical resection. Local recurrence is rare, and there are no reports of metastases.

Hibernoma
Hibernoma is a rare benign soft-tissue tumor consisting of brown fat. The tumor was originally described by Merkel in 1906, who named it a "pseudolipoma" (11). Gery (110) coined the term hibernoma in 1914 because of its resemblance to the brown fat in hibernating animals, and approximately 100 cases have been reported to date. Hibernoma has also been referred to as lipoma of immature adipose tissue, lipoma of embryonic fat, and fetal lipoma, terms that have been proposed by some authors because brown fat bears a close resemblance to immature white adipose tissue (111–124). Brown adipose tissue has been postulated to be a form of fetal fat with possible endocrine and nonshivering thermoregulatory functions in hibernating animals and newborn humans.

Brown adipose tissue was originally reported to occur in hibernating animals by Welch in 1670 (11), when he noted a glandlike structure in the mediastinum of a woodchuck. Rasmussen (118) found that brown fat occurs in more than 50 nonhibernating species, including man. It usually occurs in the vestiges where brown fat is found in fetuses and infants, such as the periscapular and interscapular region, the neck, axilla and shoulder, thorax, and retroperitoneum. Brown fat is also infrequently seen in the abdomen, thigh, buttock, popliteal fossa, and intracranial sites. Recently, the thigh has been suggested as the most common location (30% of cases) (Fig 14) (114).

View larger version (94K): [in this window] [in a new window] [Download PPT slide]   Figure 14a.  Hibernoma in a 27-year-old woman with a painless mass in the thigh. (a, b) Coronal T1-weighted (750/30) (a) and sagittal T2-weighted (2100/100) (b) MR images through the thigh show a mass with signal intensity similar to that of fat (arrows) with both sequences. Serpentine channels (arrowheads) are seen in the central portion of the lesion. (c) Anteroposterior image from angiography reveals intense diffuse staining of the mass. (d) Photograph of the axially sectioned gross specimen shows the typical yellow-brown color of hibernoma and central blood vessels (arrows). (e) Photomicrograph (original magnification, x200; H-E stain) of the specimen reveals typical granular eosinophilic cells (E) intermixed with univacuolar adult-type lipocytes (L).

View larger version (88K): [in this window] [in a new window] [Download PPT slide]   Figure 14b.  Hibernoma in a 27-year-old woman with a painless mass in the thigh. (a, b) Coronal T1-weighted (750/30) (a) and sagittal T2-weighted (2100/100) (b) MR images through the thigh show a mass with signal intensity similar to that of fat (arrows) with both sequences. Serpentine channels (arrowheads) are seen in the central portion of the lesion. (c) Anteroposterior image from angiography reveals intense diffuse staining of the mass. (d) Photograph of the axially sectioned gross specimen shows the typical yellow-brown color of hibernoma and central blood vessels (arrows). (e) Photomicrograph (original magnification, x200; H-E stain) of the specimen reveals typical granular eosinophilic cells (E) intermixed with univacuolar adult-type lipocytes (L).

View larger version (94K): [in this window] [in a new window] [Download PPT slide]   Figure 14c.  Hibernoma in a 27-year-old woman with a painless mass in the thigh. (a, b) Coronal T1-weighted (750/30) (a) and sagittal T2-weighted (2100/100) (b) MR images through the thigh show a mass with signal intensity similar to that of fat (arrows) with both sequences. Serpentine channels (arrowheads) are seen in the central portion of the lesion. (c) Anteroposterior image from angiography reveals intense diffuse staining of the mass. (d) Photograph of the axially sectioned gross specimen shows the typical yellow-brown color of hibernoma and central blood vessels (arrows). (e) Photomicrograph (original magnification, x200; H-E stain) of the specimen reveals typical granular eosinophilic cells (E) intermixed with univacuolar adult-type lipocytes (L).

View larger version (140K): [in this window] [in a new window] [Download PPT slide]   Figure 14d.  Hibernoma in a 27-year-old woman with a painless mass in the thigh. (a, b) Coronal T1-weighted (750/30) (a) and sagittal T2-weighted (2100/100) (b) MR images through the thigh show a mass with signal intensity similar to that of fat (arrows) with both sequences. Serpentine channels (arrowheads) are seen in the central portion of the lesion. (c) Anteroposterior image from angiography reveals intense diffuse staining of the mass. (d) Photograph of the axially sectioned gross specimen shows the typical yellow-brown color of hibernoma and central blood vessels (arrows). (e) Photomicrograph (original magnification, x200; H-E stain) of the specimen reveals typical granular eosinophilic cells (E) intermixed with univacuolar adult-type lipocytes (L).

View larger version (157K): [in this window] [in a new window] [Download PPT slide]   Figure 14e.  Hibernoma in a 27-year-old woman with a painless mass in the thigh. (a, b) Coronal T1-weighted (750/30) (a) and sagittal T2-weighted (2100/100) (b) MR images through the thigh show a mass with signal intensity similar to that of fat (arrows) with both sequences. Serpentine channels (arrowheads) are seen in the central portion of the lesion. (c) Anteroposterior image from angiography reveals intense diffuse staining of the mass. (d) Photograph of the axially sectioned gross specimen shows the typical yellow-brown color of hibernoma and central blood vessels (arrows). (e) Photomicrograph (original magnification, x200; H-E stain) of the specimen reveals typical granular eosinophilic cells (E) intermixed with univacuolar adult-type lipocytes (L).

Gross pathologic inspection demonstrates a well-demarcated, encapsulated soft, greasy to rubbery, brown to yellow lobulated mass. These masses usually measure 5–10 cm in diameter, although there have been reports of hibernomas reaching 20 cm in size (1,4,6). At histologic analysis, hibernomas are lobulated and composed of cells of varying degrees of differentiation. Multivacuolar adipocytes and brown fat cells with granular eosinophilic cytoplasm are interspersed with univacuolar adipocytes. The univacuolar cells have one or more large lipid droplets with peripheral nuclei, resembling lipocytes. There is marked hypervascularity, which combines with abundant mitochondria to give hibernomas their brown color. Recently, four histologic variants have been described, including typical (82% of cases), myxoid (8%), lipomalike (7%), and spindle cell (2%) (114). The typical variant is composed primarily of brown fat cells (114). The myxoid variety occurs primarily in men and shows high water content (114). The lipomalike variant contains significant amounts of adult fat and commonly affects the thigh (114). Finally, the spindle cell variant has features of spindle cell lipoma and hibernoma and occurs primarily in the subcutaneous tissues of the neck (114). Cytogenetic abnormalities commonly associated with hibernomas are rearrangements of 11q13–21 genes.

Radiography may show a radiolucent mass with no osseous abnormalities or mineralization. Sonography demonstrates a well-circumscribed hyperechoic mass, and Doppler imaging may show hypervascularity, which has also been seen with angiography. Arteriovenous shunting has been reported (116). For these reasons, core needle biopsy may be ill advised, particularly in deep lesions in which vascular control cannot be easily attained. Contrast-enhanced CT and MR imaging can direct fine needle aspiration to a location without prominent vascularity.

CT and MR imaging demonstrate a well-defined intermuscular, intramuscular, subcutaneous, or retroperitoneal lesion. The intrinsic imaging characteristics likely correspond to the histologic variants described above. Lesions are heterogeneous and are most frequently similar to but not identical to fat, corresponding to the typical variant (Fig 14) (115,117,121,124). In addition, prominent branching and serpentine high-flow (low signal intensity with all MR pulse sequences) and low-flow vascular structures are often apparent, as is contrast enhancement (115,117,121,124). Although these features may initially suggest well-differentiated liposarcoma, this type of vascularity is never apparent in liposarcomas and is an important feature for differentiation. The next most common MR and CT appearance is tissue identical to fat but with branching vascular structures; this appearance corresponds to the lipomalike or, if located in the neck, the rare spindle cell variety (Fig 14) (115, 117,121,124). The myxoid variant has nonspecific high water content imaging features, and, in our experience, the diagnosis cannot be suggested prospectively.

Bone scintigraphy may demonstrate moderate uptake on blood pool images and mild uptake on static images (122,123). Intense uptake has been reported on images obtained with technetium 99m tetrofosmin and fluorine 18 fluorodeoxyglucose positron emission tomography (122,123). These scintigraphic features are not typically seen with lipoma or well-differentiated liposarcoma and are a reflection of the hypervascularity and increased cellular activity (with glucose turnover) of hibernomas.

Treatment is complete surgical resection, and local recurrence does not occur with complete excision. There are no reports of metastases or malignant transformation.

	Bone, Joint, and Tendon Sheath Lipomatous Lesions

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Soft-tissue Lipomatous Lesions Bone, Joint, and Tendon... Conclusions References

 
Intraosseous Lipoma
Intraosseous lipoma is the most frequent lipogenic lesion of bone. The pathogenesis of intraosseous lipoma has been considerably debated in the pathology literature, but there is no doubt that this lesion represents a distinct entity and can produce both characteristic and confusing radiologic findings. Although generally considered rare, intraosseous lipoma, in our experience, occurs with much more frequency than the widely cited prevalence of less than one per 1000 cases of primary bone tumors (125–132). We believe the prevalence of intraosseous lipoma is markedly underreported for several reasons. First, its radiographic manifestations are nonspecific and can be confused with those of other entities. Second, its typically benign radiographic appearance frequently precludes further imaging with CT or MR imaging that would delineate the fatty consistency of the lesion. Finally, the histopathologic features of intraosseous lipoma can be difficult to interpret if not correlated with available radiologic studies: Fat in the lesion may be pathologically indistinguishable from normal fat in yellow marrow, and, if ischemic changes are present, it may be difficult to distinguish osteonecrosis from intraosseous lipoma.

Patients range in age from 5 to 85 years, with the lesions being most frequently discovered in the 4th and 5th decades of life (127,128,132–144). Males are slightly more commonly affected than females. Although intraosseous lesions may be discovered incidentally, pain has been reported in up to 66% of cases (132). The cause of pain is unclear, but it may be mechanical due to expansile remodeling of bone or it may be related to the ischemic changes that frequently accompany these lesions. It is also possible that the pain is referable to nearby joint disease and that the intraosseous lesion is incidentally discovered. This clinical scenario is particularly common in proximal femoral lesions in our experience. Pathologic fracture and a palpable mass are rare clinical manifestations.

Intraosseous lipomas have been reported to occur throughout the skeleton. Frequent locations include the intertrochanteric region of the proximal femur (34% of cases), calcaneus (8%) (Fig 15), ilium (8%) (particularly adjacent to the sacroiliac joint) (Fig 16), tibia (13%), fibula (10%) (Fig 17), humerus (5%), and ribs (5%) (127,128,132). Long bone lesions typically occur in the metaphysis, but diaphyseal involvement is not uncommon. Epiphyseal involvement is unusual. Lesions are usually intramedullary, although rarely they occur in an intracortical location. Lesion size varies from 2 to 13 cm, with most lesions being 5–6 cm (132). Intraosseous lipomas are typically solitary, although multiple lesions have been described (142,143). Multiple intraosseous lipomas affecting more than 10 bones in a single patient (intraosseous lipomatosis) have also been reported, both with and without associated hyperlipoproteinemia (142,143).

View larger version (91K): [in this window] [in a new window] [Download PPT slide]   Figure 15a.  Incidentally discovered intraosseous lipoma in the calcaneus of a 42-year-old man. (a) Lateral radiograph of the calcaneus shows a radiolucent lesion with sclerotic margins (arrows). A central rim of sclerosis producing a bull’s-eye appearance (arrowheads) is also seen. (b) Axial CT scan through the calcaneus shows fat in the periphery of the lesion (*). Thin rim of ossification (arrows) surrounds a central area of fluid attenuation (C). (c, d) Coronal T1-weighted (500/30) (c) and coronal T2-weighted (2000/90) (d) MR images through the calcaneus reveal the lesion, which has an outer rim of adipose signal intensity (A) surrounding a central focus of fluid signal intensity (F). (e) Photomicrograph (original magnification, x125; H-E stain) shows central rim of ossification (O) at the margin of fat necrosis (FN) and surrounding viable fat in the intraosseous lipoma (L).

View larger version (90K): [in this window] [in a new window] [Download PPT slide]   Figure 15b.  Incidentally discovered intraosseous lipoma in the calcaneus of a 42-year-old man. (a) Lateral radiograph of the calcaneus shows a radiolucent lesion with sclerotic margins (arrows). A central rim of sclerosis producing a bull’s-eye appearance (arrowheads) is also seen. (b) Axial CT scan through the calcaneus shows fat in the periphery of the lesion (*). Thin rim of ossification (arrows) surrounds a central area of fluid attenuation (C). (c, d) Coronal T1-weighted (500/30) (c) and coronal T2-weighted (2000/90) (d) MR images through the calcaneus reveal the lesion, which has an outer rim of adipose signal intensity (A) surrounding a central focus of fluid signal intensity (F). (e) Photomicrograph (original magnification, x125; H-E stain) shows central rim of ossification (O) at the margin of fat necrosis (FN) and surrounding viable fat in the intraosseous lipoma (L).

View larger version (102K): [in this window] [in a new window] [Download PPT slide]   Figure 15c.  Incidentally discovered intraosseous lipoma in the calcaneus of a 42-year-old man. (a) Lateral radiograph of the calcaneus shows a radiolucent lesion with sclerotic margins (arrows). A central rim of sclerosis producing a bull’s-eye appearance (arrowheads) is also seen. (b) Axial CT scan through the calcaneus shows fat in the periphery of the lesion (*). Thin rim of ossification (arrows) surrounds a central area of fluid attenuation (C). (c, d) Coronal T1-weighted (500/30) (c) and coronal T2-weighted (2000/90) (d) MR images through the calcaneus reveal the lesion, which has an outer rim of adipose signal intensity (A) surrounding a central focus of fluid signal intensity (F). (e) Photomicrograph (original magnification, x125; H-E stain) shows central rim of ossification (O) at the margin of fat necrosis (FN) and surrounding viable fat in the intraosseous lipoma (L).

View larger version (81K): [in this window] [in a new window] [Download PPT slide]   Figure 15d.  Incidentally discovered intraosseous lipoma in the calcaneus of a 42-year-old man. (a) Lateral radiograph of the calcaneus shows a radiolucent lesion with sclerotic margins (arrows). A central rim of sclerosis producing a bull’s-eye appearance (arrowheads) is also seen. (b) Axial CT scan through the calcaneus shows fat in the periphery of the lesion (*). Thin rim of ossification (arrows) surrounds a central area of fluid attenuation (C). (c, d) Coronal T1-weighted (500/30) (c) and coronal T2-weighted (2000/90) (d) MR images through the calcaneus reveal the lesion, which has an outer rim of adipose signal intensity (A) surrounding a central focus of fluid signal intensity (F). (e) Photomicrograph (original magnification, x125; H-E stain) shows central rim of ossification (O) at the margin of fat necrosis (FN) and surrounding viable fat in the intraosseous lipoma (L).

View larger version (120K): [in this window] [in a new window] [Download PPT slide]   Figure 15e.  Incidentally discovered intraosseous lipoma in the calcaneus of a 42-year-old man. (a) Lateral radiograph of the calcaneus shows a radiolucent lesion with sclerotic margins (arrows). A central rim of sclerosis producing a bull’s-eye appearance (arrowheads) is also seen. (b) Axial CT scan through the calcaneus shows fat in the periphery of the lesion (*). Thin rim of ossification (arrows) surrounds a central area of fluid attenuation (C). (c, d) Coronal T1-weighted (500/30) (c) and coronal T2-weighted (2000/90) (d) MR images through the calcaneus reveal the lesion, which has an outer rim of adipose signal intensity (A) surrounding a central focus of fluid signal intensity (F). (e) Photomicrograph (original magnification, x125; H-E stain) shows central rim of ossification (O) at the margin of fat necrosis (FN) and surrounding viable fat in the intraosseous lipoma (L).

View larger version (112K): [in this window] [in a new window] [Download PPT slide]   Figure 16a.  Intraosseous lipoma discovered incidentally at intravenous urography in a 71-year-old man. (a) Anteroposterior radiograph of the pelvis shows a radiolucent lesion with a sclerotic rim (arrows) in the right ilium, adjacent to the sacroiliac joint. (b) Axial CT scan reveals fat attenuation in the anterior portion of the lesion (arrowheads) and fluid attenuation (arrows) posteriorly. (c) Follow-up CT scan obtained 6 months later shows that the lesion is now entirely low attenuation and fluid filled.

View larger version (106K): [in this window] [in a new window] [Download PPT slide]   Figure 16b.  Intraosseous lipoma discovered incidentally at intravenous urography in a 71-year-old man. (a) Anteroposterior radiograph of the pelvis shows a radiolucent lesion with a sclerotic rim (arrows) in the right ilium, adjacent to the sacroiliac joint. (b) Axial CT scan reveals fat attenuation in the anterior portion of the lesion (arrowheads) and fluid attenuation (arrows) posteriorly. (c) Follow-up CT scan obtained 6 months later shows that the lesion is now entirely low attenuation and fluid filled.

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 16c.  Intraosseous lipoma discovered incidentally at intravenous urography in a 71-year-old man. (a) Anteroposterior radiograph of the pelvis shows a radiolucent lesion with a sclerotic rim (arrows) in the right ilium, adjacent to the sacroiliac joint. (b) Axial CT scan reveals fat attenuation in the anterior portion of the lesion (arrowheads) and fluid attenuation (arrows) posteriorly. (c) Follow-up CT scan obtained 6 months later shows that the lesion is now entirely low attenuation and fluid filled.

View larger version (107K): [in this window] [in a new window] [Download PPT slide]   Figure 17a.  Incidentally discovered intraosseous lipoma of the fibula in a 34-year-old woman. (a) Anteroposterior radiograph of the knee shows an expansile nonaggressive lesion in the proximal fibular diaphysis that contains central mineralization suggestive of chondroid matrix (arrows). (b) Coronal T1-weighted (460/20) MR image of the lower leg shows the fibular lesion, which has centrally increased signal intensity, similar to that of fat (*). (c) Photograph of the sagittally sectioned whole-mount specimen (H-E stain) reveals ischemic ossification (*), responsible for opacity in the lesion at radiography, and surrounding lipoma (L).

View larger version (117K): [in this window] [in a new window] [Download PPT slide]   Figure 17b.  Incidentally discovered intraosseous lipoma of the fibula in a 34-year-old woman. (a) Anteroposterior radiograph of the knee shows an expansile nonaggressive lesion in the proximal fibular diaphysis that contains central mineralization suggestive of chondroid matrix (arrows). (b) Coronal T1-weighted (460/20) MR image of the lower leg shows the fibular lesion, which has centrally increased signal intensity, similar to that of fat (*). (c) Photograph of the sagittally sectioned whole-mount specimen (H-E stain) reveals ischemic ossification (*), responsible for opacity in the lesion at radiography, and surrounding lipoma (L).

View larger version (57K): [in this window] [in a new window] [Download PPT slide]   Figure 17c.  Incidentally discovered intraosseous lipoma of the fibula in a 34-year-old woman. (a) Anteroposterior radiograph of the knee shows an expansile nonaggressive lesion in the proximal fibular diaphysis that contains central mineralization suggestive of chondroid matrix (arrows). (b) Coronal T1-weighted (460/20) MR image of the lower leg shows the fibular lesion, which has centrally increased signal intensity, similar to that of fat (*). (c) Photograph of the sagittally sectioned whole-mount specimen (H-E stain) reveals ischemic ossification (*), responsible for opacity in the lesion at radiography, and surrounding lipoma (L).

At gross pathologic examination, intraosseous lipomas are pale or bright yellow and may reveal a thin surrounding capsule (127,128,132,133). There is often lobular growth with thin septa. The lesions are composed of mature adult fat and may contain a paucity of atrophic trabeculae. Milgram (127,128) divided intraosseous lipomas into three stages. In stage 1, lesions contain viable fat without necrosis and cause trabecular resorption (127,128). Stage 2 lesions demonstrate viable fat and fat necrosis, as well as regions of dystrophic calcification (127,128). Finally, stage 3 intraosseous lipomas demonstrate involutional changes with extensive fat necrosis, cyst formation, calcification, and reactive new bone formation (127,128).

The radiologic appearance of intraosseous lipoma depends on the histologic composition of the lesion (125,127–129,131,132,134,137–139,146). Intraosseous lipomas can contain varying amounts of fat, bone, fibrous tissue, and cystic degeneration resulting in a range of radiographic manifestations (Figs 15–17).

Intraosseous lipomas composed solely of fat (Milgram stage 1 lesions) are radiolucent, well-circumscribed lesions that frequently are associated with mild focal expansile remodeling of the affected bone (50% of cases) (125,127–129,131,132,134,137–139). Thin or occasionally thick trabecular ridges may be present in the periphery of the lesion and produce a septated appearance (125,127–129,131,132,134,137–139). Expansile remodeling of bone is more prominent in thin long bones such as the fibula and ulna (Fig 17). The radiographic appearance of intraosseous lipoma containing only fat is nonspecific and shares the same features of unicameral bone cyst, fibrous dysplasia, and plasmacytoma.

Intraosseous lipoma containing only fat is easily differentiated from other primary osseous lesions at MR imaging or CT because both modalities offer the ability to document the intrinsic lesional adipose tissue (125,127–129,131,132,134,137–139,146) (Figs 15–17). CT demonstrates the low attenuation of fat (<–60 to –100 HU) and, if present, expansile remodeling of the intramedullary canal (125,127–129,131,132,134,137–139,146) (Figs 15, 16). The lesion may be easily differentiated from surrounding normal fatty marrow by a peripheral ossific rim or capsule that separates the lesion from the normal surrounding bone; this finding is best seen on long axis imaging planes. However, in other cases this differentiation is quite difficult, similar to the differentiation of nonencapsulated subcutaneous soft-tissue lipoma. On CT scans, the attenuation of normal marrow is often slightly higher than that of lipoma, owing to cellular elements in yellow marrow. On MR images, the adipose tissue of intraosseous lipomas is easily appreciated, but lesions consisting solely of fat may be difficult to differentiate from surrounding normal yellow marrow (Fig 17). The fat in the lesion shows T1 prolongation and T2 shortening similar to subcutaneous fat. Normal yellow marrow shows subtle signal intensity lower than that of the intraosseous lipoma on T1-weighted MR images, again related to cellular elements (125,127–129,131,132,134,137–139,146). Expansile remodeling of bone and a thin margin of low signal intensity secondary to peripheral ossification or capsule are helpful differentiating features when radiographs are not available for review (144,146).

Intraosseous lipomas may be associated with variable amounts of central or peripheral ossification or calcification (Milgram stage 2 or 3 lesions) (127,128) (Fig 17). Histologic evidence of ischemia in intraosseous lipomas has led to the speculation that slow enlargement of a lipoma in the enclosed space of the medullary canal may lead to increased intramedullary pressure. Increased intramedullary pressure may then compromise blood flow, particularly in venous structures. Fat necrosis may ensue with calcification about its periphery or intermixed in the lesion. Alternatively, the central calcification or peripheral ossification seen in and around an intraosseous lipoma may be an inherent product of the mesenchymal cells that create the lesion rather than reactive in nature.

Regardless of pathogenesis, the ossification and calcification in an intraosseous lipoma (Milgram stage 2 or 3 lesions) may produce a distinctive radiographic appearance (127,128). Central or ringlike calcification in a lucent lesion involving the body of the calcaneus is pathognomonic of an intraosseous lipoma and allows it to be distinguished from unicameral bone cyst (Fig 15). However, this same calcification-ossification pattern in other less common locations may cause confusion in diagnosis. The ossification in these lesions may be extensive, leading to the term ossifying lipoma. A predominantly calcified or ossified lesion can be confused with an enostosis. Partially mineralized lesions may be mistaken for a chondroid lesion or osteonecrosis on radiographs (Fig 17).

Again, CT and MR imaging are extremely useful in differentiating intraosseous lipoma with central calcification from other lesions in the differential diagnosis. Fat is seen in portions of the lesion (unless it is completely calcified or ossified), signifying that the tumor is lipogenic in origin. The fatty composition of the lesion distinguishes the intraosseous lipoma from tumors of chondroid, osteoid, or fibrous origin (125,127–129,131,132,134,137–139,146). On MR images, central or peripheral calcification is seen as areas of low signal intensity on both T1 and T2-weighted images (125,127–129,131,132,134,137–139,146). The central or peripheral ossification-calcification associated with intraosseous lipoma is readily appreciated on CT images. Intraosseous lipoma may be difficult to differentiate from osteonecrosis at MR imaging and CT because both lesions contain intrinsic fat with a rim of tissue separating the lesion from surrounding marrow. Expansile remodeling of bone, osteolysis, and a rounded rather than irregular serpentine margin are differentiating factors that support the diagnosis of intraosseous lipoma as opposed to osteonecrosis (132,144,146).

With progressive ischemia and involution, fibrous proliferation and cystic degeneration can be seen in an intraosseous lipoma (Milgram stage 3 lesions) (127,128). Cystic degeneration may be the predominant feature depicted with CT or MR imaging (Figs 15, 16). The observation that lipomas may evolve into a purely cystic lesion and the similar skeletal distribution of intraosseous lipomas and unicameral bone cysts in adults have led some authors to postulate that unicameral bone cysts may represent completely involuted intraosseous lipomas (Fig 16). On radiographs, severely involuted lesions demonstrate a thick peripheral rind of ossification at the lesion margin with variable amounts of central ossification-calcification. The cystic consistency of the lesion on cross-sectional images may cause confusion, as may the presence of fibrous tissue. Central cystic areas may be surrounded by a rim of ossification. This central ring of ossification may then be surrounded by fat that in turn is surrounded by a rim of ossification or fibrous capsule demarcating the periphery of the lesion. The resulting bull’s-eye appearance is distinctive for intraosseous lipoma (Fig 15) (125,127–129,131,132,134,137–139,146). Despite the heterogeneous appearance of a severely involuted intraosseous lipoma on both CT and MR images, the identification of fat in the lesion permits definitive diagnosis of intraosseous lipoma.

Bone scintigraphy of intraosseous lipomas demonstrates radionuclide uptake ranging from absent to moderate (132). Marked uptake of radiotracer on bone scintigrams is distinctly unusual.

Treatment of intraosseous lipomas is often not indicated with asymptomatic lesions or those discovered incidentally (127,128). Symptomatic lipomas may be treated with curettage and bone graft placement (127,128). Recurrence and malignant transformation are rare (127,128,147).

Parosteal Lipoma
Parosteal lipoma is a rare benign neoplasm consisting of mature adipose tissue intimately associated with the periosteum of bone. Originally described in 1836 by Seering (148), the lesion was initially referred to as "periosteal lipoma." The designation of parosteal lipoma was suggested by Power in 1888 (149) to indicate that the lesion does not arise in the periosteum because the periosteum contains no fat cells. To date, approximately 150 cases of parosteal lipoma have been reported, constituting 0.3% of all lipomas (129,150–159).

The most common sites of origin for parosteal lipoma are in the thigh adjacent to the femur or in the upper extremity near the proximal radius (129,155–159). The lesion has also been reported in the tibia, humerus (Fig 18), scapula, clavicle, ribs, pelvis, metacarpals, metatarsals, mandible, and skull. Patients with parosteal lipoma range in age from 40 to 60 years old and usually present with a history of a slowly growing, large, painless and nontender immobile mass not fixed to the skin (129,155–159).

View larger version (86K): [in this window] [in a new window] [Download PPT slide]   Figure 18a.  Parosteal lipoma in a 47-year-old woman with a slowly enlarging, painless mass in the anteromedial upper arm. (a) Radiograph shows ossification (arrows) with surrounding radiolucency that represents fat (F) projecting over the proximal humeral diaphysis. (b) Axial CT scan shows fat attenuation (F) surrounding the irregular ossification arising from the anteromedial aspect of the proximal humerus (O). No medullary continuity is seen between underlying bone and surface bone formation. Bone scan (not shown) revealed moderate radionuclide uptake in this region. (c) Sagittal T1-weighted (500/30) MR image shows hyperintense signal of fat in the lesion (F) and low signal intensity in the surface bone formation (arrow). (d, e) Photograph of the sagittally sectioned gross specimen (d) and sagittally sectioned whole-mount specimen (H-E stain) (e) reveals central ossification (O) attached to the humeral cortex (C) without marrow continuity and surrounded by yellow adipose tissue (A). Humeral marrow (M) is normal.

View larger version (113K): [in this window] [in a new window] [Download PPT slide]   Figure 18b.  Parosteal lipoma in a 47-year-old woman with a slowly enlarging, painless mass in the anteromedial upper arm. (a) Radiograph shows ossification (arrows) with surrounding radiolucency that represents fat (F) projecting over the proximal humeral diaphysis. (b) Axial CT scan shows fat attenuation (F) surrounding the irregular ossification arising from the anteromedial aspect of the proximal humerus (O). No medullary continuity is seen between underlying bone and surface bone formation. Bone scan (not shown) revealed moderate radionuclide uptake in this region. (c) Sagittal T1-weighted (500/30) MR image shows hyperintense signal of fat in the lesion (F) and low signal intensity in the surface bone formation (arrow). (d, e) Photograph of the sagittally sectioned gross specimen (d) and sagittally sectioned whole-mount specimen (H-E stain) (e) reveals central ossification (O) attached to the humeral cortex (C) without marrow continuity and surrounded by yellow adipose tissue (A). Humeral marrow (M) is normal.

View larger version (109K): [in this window] [in a new window] [Download PPT slide]   Figure 18c.  Parosteal lipoma in a 47-year-old woman with a slowly enlarging, painless mass in the anteromedial upper arm. (a) Radiograph shows ossification (arrows) with surrounding radiolucency that represents fat (F) projecting over the proximal humeral diaphysis. (b) Axial CT scan shows fat attenuation (F) surrounding the irregular ossification arising from the anteromedial aspect of the proximal humerus (O). No medullary continuity is seen between underlying bone and surface bone formation. Bone scan (not shown) revealed moderate radionuclide uptake in this region. (c) Sagittal T1-weighted (500/30) MR image shows hyperintense signal of fat in the lesion (F) and low signal intensity in the surface bone formation (arrow). (d, e) Photograph of the sagittally sectioned gross specimen (d) and sagittally sectioned whole-mount specimen (H-E stain) (e) reveals central ossification (O) attached to the humeral cortex (C) without marrow continuity and surrounded by yellow adipose tissue (A). Humeral marrow (M) is normal.

View larger version (154K): [in this window] [in a new window] [Download PPT slide]   Figure 18d.  Parosteal lipoma in a 47-year-old woman with a slowly enlarging, painless mass in the anteromedial upper arm. (a) Radiograph shows ossification (arrows) with surrounding radiolucency that represents fat (F) projecting over the proximal humeral diaphysis. (b) Axial CT scan shows fat attenuation (F) surrounding the irregular ossification arising from the anteromedial aspect of the proximal humerus (O). No medullary continuity is seen between underlying bone and surface bone formation. Bone scan (not shown) revealed moderate radionuclide uptake in this region. (c) Sagittal T1-weighted (500/30) MR image shows hyperintense signal of fat in the lesion (F) and low signal intensity in the surface bone formation (arrow). (d, e) Photograph of the sagittally sectioned gross specimen (d) and sagittally sectioned whole-mount specimen (H-E stain) (e) reveals central ossification (O) attached to the humeral cortex (C) without marrow continuity and surrounded by yellow adipose tissue (A). Humeral marrow (M) is normal.

View larger version (129K): [in this window] [in a new window] [Download PPT slide]   Figure 18e.  Parosteal lipoma in a 47-year-old woman with a slowly enlarging, painless mass in the anteromedial upper arm. (a) Radiograph shows ossification (arrows) with surrounding radiolucency that represents fat (F) projecting over the proximal humeral diaphysis. (b) Axial CT scan shows fat attenuation (F) surrounding the irregular ossification arising from the anteromedial aspect of the proximal humerus (O). No medullary continuity is seen between underlying bone and surface bone formation. Bone scan (not shown) revealed moderate radionuclide uptake in this region. (c) Sagittal T1-weighted (500/30) MR image shows hyperintense signal of fat in the lesion (F) and low signal intensity in the surface bone formation (arrow). (d, e) Photograph of the sagittally sectioned gross specimen (d) and sagittally sectioned whole-mount specimen (H-E stain) (e) reveals central ossification (O) attached to the humeral cortex (C) without marrow continuity and surrounded by yellow adipose tissue (A). Humeral marrow (M) is normal.

At gross pathologic examination, parosteal lipomas are adherent to the underlying periosteum and are greasy yellowish masses (Fig 18). These lesions are composed of mature adult fat identical to soft-tissue lipomas. Cartilage, osteoid metaplasia, and areas of osseous excrescences or cortical thickening extending from and attaching the lesion to the bone surface (Fig 18) are common (11,132,133). These osseous excrescences do not show cortical or medullary continuity with the adjacent bone. It is this relationship to the underlying bone that distinguishes this lesion from a soft-tissue lipoma. The cartilage (usually hyaline, but also with small foci of fibrocartilage) and osteoid metaplasia typically occur adjacent to the osseous excrescences. As with other lipomas, parosteal lipomas demonstrate lobular growth, commonly with intervening thin septations. Recent cytogenetic analysis has shown a 3;12 translocation in parosteal lipomas, similar to that evident in soft-tissue lipomas, a finding suggestive of a common pathogenesis (154).

The imaging features of parosteal lipoma are usually distinctive. In a study of 32 cases of parosteal lipoma by Fleming et al (153), nearly 70% of patients had abnormal underlying bone and 50% had osseous reaction. The most frequent osseous reactive changes included bowing of bone or cortical erosion secondary to the adjacent lipomatous mass (153). In a study of eight patients by Murphey et al in 1994 (150), the major radiographic features of parosteal lipoma included a juxtacortical radiolucent lipomatous mass with varying degrees of septation associated with surface bone productive changes ranging from very subtle to obvious cortical thickening and variably sized ossific protuberances or excrescences (Fig 18). These areas of cortical abnormality are best evaluated with radiography and do not show medullary or cortical continuity with the underlying bone, as would be expected in an osteochondroma (Fig 18). Bone scintigraphy almost invariably shows increased radiotracer accumulation limited to this site of bone production.

The CT features of parosteal lipoma also include identification of these surface bone productive changes (150). However, appreciation of these subtle findings may require narrow window and level settings for optimal detection. CT also accurately delineates the lipomatous component of the mass (which generally measures between –60 and –125 HU), the variable degree of septation, and the relationship of the mass to the underlying cortex (150). The osseous protuberances may be quite prominent and show both cortical and marrow components, but again no continuity with the underlying bone is seen (Fig 18). Contrast material administration (at CT or MR imaging) may show mild enhancement in the fibrous tissue rim of the parosteal lipoma, although this feature is uncommon (150).

MR imaging is considered superior to CT for evaluation of parosteal lipoma. The tumor is identified on MR images as a juxtacortical mass with signal intensity identical to that of subcutaneous fat, regardless of pulse sequence (Fig 18) (150). Heterogeneity in these lesions is invariably present and corresponds to the pathologic components in the lesion. Areas with intermediate signal intensity on T1-weighted images that are high signal intensity on T2-weighted images represent the cartilaginous components in parosteal lipoma (150). Fibrovascular septa may cause a lobulated appearance of the fat component, with low-signal-intensity strands on T1-weighted images that become higher in signal intensity on the long TR images (particularly with fat suppression). Larger areas of bone production surrounded by the lipomatous components are also well demonstrated with MR imaging. Adjacent muscle atrophy, poorly demonstrated by CT, is identified on MR images as increased striations of fat in the affected muscle and is caused by associated nerve entrapment (150). This finding is best appreciated on T2-weighted images because of the decreased signal intensity of normal muscle relative to fat. Finally, MR imaging best demonstrates the relationship of the tumor to the underlying native bone and muscle, and this information is important for surgical planning because parosteal lipoma is usually firmly adherent to the underlying cortex at the site of surface bone production.

Treatment of parosteal lipoma is complete surgical resection. In cases with nerve entrapment, the tumor must be removed before irreversible muscle atrophy to maintain function (150–152). The nerve must also be separated from the parosteal lipoma and spared during surgical excision. At surgery, parosteal lipomas are characteristically encapsulated and strongly adherent to the underlying periosteum (Fig 18). The sites at which the tumors are most strongly attached to underlying bone are those areas of most prominent osseous proliferation. Because of this characteristic, adequate surgical removal of a parosteal lipoma requires either subperiosteal dissection, an osteotome to separate the lesion from underlying bone, or segmental resection of bone, in contrast to the relatively easy dissection of a soft-tissue lipoma lying adjacent to bone (150–152). Local recurrence is unusual but has been reported. There are no reports of malignant transformation.

Liposclerosing Myxofibrous Tumor of Bone
Liposclerosing myxofibrous tumor of bone is a benign fibroosseous lesion that is characterized by a complex mixture of histologic elements that may include lipoma, fibroxanthoma, myxoma, myxofibroma, fibrous dysplasia–like features, cyst formation, fat necrosis, ischemic ossification, and rarely cartilage (130,147,160–164). Despite its histologic complexity, liposclerosing myxofibrous tumor has a relatively characteristic radiologic appearance and skeletal distribution (Fig 19). Recent genetic analysis suggests that the lesion may represent a variant of fibrous dysplasia (163). Lesions are often discovered incidentally (41% of cases) in patients without symptoms referable to the lesion (164). Nonspecific pain (48% of cases) or pathologic fracture (10%) is seen in symptomatic patients (164).

View larger version (106K): [in this window] [in a new window] [Download PPT slide]   Figure 19a.  Liposclerosing myxofibrous tumor of the intertrochanteric femur in a 39-year-old man with mild hip pain. (a) Anteroposterior radiograph of the left hip shows a geographic lytic lesion involving the femur with a thin well-defined sclerotic margin (arrows) and containing globular amorphous mineralized matrix (*). (b) Coronal T1-weighted (700/33) MR image demonstrates the lesion (arrows), which has signal intensity similar to that of skeletal muscle. (c) Coronal T2-weighted (2000/100) MR image shows the moderately heterogeneous and predominantly high-signal-intensity tumor (arrows) with a well-defined margin. (d) Technetium 99m methylene diphosphonate bone scan of the pelvis shows mild focal increased radionuclide uptake in the lesion (arrowheads). (e) Photomicrograph (original magnification, x175; H-E stain) shows an area of prominent fibroxanthomatous and myxoid tissue (amorphous pink areas) intermixed with a small area of focal adipocytes (A).

View larger version (120K): [in this window] [in a new window] [Download PPT slide]   Figure 19b.  Liposclerosing myxofibrous tumor of the intertrochanteric femur in a 39-year-old man with mild hip pain. (a) Anteroposterior radiograph of the left hip shows a geographic lytic lesion involving the femur with a thin well-defined sclerotic margin (arrows) and containing globular amorphous mineralized matrix (*). (b) Coronal T1-weighted (700/33) MR image demonstrates the lesion (arrows), which has signal intensity similar to that of skeletal muscle. (c) Coronal T2-weighted (2000/100) MR image shows the moderately heterogeneous and predominantly high-signal-intensity tumor (arrows) with a well-defined margin. (d) Technetium 99m methylene diphosphonate bone scan of the pelvis shows mild focal increased radionuclide uptake in the lesion (arrowheads). (e) Photomicrograph (original magnification, x175; H-E stain) shows an area of prominent fibroxanthomatous and myxoid tissue (amorphous pink areas) intermixed with a small area of focal adipocytes (A).

View larger version (96K): [in this window] [in a new window] [Download PPT slide]   Figure 19c.  Liposclerosing myxofibrous tumor of the intertrochanteric femur in a 39-year-old man with mild hip pain. (a) Anteroposterior radiograph of the left hip shows a geographic lytic lesion involving the femur with a thin well-defined sclerotic margin (arrows) and containing globular amorphous mineralized matrix (*). (b) Coronal T1-weighted (700/33) MR image demonstrates the lesion (arrows), which has signal intensity similar to that of skeletal muscle. (c) Coronal T2-weighted (2000/100) MR image shows the moderately heterogeneous and predominantly high-signal-intensity tumor (arrows) with a well-defined margin. (d) Technetium 99m methylene diphosphonate bone scan of the pelvis shows mild focal increased radionuclide uptake in the lesion (arrowheads). (e) Photomicrograph (original magnification, x175; H-E stain) shows an area of prominent fibroxanthomatous and myxoid tissue (amorphous pink areas) intermixed with a small area of focal adipocytes (A).

View larger version (75K): [in this window] [in a new window] [Download PPT slide]   Figure 19d.  Liposclerosing myxofibrous tumor of the intertrochanteric femur in a 39-year-old man with mild hip pain. (a) Anteroposterior radiograph of the left hip shows a geographic lytic lesion involving the femur with a thin well-defined sclerotic margin (arrows) and containing globular amorphous mineralized matrix (*). (b) Coronal T1-weighted (700/33) MR image demonstrates the lesion (arrows), which has signal intensity similar to that of skeletal muscle. (c) Coronal T2-weighted (2000/100) MR image shows the moderately heterogeneous and predominantly high-signal-intensity tumor (arrows) with a well-defined margin. (d) Technetium 99m methylene diphosphonate bone scan of the pelvis shows mild focal increased radionuclide uptake in the lesion (arrowheads). (e) Photomicrograph (original magnification, x175; H-E stain) shows an area of prominent fibroxanthomatous and myxoid tissue (amorphous pink areas) intermixed with a small area of focal adipocytes (A).

View larger version (173K): [in this window] [in a new window] [Download PPT slide]   Figure 19e.  Liposclerosing myxofibrous tumor of the intertrochanteric femur in a 39-year-old man with mild hip pain. (a) Anteroposterior radiograph of the left hip shows a geographic lytic lesion involving the femur with a thin well-defined sclerotic margin (arrows) and containing globular amorphous mineralized matrix (*). (b) Coronal T1-weighted (700/33) MR image demonstrates the lesion (arrows), which has signal intensity similar to that of skeletal muscle. (c) Coronal T2-weighted (2000/100) MR image shows the moderately heterogeneous and predominantly high-signal-intensity tumor (arrows) with a well-defined margin. (d) Technetium 99m methylene diphosphonate bone scan of the pelvis shows mild focal increased radionuclide uptake in the lesion (arrowheads). (e) Photomicrograph (original magnification, x175; H-E stain) shows an area of prominent fibroxanthomatous and myxoid tissue (amorphous pink areas) intermixed with a small area of focal adipocytes (A).

The differential diagnosis of liposclerosing myxofibrous tumor is relatively limited and includes intraosseous lipoma and fibrous dysplasia (often monostotic). A liposclerosing myxofibrous tumor can be distinguished from a lipoma on CT or MR images by identifying fat within the latter lesion. The distinction between liposclerosing myxofibrous tumor and fibrous dysplasia is more difficult and may not be possible in some cases. In general, fibrous dysplasia reveals less sclerosis and greater radionuclide accumulation on bone scintigrams. The signal intensity of fibrous dysplasia on fluid-sensitive MR images is variable, but the pattern of intermediate or decreased signal intensity often seen in fibrous dysplasia has not been identified in liposclerosing myxofibrous tumor (132,133).

Treatment is variable depending on the clinical scenario. Asymptomatic lesions discovered incidentally may not require therapy. Symptomatic lesions are treated with curettage, bone grafting, and fixation. Lesions manifesting with pathologic fracture of the proximal femur usually require joint arthroplasty. These tumors have an increased prevalence of malignant transformation, compared with fibrous dysplasia and other benign fibro-osseous lesions (147,164). This increased propensity for malignant transformation is likely secondary to the extensive involutional and ischemic change of liposclerosing myxofibrous tumor, with the associated sarcoma arising from areas of ischemic ossification in the lesion or from progressive atypism of the altered lipomatous elements. The risk of malignant transformation to malignant fibrous histiocytoma or osteosarcoma was estimated to be 10% by Kransdorf et al (164). This percentage is likely a marked overestimate because of the referral population in the study. Malignant transformation manifests with aggressive characteristics, including cortical destruction and an associated soft-tissue mass.

Lipoma of the Joint or Tendon Sheath
Lipomas of the joint or tendon sheath are rare and manifest with two variants: (a) a discrete solid fatty mass in the affected joint or tendon sheath and (b) a "lipoma-like" lesion composed of hypertrophic synovial villi distended with fat, typically seen in the knee, and called lipoma arborescens (1,165).

True discrete intraarticular or tendon sheath lipoma is quite rare and must be distinguished from the diffuse synovial lipoma, which is considerably more common. Tendon sheath lipomas most commonly affect the hand and wrist, with less frequent involvement of the foot and ankle (165). Discrete intraarticular (synovial) lipoma most commonly affects the suprapatellar bursa of the knee (75% of cases), with additional cases in the hip and lumbar facet joint reported recently by Marui et al (165).

A lipoma of the tendon sheath or a discrete synovial lipoma is a focal lipomatous mass, with imaging features similar to those of a superficial or deep lipoma. However, focal intraarticular lipoma may show a nonspecific appearance with more fluidlike signal intensity that has been attributed to mucoid degeneration in the tumor.

Lipoma arborescens, or diffuse lipoma of the joint, is a "lipoma-like" lesion in which the subsynovial connective tissue is infiltrated by mature adipocytes, often associated with scattered inflammatory cells. Also referred to as "diffuse synovial lipoma", lipoma arborescens is frequently a secondary reactive process associated with degenerative joint disease, chronic rheumatoid arthritis, or prior trauma (166–172). However, primary cases without underlying chronic intraarticular pathologic conditions have been reported. Synovial lipoma is relatively rare; in a review of 707 lipomas, Myhre-Jensen (2) found two (0.3%) intraarticular cases of lipoma arborescens. We believe this prevalence is an underestimation; with the advent of MR imaging for evaluation of internal joint derangement, lipoma arborescens is now more frequently recognized. Hallel et al (166) suggested that the lesion be called villous lipomatous proliferation of the synovial membrane, because the designation lipoma arborescens implies a tumor.

The majority of cases occur in the knee (Fig 20). Affected individuals have symptoms that progress over several years to as long as 30 years and that include painless synovial thickening and intermittent effusions. The clinical course is typically marked by intermittent exacerbations. Patients range in age from 9 to 66 years, with males affected much more commonly than females (166–172). In reviewing the literature in 1989, Armstrong and Watt (167) noted nine cases in the knee, six unilateral and three bilateral. They also noted one patient with bilateral wrist involvement and one final case involving the hip unilaterally (167). We have recently seen a patient with bilateral knee and bilateral elbow involvement. Although lipoma arborescens is typically observed as an intraarticular lesion involving the synovial lining of the joint, bursal involvement has also been described (169).

View larger version (146K): [in this window] [in a new window] [Download PPT slide]   Figure 20a.  Lipoma arborescens in a 57-year-old man with a swollen, painful left knee and osteoarthritis. (a, b) Sagittal T1-weighted (500/30) (a) and axial T2-weighted (2000/90) (b) MR images of the knee show frondlike projections of hypertrophied synovium extending into a large effusion in the suprapatellar bursa (arrowheads). The hypertrophied synovium has signal intensity identical to that of subcutaneous fat with both pulse sequences. (c) Photograph of resected synovial tissue shows yellow fat in villous fronds (*). (d) Photomicrograph of synovial tissue (original magnification, x80, H-E stain) shows vacuolated appearance of fat tissue in a central core of hypertrophied synovial tissue (*) and superficial synovial cell lining (arrows).

View larger version (148K): [in this window] [in a new window] [Download PPT slide]   Figure 20b.  Lipoma arborescens in a 57-year-old man with a swollen, painful left knee and osteoarthritis. (a, b) Sagittal T1-weighted (500/30) (a) and axial T2-weighted (2000/90) (b) MR images of the knee show frondlike projections of hypertrophied synovium extending into a large effusion in the suprapatellar bursa (arrowheads). The hypertrophied synovium has signal intensity identical to that of subcutaneous fat with both pulse sequences. (c) Photograph of resected synovial tissue shows yellow fat in villous fronds (*). (d) Photomicrograph of synovial tissue (original magnification, x80, H-E stain) shows vacuolated appearance of fat tissue in a central core of hypertrophied synovial tissue (*) and superficial synovial cell lining (arrows).

View larger version (110K): [in this window] [in a new window] [Download PPT slide]   Figure 20c.  Lipoma arborescens in a 57-year-old man with a swollen, painful left knee and osteoarthritis. (a, b) Sagittal T1-weighted (500/30) (a) and axial T2-weighted (2000/90) (b) MR images of the knee show frondlike projections of hypertrophied synovium extending into a large effusion in the suprapatellar bursa (arrowheads). The hypertrophied synovium has signal intensity identical to that of subcutaneous fat with both pulse sequences. (c) Photograph of resected synovial tissue shows yellow fat in villous fronds (*). (d) Photomicrograph of synovial tissue (original magnification, x80, H-E stain) shows vacuolated appearance of fat tissue in a central core of hypertrophied synovial tissue (*) and superficial synovial cell lining (arrows).

View larger version (167K): [in this window] [in a new window] [Download PPT slide]   Figure 20d.  Lipoma arborescens in a 57-year-old man with a swollen, painful left knee and osteoarthritis. (a, b) Sagittal T1-weighted (500/30) (a) and axial T2-weighted (2000/90) (b) MR images of the knee show frondlike projections of hypertrophied synovium extending into a large effusion in the suprapatellar bursa (arrowheads). The hypertrophied synovium has signal intensity identical to that of subcutaneous fat with both pulse sequences. (c) Photograph of resected synovial tissue shows yellow fat in villous fronds (*). (d) Photomicrograph of synovial tissue (original magnification, x80, H-E stain) shows vacuolated appearance of fat tissue in a central core of hypertrophied synovial tissue (*) and superficial synovial cell lining (arrows).

Radiographs of patients with lipoma arborescens reveal soft-tissue swelling around the joint that may or may not be radiolucent. Sonography is useful for documenting the joint effusion as well as the villous nature of the mass. Associated imaging findings include degenerative changes, meniscal tear, synovial cysts, and bone erosions. Arthrography shows multiple or lobulated intraarticular filling defects.

MR imaging demonstrates large villous, frondlike masses with an associated joint effusion (Fig 20) (11,167,169,171,172). The signal intensity of the frondlike projections mirrors the signal characteristics of fat, regardless of pulse sequence (Fig 20). Enhancement following intravenous administration of contrast material may be seen in the overlying inflamed synovium. Lesions have been mistaken for liposarcoma, and the key to diagnosis is recognition of the diffuse synovial (joint or bursal) origin, as well as the multilobulated frondlike contour. CT also shows a frondlike mass of fat attenuation, although the villous nature may be more difficult to recognize at CT, compared with MR imaging.

The suggested treatment of lipoma arborescens remains synovectomy. Synovectomy alleviates the synovial thickening and effusions but not the associated osteoarthritis (1,4,6). Recurrence following synovectomy has been reported.

	Conclusions

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Soft-tissue Lipomatous Lesions Bone, Joint, and Tendon... Conclusions References

 
Benign musculoskeletal lipomatous lesions are common in both soft tissue and bone. The radiologic manifestations vary widely. We have reviewed, illustrated, and correlated the pathologic and radiologic features of these lesions, including soft-tissue lipoma, lipomatosis, lipomatosis of nerve, angiolipoma, myolipoma of soft tissue, chondroid lipoma, spindle cell lipoma or pleomorphic lipoma, hibernoma, intraosseous lipoma, parosteal lipoma, liposclerosing myxofibrous tumor of bone, and lipoma of the joint or tendon sheath. The unifying radiologic feature of these lesions is the presence of fat, which corresponds to adipose tissue pathologically. The identification of fat is best performed with CT or MR imaging. Imaging also often demonstrates nonlipomatous components. The frequency of these nonlipomatous elements, their appearances, pathologic basis, and implication for patient evaluation vary by specific diagnosis. Understanding and recognizing the spectrum of the various types of benign musculoskeletal lipomatous lesions allows improved patient assessment and is vital for optimal clinical management including diagnosis, biopsy, and treatment.

	Acknowledgments

 
The authors gratefully acknowledge the support of Anika Ismel Torruella for manuscript preparation. In addition, we thank the residents who attend the Armed Forces Institute of Pathology’s radiologic pathology courses (past, present, and future) for their contributions to the AFIP series of patients. Without them, this project would not have been possible.

	Footnotes

 
Abbreviation: H-E = hematoxylin-eosin

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official nor as reflecting the views of the Departments of the Army, Navy, or Defense.

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