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Pitfalls in MR Image Interpretation Prompting Referrals to an Orthopedic Oncology Clinic¹

¹ From the Department of Radiology, University of Chicago Hospitals, 5841 S Maryland Ave, MC 2026, Chicago, IL 60637. Recipient of a Certificate of Merit award for an education exhibit at the 2005 RSNA Annual Meeting. Received March 17, 2006; revision requested August 14 and received August 25; accepted August 28. Authors have no financial relationships to disclose. Address correspondence to G.S.S. (e-mail: sstacy{at}radiology.bsd.uchicago.edu).

	Abstract

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

 
Patients referred to the authors’ hospital for evaluation on suspicion of a bone or soft-tissue malignancy frequently present to the Orthopaedic Oncology Clinic with magnetic resonance (MR) images that show typical features of nonmalignant or nonneoplastic entities. The purpose of this article is to review the benign entities that may be mistaken by the radiologist for a malignancy and thus lead to needless referral to an orthopedic oncologist. Normal hematopoietic marrow and marrow edema due to a stress reaction may mimic a neoplasm at MR imaging, but knowledge of the typical patterns and locations of these features allows an accurate radiologic interpretation. The MR imaging appearance of osteonecrosis, Paget disease, benign bone lesions, and rheumatologic conditions may be confusing; in such circumstances, radiographic findings may help formulate a correct diagnosis. Knowledge of the common locations and appearances of bursae and ganglia is necessary so that radiologists do not misinterpret these benign entities as soft-tissue sarcomas. Soft-tissue trauma and inflammation also may mimic tumors at MR imaging, but a familiarity with the imaging patterns of nonneoplastic change in muscle allows the avoidance of misinterpretation. The clinical history, as always, is an important component of proper diagnosis. The radiologist can be especially useful to both the clinician and the patient by recognizing entities that are highly unlikely to represent malignancy and by confidently reporting those entities as benign, thereby sparing the patient an unnecessary trip to the orthopedic oncologist.

© RSNA, 2007

	LEARNING OBJECTIVES FOR TEST 4

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

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

• Recognize the importance of accurate radiologic interpretation of benign and nonneoplastic entities to avoid unnecessary referrals to an orthopedic oncologist.
  
• Describe the MR imaging features of nonneoplastic or benign marrow, articular, and soft-tissue processes that may mimic malignancy but that do not require referral to an orthopedic oncologist.
  
• Identify the MR imaging features specific to marrow, articular, and soft-tissues processes that warrant referral to an orthopedic oncologist.
  

	Introduction

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

 
The orthopedic oncology team at our hospital includes surgeons, oncologists, pathologists, and radiologists who specialize in the diagnosis and treatment of tumors of bone and soft tissue. As the radiologists on this team, we are fortunate to be able to attend the Orthopaedic Oncology Clinic, where the surgeons see both new and returning patients. Most of the new patients are referred to the clinic from another institution, and they are asked to provide all pertinent imaging studies performed at the referring institution. We review these images with the orthopedic surgeons to decide whether the lesion truly represents a neoplasm that requires further work-up and possibly treatment. New patients also are asked to provide for our review the reports of imaging studies performed at the referring hospitals and clinics. While reviewing these reports, we noticed that radiologists often misinterpreted certain imaging patterns and nonneoplastic disease entities as representative of a possible malignancy (Table 1).

	Marrow Pitfalls

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

On T1-weighted magnetic resonance (MR) images, the signal intensity of normal yellow marrow is like that of subcutaneous fat. Yellow marrow appears darker on MR images obtained with fat suppression. On T1-weighted MR images, the signal intensity of normal red marrow is lower than that of fat (and, hence, of yellow marrow) but generally higher than that of skeletal muscle because of an admixture of fatty elements with hematopoietic elements. On T2-weighted fat-suppressed MR images, the signal intensity of normal red marrow is higher than that of yellow marrow and often is similar to or slightly higher than that of skeletal muscle.

"Residual" red marrow is a typical component of the bones of the axial skeleton and frequently is present in the proximal metaphyses of the femora and humeri in normal adults (3). Residual red marrow is symmetric in distribution, and hence its observation in both proximal femora at pelvic MR imaging does not usually cause any diagnostic confusion. However, if one of the two extremities is unavailable for comparison, the nonfatty signal of residual red marrow may be mistaken for that of a neoplasm. Residual red marrow is seen fairly often in the distal femur at MR imaging of the knee (4), particularly in adolescents and women of menstruating age. Red marrow may persist in a focal, geographic, patchy pattern that occasionally mimics an intramedullary lesion (Fig 1). Knowledge of the typical distribution of residual red marrow in adults and the signal characteristics of normal red marrow should allow the avoidance of misdiagnosis.

View larger version (156K): [in this window] [in a new window] [Download PPT slide]   Figure 1a.  Normal hematopoietic (or red) marrow in a 21-year-old woman with knee pain and no additional medical problems. (a) Coronal T1-weighted MR image shows patchy regions of intermediate signal intensity (arrows) in the distal femur. These regions have signal intensity higher than that in nearby skeletal muscle (*), and this characteristic is suggestive of residual red marrow. (b) Coronal T2-weighted fat-suppressed MR image shows a patchy area (arrow) with signal intensity similar to that of skeletal muscle (*). MR images from 1%–2% of patients referred to the Orthopaedic Oncology Clinic showed similar features that are characteristic of residual red marrow but that were misinterpreted as potential malignancy.

View larger version (115K): [in this window] [in a new window] [Download PPT slide]   Figure 1b.  Normal hematopoietic (or red) marrow in a 21-year-old woman with knee pain and no additional medical problems. (a) Coronal T1-weighted MR image shows patchy regions of intermediate signal intensity (arrows) in the distal femur. These regions have signal intensity higher than that in nearby skeletal muscle (*), and this characteristic is suggestive of residual red marrow. (b) Coronal T2-weighted fat-suppressed MR image shows a patchy area (arrow) with signal intensity similar to that of skeletal muscle (*). MR images from 1%–2% of patients referred to the Orthopaedic Oncology Clinic showed similar features that are characteristic of residual red marrow but that were misinterpreted as potential malignancy.

Osteoid osteomas and stress fractures in the cortex may lead to edema in the adjacent marrow that may mimic the patchy pattern of normal hematopoietic red marrow on MR images; hence, careful evaluation of the cortex is recommended in cases where peripheral nonfatty marrow is evident.

Stress Fractures and Stress Reaction
Bone marrow edema may result from a variety of nonneoplastic disorders. In some cases, an associated abnormality, such as erosion (eg, from rheumatoid arthritis) or an abscess, may be evident. In other cases (eg, in bone contusion, early-stage osteonecrosis, and transient migratory edema), there may be no discrete underlying or adjacent lesion. Stress reaction and stress fracture were the most common causes of nonneoplastic edema seen at MR imaging in patients referred to the Orthopaedic Oncology Clinic.

The detection and interpretation of stress fractures pose challenges to radiologists. The radiographic features of stress fracture vary in accordance with the location and chronicity of the injury and may include periosteal reaction, endosteal sclerosis, cortical thickening, and a lucent fracture line (5). Often radiographs appear normal in the early stages of a fracture.

Even before they can be seen on radiographs, stress fractures often produce marrow edema that is visible at MR imaging. Although such edema may mimic a malignancy, a discrete hypointense fracture line within the area of edema in a patient suspected of having a stress fracture allows an accurate diagnosis (Fig 2). A focus of increased signal intensity in the adjacent cortex also may be visible, particularly in the long bones (6). MR images also may show periosteal and marrow edema, which have been referred to as a "stress reaction," before the development of a discrete hypointense fracture line (7). Initially, the edema may be apparent only on T2-weighted images; however, marrow edema becomes visible also on T1-weighted images as the stress reaction progresses, and a fracture line eventually develops. Adjacent soft-tissue edema and enhancement (Fig 3) may lead the interpreting radiologist to suspect a neoplasm. In some instances, computed tomography (CT) may be necessary to provide the specificity needed to diagnose a stress fracture, particularly in the sacrum (8). Radiologists should be familiar with the common locations of stress injuries (Table 3). Such familiarity is particularly important in cases where the injury is manifested on MR images as edema without a discrete fracture line (9). In our experience, the edema associated with stress fracture and stress reaction is frequently much more pronounced on T2-weighted fat-suppressed images than on T1-weighted images and is often ill defined, particularly on T1-weighted images (Fig 3b); in contrast, a well-defined hypointense rounded lesion is usually evident on T1-weighted images in patients with a neoplasm.

View larger version (193K): [in this window] [in a new window] [Download PPT slide]   Figure 2a.  Fatigue fracture in a 25-year-old woman runner with low back pain. Coronal T1-weighted (a) and coronal T2-weighted fat-suppressed (b) MR images show edema in the left sacral ala as well as a linear band (arrow) that extends through the region of edema, a typical finding of stress fracture. MR images from nearly 2% of patients referred to the Orthopaedic Oncology Clinic showed features characteristic of stress fracture or stress reaction that were misinterpreted as a potential malignancy.

View larger version (168K): [in this window] [in a new window] [Download PPT slide]   Figure 2b.  Fatigue fracture in a 25-year-old woman runner with low back pain. Coronal T1-weighted (a) and coronal T2-weighted fat-suppressed (b) MR images show edema in the left sacral ala as well as a linear band (arrow) that extends through the region of edema, a typical finding of stress fracture. MR images from nearly 2% of patients referred to the Orthopaedic Oncology Clinic showed features characteristic of stress fracture or stress reaction that were misinterpreted as a potential malignancy.

View larger version (94K): [in this window] [in a new window] [Download PPT slide]   Figure 3a.  Metatarsal stress fracture in a 53-year-old woman with forefoot pain. (a) Short-axis T1-weighted fat-suppressed MR image, obtained after the intravenous administration of a gadolinium chelate, shows enhancement of the marrow of the fourth metatarsal (arrowhead) as well as the surrounding soft tissues (arrow). (b) Sagittal T1-weighted MR image shows mild and poorly defined low-signal-intensity edema (arrowhead) in the marrow of the distal fourth metatarsal, with a slight angulation of the dorsal cortex (arrow), findings suggestive of a stress fracture. Follow-up radiographs (not shown) revealed callus formation at the site, a finding that helped confirm the diagnosis. Stress fractures and stress reaction often result in relatively mild marrow edema on T1-weighted images, compared with more pronounced marrow and soft-tissue edema on T2-weighted images. Marrow and soft-tissue enhancement after gadolinium administration also is fairly common.

View larger version (137K): [in this window] [in a new window] [Download PPT slide]   Figure 3b.  Metatarsal stress fracture in a 53-year-old woman with forefoot pain. (a) Short-axis T1-weighted fat-suppressed MR image, obtained after the intravenous administration of a gadolinium chelate, shows enhancement of the marrow of the fourth metatarsal (arrowhead) as well as the surrounding soft tissues (arrow). (b) Sagittal T1-weighted MR image shows mild and poorly defined low-signal-intensity edema (arrowhead) in the marrow of the distal fourth metatarsal, with a slight angulation of the dorsal cortex (arrow), findings suggestive of a stress fracture. Follow-up radiographs (not shown) revealed callus formation at the site, a finding that helped confirm the diagnosis. Stress fractures and stress reaction often result in relatively mild marrow edema on T1-weighted images, compared with more pronounced marrow and soft-tissue edema on T2-weighted images. Marrow and soft-tissue enhancement after gadolinium administration also is fairly common.

Osteonecrosis that occurs in subarticular locations (eg, in the femoral head) is easily recognized by most radiologists. Metadiaphyseal osteonecrosis, on the other hand, has received relatively little attention in the radiology literature and is more likely to be a source of uncertainty for radiologists, judging from the radiologic reports of imaging studies in patients referred to our clinic. The characteristic radiographic pattern of metadiaphyseal osteonecrosis is that of a serpentine ringlike band of sclerosis that separates a central necrotic zone of variable lucency from surrounding normal marrow; however, this pattern is a relatively late manifestation of osteonecrosis. Earlier in the course of disease, osteonecrosis may result in a poorly defined region of lucency within the medullary space, a feature that may be indistinguishable from a lytic neoplastic process at radiography (Fig 4) (11,12). In such cases, a correct diagnosis of osteonecrosis often can be rendered easily with the aid of MR imaging.

View larger version (117K): [in this window] [in a new window] [Download PPT slide]   Figure 4a.  Myelodysplastic syndrome in a 43-year-old woman with ankle pain. (a) Lateral ankle radiograph shows a poorly defined region of lucency (arrowheads) centrally within the distal tibia. (b) Sagittal T1-weighted MR image shows a curvilinear band of low signal intensity (arrowhead) that surrounds a central region of fat signal intensity (*). (c) Sagittal T2-weighted fat-suppressed MR image shows a high-signal-intensity curvilinear band that surrounds a low-signal-intensity central region of necrotic bone. Note the double line sign (arrow), a feature that is virtually pathognomonic of osteonecrosis. Images from 2%–3% of patients referred to the Orthopaedic Oncology Clinic showed findings characteristic of osteonecrosis but interpreted as potential malignancy. MR imaging should allow a confident diagnosis in most cases.

View larger version (155K): [in this window] [in a new window] [Download PPT slide]   Figure 4b.  Myelodysplastic syndrome in a 43-year-old woman with ankle pain. (a) Lateral ankle radiograph shows a poorly defined region of lucency (arrowheads) centrally within the distal tibia. (b) Sagittal T1-weighted MR image shows a curvilinear band of low signal intensity (arrowhead) that surrounds a central region of fat signal intensity (*). (c) Sagittal T2-weighted fat-suppressed MR image shows a high-signal-intensity curvilinear band that surrounds a low-signal-intensity central region of necrotic bone. Note the double line sign (arrow), a feature that is virtually pathognomonic of osteonecrosis. Images from 2%–3% of patients referred to the Orthopaedic Oncology Clinic showed findings characteristic of osteonecrosis but interpreted as potential malignancy. MR imaging should allow a confident diagnosis in most cases.

View larger version (131K): [in this window] [in a new window] [Download PPT slide]   Figure 4c.  Myelodysplastic syndrome in a 43-year-old woman with ankle pain. (a) Lateral ankle radiograph shows a poorly defined region of lucency (arrowheads) centrally within the distal tibia. (b) Sagittal T1-weighted MR image shows a curvilinear band of low signal intensity (arrowhead) that surrounds a central region of fat signal intensity (*). (c) Sagittal T2-weighted fat-suppressed MR image shows a high-signal-intensity curvilinear band that surrounds a low-signal-intensity central region of necrotic bone. Note the double line sign (arrow), a feature that is virtually pathognomonic of osteonecrosis. Images from 2%–3% of patients referred to the Orthopaedic Oncology Clinic showed findings characteristic of osteonecrosis but interpreted as potential malignancy. MR imaging should allow a confident diagnosis in most cases.

One of the earliest MR imaging findings of osteonecrosis is nonspecific marrow edema (13). Marrow edema caused by osteonecrosis may be difficult or impossible to differentiate from marrow edema with other causes, including transient migratory edema. If the edema does not appear to be associated with a discrete lesion and if osteonecrosis is suspected clinically, then follow-up MR imaging may be indicated to monitor the regression of edema or its progression to a pattern more specific to osteonecrosis.

Metadiaphyseal osteonecrosis rarely may undergo sarcomatous degeneration. Such degeneration should be suspected if cortical destruction or a soft-tissue mass is seen (Fig 5). It is appropriate to refer patients with such findings to an orthopedic oncologist.

View larger version (118K): [in this window] [in a new window] [Download PPT slide]   Figure 5.  Sarcomatous degeneration of bone in a 57-year-old man with right knee pain that did not respond to physical therapy. Anteroposterior radiograph of the knee shows sclerotic lesions typical of multifocal osteonecrosis (arrowheads). Destruction of the lateral cortex of the proximal tibia (arrow) is indicative of sarcomatous degeneration, a finding that was confirmed at MR imaging. This patient was appropriately referred to the Orthopaedic Oncology Clinic.

Paget disease usually produces specific features on radiographs. In long bones, the initial "active" phase of disease manifests as osteolysis that extends from the end of the bone into the diaphysis with a characteristic flame-shaped advancing front. Cortical and trabecular thickening denote an "inactive" phase (14). Both phases may be evident at the same time. Radiologists may be hesitant to offer a diagnosis of Paget disease if the patient is relatively young or if the process is observed in an atypical location (Fig 6).

View larger version (78K): [in this window] [in a new window] [Download PPT slide]   Figure 6.  Paget disease of bone in a 64-year-old woman. Lateral radiograph of the distal forearm shows the advancing lytic front typical of the "active" phase of disease (arrowhead) and cortical and trabecular thickening characteristic of the "sclerotic" phase (arrow), as well as bowing and generalized expansile remodeling of the distal radius. Patients with images that showed characteristic features of Paget disease in unusual locations (eg, bones of the upper extremity) occasionally were referred to the Orthopaedic Oncology Clinic with a presumptive diagnosis of neoplasm.

View larger version (155K): [in this window] [in a new window] [Download PPT slide]   Figure 7a.  Paget disease of bone in a 50-year-old woman. (a) Axial T1-weighted MR image of the pelvis shows abnormally low-signal-intensity marrow in the left ilium (arrowhead) and thickened cortex (arrow). (b) Axial CT image of the pelvis better illustrates the cortical thickening (arrow), a finding typical of Paget disease. Approximately 1% of patients referred to the Orthopaedic Oncology Clinic had abnormalities at MR imaging that could have been easily diagnosed as Paget disease at radiography or CT.

View larger version (111K): [in this window] [in a new window] [Download PPT slide]   Figure 7b.  Paget disease of bone in a 50-year-old woman. (a) Axial T1-weighted MR image of the pelvis shows abnormally low-signal-intensity marrow in the left ilium (arrowhead) and thickened cortex (arrow). (b) Axial CT image of the pelvis better illustrates the cortical thickening (arrow), a finding typical of Paget disease. Approximately 1% of patients referred to the Orthopaedic Oncology Clinic had abnormalities at MR imaging that could have been easily diagnosed as Paget disease at radiography or CT.

	Fibroxanthoma (Nonossifying Fibroma) and Other Benign Lesions

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

 
Many patients who were referred to the Orthopaedic Oncology Clinic had benign bone tumors that were correctly diagnosed by radiologists at the referring institution on the basis of typical imaging features. Tumors of this kind are often locally aggressive and require surgery, and therefore the referral of these patients to an orthopedic oncologist is appropriate. However, some of the patients who were referred to the clinic had imaging studies that showed typical "latent" or inactive lesions that were mistaken by radiologists for more aggressive lesions. In the latter group, the most common lesion was fibroxanthoma (nonossifying fibroma).

Fibroxanthomas are common in growing children and generally do not require referral to an orthopedic surgeon unless a fracture is present or the lesion is unusually large. They typically occur in the metadiaphyseal regions of the long bones, most commonly in the lower extremities, and are cortically based or eccentrically positioned within the bone (18). They have a characteristic radiographic appearance and should not be mistaken for a malignancy. The lesions are lucent early in their development, but they maintain well-defined and usually sclerotic margins. A multilocular appearance is typical (Fig 8a), and mild expansile remodeling of the bone may occur. As the lesions mature, sclerotic components develop. Most lesions eventually disappear.

View larger version (132K): [in this window] [in a new window] [Download PPT slide]   Figure 8a.  Fibroxanthoma (nonossifying fibroma) in a 16-year-old girl. (a) Lateral radiograph of the knee shows a mixed lucent and sclerotic intramedullary lesion (arrowheads) along the posterior cortex of the distal femur. The sclerotic margin and multilocular appearance are typical of a fibroxanthoma. (b) Sagittal T2-weighted MR image of the knee shows areas of heterogeneous signal intensity (arrowheads) within the lesion. Images from 2%–3% of patients referred to the Orthopaedic Oncology Clinic showed features characteristic of fibroxanthoma but interpreted as a potential malignancy. Several of these patients arrived with only MR images and probably would not have been referred if a radiograph had been obtained for correlation.

View larger version (158K): [in this window] [in a new window] [Download PPT slide]   Figure 8b.  Fibroxanthoma (nonossifying fibroma) in a 16-year-old girl. (a) Lateral radiograph of the knee shows a mixed lucent and sclerotic intramedullary lesion (arrowheads) along the posterior cortex of the distal femur. The sclerotic margin and multilocular appearance are typical of a fibroxanthoma. (b) Sagittal T2-weighted MR image of the knee shows areas of heterogeneous signal intensity (arrowheads) within the lesion. Images from 2%–3% of patients referred to the Orthopaedic Oncology Clinic showed features characteristic of fibroxanthoma but interpreted as a potential malignancy. Several of these patients arrived with only MR images and probably would not have been referred if a radiograph had been obtained for correlation.

Occasionally, a fibroxanthoma may be unusually large and may cause so much expansile remodeling that it becomes difficult to distinguish from lesions such as an aneurysmal bone cyst or a chondromyxoid fibroma (19). Referral to an orthopedic oncologist in such cases is reasonable.

Several of the patients referred to our clinic had bone lesions with imaging features characteristic of benign entities that did not require further work-up or treatment. The images depicted classic examples of fibrous dysplasia, intraosseous lipoma, calcaneal solitary bone cyst, vertebral hemangioma, and other lesion types. These entities generally do not require referral to an orthopedic oncologist unless a fracture is present.

Patients with enchondromas were commonly referred to our clinic. Because it may be difficult to distinguish a large enchondroma from a low-grade chondrosarcoma on the basis of imaging features alone (ie, without a precise correlation with clinical symptoms), the referral of such cases to an orthopedic oncologist may be well founded; referral at least allows appropriate examination and follow-up. There were, however, a few cases of small enchondromas of the finger seen on radiographs and small enchondromas with the classic lobular appearance seen on MR images (Fig 9) that were misinterpreted as likely malignancies, and these patients were needlessly referred to the clinic (20).

View larger version (151K): [in this window] [in a new window] [Download PPT slide]   Figure 9.  Enchondroma in a 40-year-old woman. Coronal T1-weighted MR image of the knee shows a small lobulated lesion (arrow) in the medullary cavity of the proximal tibia, a typical appearance of a benign enchondroma. Patients with a small, painless, lobulated lesion such as this need not be referred to an orthopedic oncologist.

	Articular and Juxta-articular Pitfalls

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

In our opinion, the key to diagnosing a rheumatologic condition at MR imaging is to search for the same features that would establish the diagnosis at radiography. Does the process result in abnormalities on both sides of the joint? Are multiple joints involved? Is the articular cartilage abnormally thinned, with resultant narrowing of the joint?

Subchondral degenerative cysts are common in patients with osteoarthritis. These cysts can become quite large, particularly those in the femoral head and acetabulum. In our experience, however, such cysts are invariably associated with other features of osteoarthritis, including narrowing of the joint and osteophyte formation; these other features may be more readily appreciated on radiographs than on large-field-of-view MR images. Furthermore, subchondral cysts are often multiple and located on both sides of the joint. Bone proliferation from osteophyte formation, particularly at the sternoclavicular joint, occasionally produces an apparent mass both at physical examination and at MR imaging; CT may help secure the diagnosis of osteoarthritis in such questionable cases.

Synovitis is the hallmark of inflammatory arthritides, and radiologists who are unfamiliar with the MR imaging appearance of a thickened enhancing pannus may mistake it for a tumor. Gadolinium eventually passes from the synovium into the joint fluid, where it may produce enhancement of the entire joint and its contents, enhancement that is suggestive of a solid intra-articular mass. Hence, delayed imaging after gadolinium administration is generally discouraged. Observation of synovitis in other joints or adjacent tendon sheaths supports the diagnosis of inflammatory arthritis. Like degenerative cysts, subchondral erosions in inflammatory arthritis may become quite large, may be surrounded by marrow edema, and occasionally may mimic a tumor. The presence of multiple subchondral lesions with accompanying synovitis in a joint, or the involvement of multiple joints, opposes a diagnosis of malignancy (Fig 10).

View larger version (119K): [in this window] [in a new window] [Download PPT slide]   Figure 10.  Rheumatoid arthritis in a 61-year-old man. Coronal T2-weighted fat-suppressed image of the ankle shows a high-signal-intensity lesion with a surrounding hypointense rim (large arrow) in the inferior aspect of the talar body, features that represent an area of subcortical erosion with adjacent bone marrow edema. Several smaller areas of erosion in the calcaneus (small arrows) and high-signal-intensity synovitis in the tarsal sinus (*) support a diagnosis of erosive inflammatory arthritis rather than a neoplasm. Subchondral erosions and degenerative cysts, particularly larger (1–3-cm) lesions with adjacent edema and enhancement, were commonly misinterpreted as a neoplasm and resulted in needless referrals to the Orthopaedic Oncology Clinic. This case demonstrates the importance of recognizing an arthritic process.

Other entities, such as pigmented villonodular synovitis and "primary" synovial (osteo)chondromatosis, also result in intra-articular masses that may require surgical debulking or biopsy. As patients with these disorders are treated at our institution by orthopedic oncologists, we considered their referral appropriate. However, patients with synovitis due to a rheumatologic condition or loose bodies in the joint secondary to prior trauma or osteoarthritis do not need to be referred to an orthopedic oncologist.

Recesses, Bursae, Cysts, and Ganglia
Distention of a synovial recess or bursa can produce a soft-tissue mass that may be misinterpreted as a neoplasm on MR examinations, particularly if it contains heterogeneous material (eg, loose bodies, nodular synovitis) or if the radiologist is not familiar with normal articular-bursal anatomy. A synovial recess is generally defined as a direct extension of a joint cavity, whereas a bursa is a synovium-lined compartment that exists separately from a joint (23). However, this distinction is not always clear, because what constitutes a bursa embryologically may appear at later imaging studies as a synovial recess, and vice versa. Articular communication with periarticular bursae is not uncommon (24). A list of commonly encountered recesses and bursae, compiled from various sources (23–33), is provided in Table 4.

View larger version (183K): [in this window] [in a new window] [Download PPT slide]   Figure 11a.  Popliteal cyst with ossified loose bodies in a 64-year-old woman. (a) Sagittal T2-weighted MR image of the knee shows an apparent mass with small internal foci of low signal intensity (arrowhead) posterior to the medial femoral condyle. (b) Axial intermediate-weighted fat-suppressed MR image shows round foci of low signal intensity (arrowhead) in the popliteal cyst. The "neck" of the cyst extends between the tendons of the semi-membranosus muscle (small arrow) and the medial head of the gastrocnemius muscle (large arrow). (c) Lateral radiograph shows ossified loose bodies (arrow) that correspond to the round low-signal-intensity foci seen on MR images. This case emphasizes the importance of knowing the bursal anatomy and correlating MR images with radiographs.

View larger version (166K): [in this window] [in a new window] [Download PPT slide]   Figure 11b.  Popliteal cyst with ossified loose bodies in a 64-year-old woman. (a) Sagittal T2-weighted MR image of the knee shows an apparent mass with small internal foci of low signal intensity (arrowhead) posterior to the medial femoral condyle. (b) Axial intermediate-weighted fat-suppressed MR image shows round foci of low signal intensity (arrowhead) in the popliteal cyst. The "neck" of the cyst extends between the tendons of the semi-membranosus muscle (small arrow) and the medial head of the gastrocnemius muscle (large arrow). (c) Lateral radiograph shows ossified loose bodies (arrow) that correspond to the round low-signal-intensity foci seen on MR images. This case emphasizes the importance of knowing the bursal anatomy and correlating MR images with radiographs.

View larger version (144K): [in this window] [in a new window] [Download PPT slide]   Figure 11c.  Popliteal cyst with ossified loose bodies in a 64-year-old woman. (a) Sagittal T2-weighted MR image of the knee shows an apparent mass with small internal foci of low signal intensity (arrowhead) posterior to the medial femoral condyle. (b) Axial intermediate-weighted fat-suppressed MR image shows round foci of low signal intensity (arrowhead) in the popliteal cyst. The "neck" of the cyst extends between the tendons of the semi-membranosus muscle (small arrow) and the medial head of the gastrocnemius muscle (large arrow). (c) Lateral radiograph shows ossified loose bodies (arrow) that correspond to the round low-signal-intensity foci seen on MR images. This case emphasizes the importance of knowing the bursal anatomy and correlating MR images with radiographs.

Cysts and ganglia are fluid- or mucin-filled masses that typically occur around joints. They may cause symptoms and may require orthopedic surgical intervention; however, referral to an orthopedic oncologist is usually unnecessary. The distinction between what represents a cyst and what represents a ganglion is not clear, in large part because the pathogenesis of cysts and ganglia is uncertain (24,34). The terms often are used interchangeably.

Cysts and ganglia typically have homogeneous signal intensity similar to that of fluid (Fig 12a), although lobulation and septation frequently are evident. On MR images obtained after intravenous gadolinium is administered, a thin rim of peripheral enhancement is commonly seen, a feature that is occasionally accompanied by enhanced septal lines (Fig 12b). Paralabral cysts of the shoulder and acetabulum and parameniscal cysts of the knee are typically associated with underlying labral tears and meniscal tears, respectively (24,31). Identification of the underlying tear is essential for the diagnosis of a paralabral or parameniscal cyst, although the tear may not always be evident. In the knee, cysts and ganglia adjacent to the cruciate ligaments and the tibio-fibular joint are fairly common. In the shoulder, cysts arising above the acromioclavicular joint often represent an extension of fluid from the glenohumeral joint through a torn rotator cuff and acromioclavicular joint capsule (35). Cysts also may arise within muscle, and these, too, show signal intensity similar to that of fluid. Cysts in the rotator cuff musculature have been shown to be associated with underlying cuff tears (36). Ganglia are commonly found in the hands and feet, often adjacent to tendons. Cysts and ganglia may extend into adjacent bone; occasionally, these are seen as lucent lesions on radiographs. The keys to diagnosing benign cysts and ganglia are knowing their common locations and recognizing the characteristic homogeneous fluidlike signal intensity and enhanced thin peripheral rim (and occasionally enhanced septa).

View larger version (134K): [in this window] [in a new window] [Download PPT slide]   Figure 12a.  Ganglion of the Guyon canal in a 38-year-old man. (a) Axial T2-weighted MR image of the wrist shows a lobulated homogeneous mass (arrowhead) with fluid signal intensity volar to the hook of the hamate. (b) Axial T1-weighted fat-suppressed image of the wrist after intravenous administration of a gadolinium chelate shows only peripheral enhancement of the mass (arrowhead), a feature typical of a ganglion. Nearly 2% of patients referred to the Orthopaedic Oncology Clinic arrived with MR images that showed characteristic features of a juxta-articular cyst or ganglion that was misinterpreted as a potential malignancy. The signal intensity and enhancement patterns usually allow a confident diagnosis.

View larger version (138K): [in this window] [in a new window] [Download PPT slide]   Figure 12b.  Ganglion of the Guyon canal in a 38-year-old man. (a) Axial T2-weighted MR image of the wrist shows a lobulated homogeneous mass (arrowhead) with fluid signal intensity volar to the hook of the hamate. (b) Axial T1-weighted fat-suppressed image of the wrist after intravenous administration of a gadolinium chelate shows only peripheral enhancement of the mass (arrowhead), a feature typical of a ganglion. Nearly 2% of patients referred to the Orthopaedic Oncology Clinic arrived with MR images that showed characteristic features of a juxta-articular cyst or ganglion that was misinterpreted as a potential malignancy. The signal intensity and enhancement patterns usually allow a confident diagnosis.

View larger version (176K): [in this window] [in a new window] [Download PPT slide]   Figure 13a.  Synovial sarcoma in an 18-year-old man. (a) Axial T2-weighted fat-suppressed MR image of the knee shows a relatively homogeneously hyperintense mass (arrowhead) deep to the medial retinaculum. The mass mimics a large cyst. (b) Axial T1-weighted fat-suppressed MR image of the knee, obtained after the intravenous administration of a gadolinium chelate, shows diffuse enhancement of the mass (arrowhead), a finding indicative of a solid tumor. This patient was appropriately referred to the Orthopaedic Oncology Clinic.

View larger version (163K): [in this window] [in a new window] [Download PPT slide]   Figure 13b.  Synovial sarcoma in an 18-year-old man. (a) Axial T2-weighted fat-suppressed MR image of the knee shows a relatively homogeneously hyperintense mass (arrowhead) deep to the medial retinaculum. The mass mimics a large cyst. (b) Axial T1-weighted fat-suppressed MR image of the knee, obtained after the intravenous administration of a gadolinium chelate, shows diffuse enhancement of the mass (arrowhead), a finding indicative of a solid tumor. This patient was appropriately referred to the Orthopaedic Oncology Clinic.

	Soft-Tissue Pitfalls

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

View larger version (165K): [in this window] [in a new window] [Download PPT slide]   Figure 14a.  Muscle strain injury in a 20-year-old man. (a) Axial T2-weighted MR image of the thigh shows a mass with high signal intensity (arrowhead) within the rectus femoris. (b) Axial T1-weighted MR image shows the location of the high-signal-intensity mass (arrowhead) adjacent to the distal tendon (arrow) of the rectus femoris muscle, findings compatible with a subacute hematoma from a previous myotendinous strain injury. The low-signal-intensity rim that partially surrounds the hematoma represents hemosiderin deposition. Knowledge of the MR imaging appearance of hematomas should allow the avoidance of a mistaken diagnosis of malignancy in most cases.

View larger version (167K): [in this window] [in a new window] [Download PPT slide]   Figure 14b.  Muscle strain injury in a 20-year-old man. (a) Axial T2-weighted MR image of the thigh shows a mass with high signal intensity (arrowhead) within the rectus femoris. (b) Axial T1-weighted MR image shows the location of the high-signal-intensity mass (arrowhead) adjacent to the distal tendon (arrow) of the rectus femoris muscle, findings compatible with a subacute hematoma from a previous myotendinous strain injury. The low-signal-intensity rim that partially surrounds the hematoma represents hemosiderin deposition. Knowledge of the MR imaging appearance of hematomas should allow the avoidance of a mistaken diagnosis of malignancy in most cases.

View larger version (134K): [in this window] [in a new window] [Download PPT slide]   Figure 15a.  Avulsion injury and tendon retraction in a 58-year-old woman. (a) Axial T2-weighted MR image of the proximal thigh shows an apparent mass with a rim of high signal intensity (arrowhead) in the posterior soft tissues. Note the absence of the normal hamstring musculature. (b) Coronal T2-weighted fat-suppressed MR image shows the retracted hamstring musculature surrounded by fluid (arrowhead).

View larger version (108K): [in this window] [in a new window] [Download PPT slide]   Figure 15b.  Avulsion injury and tendon retraction in a 58-year-old woman. (a) Axial T2-weighted MR image of the proximal thigh shows an apparent mass with a rim of high signal intensity (arrowhead) in the posterior soft tissues. Note the absence of the normal hamstring musculature. (b) Coronal T2-weighted fat-suppressed MR image shows the retracted hamstring musculature surrounded by fluid (arrowhead).

View larger version (144K): [in this window] [in a new window] [Download PPT slide]   Figure 16a.  Avulsion injury in a 15-year-old boy. (a) Anteroposterior radiograph of the right hip shows lobulated ossification (arrow) along the proximal ischium. (b) Coronal T2-weighted MR image of the pelvis shows high-signal-intensity edema within the right ischium (arrowhead) as well as at the origin of the hamstring musculature (arrow). (c) Transverse CT image through the pelvis shows an avulsion fracture (arrowhead) arising from the right ischium at the hamstring origin. Healing avulsion injuries in children may mimic an ossifying neoplasm on radiographs; MR imaging or CT may be useful for follow-up evaluation.

View larger version (147K): [in this window] [in a new window] [Download PPT slide]   Figure 16b.  Avulsion injury in a 15-year-old boy. (a) Anteroposterior radiograph of the right hip shows lobulated ossification (arrow) along the proximal ischium. (b) Coronal T2-weighted MR image of the pelvis shows high-signal-intensity edema within the right ischium (arrowhead) as well as at the origin of the hamstring musculature (arrow). (c) Transverse CT image through the pelvis shows an avulsion fracture (arrowhead) arising from the right ischium at the hamstring origin. Healing avulsion injuries in children may mimic an ossifying neoplasm on radiographs; MR imaging or CT may be useful for follow-up evaluation.

View larger version (86K): [in this window] [in a new window] [Download PPT slide]   Figure 16c.  Avulsion injury in a 15-year-old boy. (a) Anteroposterior radiograph of the right hip shows lobulated ossification (arrow) along the proximal ischium. (b) Coronal T2-weighted MR image of the pelvis shows high-signal-intensity edema within the right ischium (arrowhead) as well as at the origin of the hamstring musculature (arrow). (c) Transverse CT image through the pelvis shows an avulsion fracture (arrowhead) arising from the right ischium at the hamstring origin. Healing avulsion injuries in children may mimic an ossifying neoplasm on radiographs; MR imaging or CT may be useful for follow-up evaluation.

View larger version (128K): [in this window] [in a new window] [Download PPT slide]   Figure 17a.  Posttraumatic heterotopic ossification in a 15-year-old boy. (a) Anteroposterior radiograph of the left hip shows soft-tissue ossification (arrows) overlying the superolateral acetabulum and proximal femur. (b) Axial T1-weighted MR image of the hip shows a mass (arrowhead) of heterogeneous signal intensity in the approximate location of the rectus femoris muscle. (c) Axial T2-weighted MR image reveals that the mass (arrowhead) is of heterogeneously high signal intensity. (d) Axial T1-weighted MR image obtained after the intravenous administration of a gadolinium chelate shows enhancement of the mass (arrowhead). (e) Axial CT image reveals peripheral ossification of the mass (arrowhead), a typical feature of myositis ossificans traumatica. Myositis ossificans may mimic a soft-tissue sarcoma on MR images, but CT often reveals the true nature of the mass.

View larger version (129K): [in this window] [in a new window] [Download PPT slide]   Figure 17b.  Posttraumatic heterotopic ossification in a 15-year-old boy. (a) Anteroposterior radiograph of the left hip shows soft-tissue ossification (arrows) overlying the superolateral acetabulum and proximal femur. (b) Axial T1-weighted MR image of the hip shows a mass (arrowhead) of heterogeneous signal intensity in the approximate location of the rectus femoris muscle. (c) Axial T2-weighted MR image reveals that the mass (arrowhead) is of heterogeneously high signal intensity. (d) Axial T1-weighted MR image obtained after the intravenous administration of a gadolinium chelate shows enhancement of the mass (arrowhead). (e) Axial CT image reveals peripheral ossification of the mass (arrowhead), a typical feature of myositis ossificans traumatica. Myositis ossificans may mimic a soft-tissue sarcoma on MR images, but CT often reveals the true nature of the mass.

View larger version (141K): [in this window] [in a new window] [Download PPT slide]   Figure 17c.  Posttraumatic heterotopic ossification in a 15-year-old boy. (a) Anteroposterior radiograph of the left hip shows soft-tissue ossification (arrows) overlying the superolateral acetabulum and proximal femur. (b) Axial T1-weighted MR image of the hip shows a mass (arrowhead) of heterogeneous signal intensity in the approximate location of the rectus femoris muscle. (c) Axial T2-weighted MR image reveals that the mass (arrowhead) is of heterogeneously high signal intensity. (d) Axial T1-weighted MR image obtained after the intravenous administration of a gadolinium chelate shows enhancement of the mass (arrowhead). (e) Axial CT image reveals peripheral ossification of the mass (arrowhead), a typical feature of myositis ossificans traumatica. Myositis ossificans may mimic a soft-tissue sarcoma on MR images, but CT often reveals the true nature of the mass.

View larger version (141K): [in this window] [in a new window] [Download PPT slide]   Figure 17d.  Posttraumatic heterotopic ossification in a 15-year-old boy. (a) Anteroposterior radiograph of the left hip shows soft-tissue ossification (arrows) overlying the superolateral acetabulum and proximal femur. (b) Axial T1-weighted MR image of the hip shows a mass (arrowhead) of heterogeneous signal intensity in the approximate location of the rectus femoris muscle. (c) Axial T2-weighted MR image reveals that the mass (arrowhead) is of heterogeneously high signal intensity. (d) Axial T1-weighted MR image obtained after the intravenous administration of a gadolinium chelate shows enhancement of the mass (arrowhead). (e) Axial CT image reveals peripheral ossification of the mass (arrowhead), a typical feature of myositis ossificans traumatica. Myositis ossificans may mimic a soft-tissue sarcoma on MR images, but CT often reveals the true nature of the mass.

View larger version (124K): [in this window] [in a new window] [Download PPT slide]   Figure 17e.  Posttraumatic heterotopic ossification in a 15-year-old boy. (a) Anteroposterior radiograph of the left hip shows soft-tissue ossification (arrows) overlying the superolateral acetabulum and proximal femur. (b) Axial T1-weighted MR image of the hip shows a mass (arrowhead) of heterogeneous signal intensity in the approximate location of the rectus femoris muscle. (c) Axial T2-weighted MR image reveals that the mass (arrowhead) is of heterogeneously high signal intensity. (d) Axial T1-weighted MR image obtained after the intravenous administration of a gadolinium chelate shows enhancement of the mass (arrowhead). (e) Axial CT image reveals peripheral ossification of the mass (arrowhead), a typical feature of myositis ossificans traumatica. Myositis ossificans may mimic a soft-tissue sarcoma on MR images, but CT often reveals the true nature of the mass.

Malignant soft-tissue tumors may bleed, and it may be difficult to distinguish between a simple hematoma and a hemorrhagic neoplasm at MR imaging (38). Nodular or masslike enhancement following intravenous gadolinium administration is suggestive of a neoplasm, whereas a lack of enhancement essentially excludes that diagnosis. However, fibrovascular tissue within a hematoma may show some enhancement following gadolinium administration; furthermore, delayed imaging after gadolinium administration may allow diffusion of the contrast material into the hematoma, with resultant enhancement mimicking that in a tumor. If a hematoma is observed but an underlying soft-tissue neoplasm is suspected, then the hematoma should be followed up until it resolves, or biopsy should be considered.

Nontraumatic Muscle Edema and Inflammation
Edema in a muscle or a group of muscles may be seen at MR imaging in various nontraumatic and nonneoplastic conditions, including autoimmune myositis, subacute muscle denervation, recent surgery or radiation therapy, rhabdomyolysis, and compartment syndrome (38,43). Although myositis typically results in muscle edema and enlargement rather than a discrete tumoral mass, occasionally an infiltrative and aggressive masslike appearance may manifest at MR imaging and mimic a sarcoma (43–45), particularly in cases of infectious myositis. What appears to be a mass in one imaging plane may be more confidently diagnosed as an inflamed muscle in another plane (Fig 18) if the muscle maintains its normal fusiform contour. Persistence of the normal feathery pattern of muscle signal intensity (despite signal alterations caused by edema) also argues against the presence of a tumor. More extensive inflammation results in the disappearance of this pattern, with enlargement of the affected muscle and edema of the fascial planes; stranding of the subcutaneous fat from cellulitis may be seen in cases of infectious myositis. An examination of the myotendinous junction may help distinguish inflammation from a neoplasm: Tendons typically are displaced by tumors but may retain their normal position within an inflamed muscle.

View larger version (111K): [in this window] [in a new window] [Download PPT slide]   Figure 18a.  Inflammatory myopathy in a 39-year-old man. (a) Axial T1-weighted fat-suppressed MR image of the arm shows enhancement of the brachialis muscle (arrowhead), a finding that mimics a mass. (b) Sagittal T1-weighted fat-suppressed MR image shows diffuse enhancement of the distal brachialis muscle (arrowheads); the muscle maintains its normal contour, and no mass is evident, two features that oppose a diagnosis of neoplasm. Muscle injury may have a similar appearance on MR images.

View larger version (113K): [in this window] [in a new window] [Download PPT slide]   Figure 18b.  Inflammatory myopathy in a 39-year-old man. (a) Axial T1-weighted fat-suppressed MR image of the arm shows enhancement of the brachialis muscle (arrowhead), a finding that mimics a mass. (b) Sagittal T1-weighted fat-suppressed MR image shows diffuse enhancement of the distal brachialis muscle (arrowheads); the muscle maintains its normal contour, and no mass is evident, two features that oppose a diagnosis of neoplasm. Muscle injury may have a similar appearance on MR images.

Necrotic or cystic tumors may mimic abscesses at MR imaging. Gadolinium administration in such cases often helps demonstrate nodular or masslike solid enhancing components in the tumor. It may be possible to distinguish neoplastic from nonneoplastic muscle abnormalities by using MR imaging, even without knowledge of the patient’s condition (eg, pain, fever, underlying disease processes). However, aspiration or tissue biopsy may be required in some instances, and referral to an orthopedic oncologist is certainly appropriate if there is a reasonable probability that the abnormality is a neoplasm (48).

Lipoma
Fat-containing soft-tissue masses of the extremities are one of the most common reasons for referral to the Orthopaedic Oncology Clinic. This is not surprising, as lipoma is the most common soft-tissue neoplasm (49). Although many of the patients who presented to our clinic with benign lipomas were referred for elective resection, a few were referred with reports that erroneously suggested a diagnosis of liposarcoma. A lipoma can be confidently diagnosed at MR imaging if all pulse sequences depict a mass with signal that is completely isointense to the signal of normal subcutaneous fat (Fig 19). The presence of a few nonenhancing thin (<2-mm) septa within the mass need not undermine confidence in a diagnosis of lipoma. In addition, the interdigitation of an intramuscular lipoma with skeletal muscle may create a striated appearance; this finding has not been described as occurring in liposarcomas, and it, too, allows a confident diagnosis of lipoma.

View larger version (150K): [in this window] [in a new window] [Download PPT slide]   Figure 19.  Lipoma in a 49-year-old woman. Axial T1-weighted MR image shows a mass (arrowhead) in the musculature of the medial aspect of the right arm. The mass had homogeneous signal intensity characteristic of fat on all MR images, a finding indicative of a lipoma. Liposarcoma should not be included in the differential diagnosis.

View larger version (142K): [in this window] [in a new window] [Download PPT slide]   Figure 20.  Well-differentiated liposarcoma in a 63-year-old woman. Axial T1-weighted MR image of the arm shows a predominantly fatty mass (arrowhead) with prominent septa. Focal areas with signal that is slightly hypointense to that of fat are indicative of nonadipose elements (*). Although such findings might be representative also of a benign lipoma with nonadipose elements, referral to an orthopedic oncologist is appropriate.

	Summary

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

 
A couple of observations can be made on the basis of our experience in the Orthopaedic Oncology Clinic. First, radiologists tend to overdiagnose nonfatty marrow on MR examinations as a potential neoplasm. Normal hematopoietic marrow and osteonecrosis have typical MR imaging features that should allow a confident diagnosis in most cases. The MR imaging appearances of Paget disease and fibroxanthoma (nonossifying fibroma) may not be familiar to radiologists and hence may lead to a diagnosis of malignancy; this underscores the importance of correlating MR images with radiographs, which show the usually pathognomonic appearance of these lesions. MR images of a suspected malignant lesion should never be interpreted without radiographic correlation. In addition, marrow edema may be due to trauma, osteonecrosis, or a host of other processes and should not invoke an automatic finding of neoplasm.

Second, radiologists tend to overdiagnose benign soft-tissue and juxta-articular conditions (eg, muscle edema, distended bursae, and cysts) as potential malignancies on MR images. MR imaging is generally the best modality for evaluating soft-tissue abnormalities; however, knowledge of the anatomy of joints and bursae and an appreciation of signal intensity and enhancement patterns that may be seen in nonneoplastic disease processes are essential for accurate diagnosis.

Our consideration of the entities described in this article was no doubt aided by the fact that when we reviewed the imaging studies, we were situated in the Orthopaedic Oncology Clinic, where we almost certainly had greater access to pertinent clinical data than did the radiologists at the referring institutions. Furthermore, other factors unrelated to the radiologist’s interpretation (eg, cosmetic deformity) may lead to referral to an orthopedic oncologist. Finally, very few of these entities undergo biopsy, and hence histologic confirmation of the consensus diagnoses made by the orthopedic oncology team is not usually available. Nevertheless, a radiologist’s report stating that a malignant neoplasm cannot be excluded but describing an entity with classically benign imaging features may leave the clinician with little choice but to refer the patient to a tumor specialist. The evaluation of solitary lesions in bone or soft tissue may be a daunting task, particularly for radiologists who do not see many bone and soft-tissue tumors. In many cases, the lesion clearly appears aggressive, and a confident diagnosis of malignancy can be rendered. In other cases, the lesion may be described as indeterminate despite evaluation with multiple modalities, and referral of the patient to an orthopedic oncologist is appropriate. The radiologist can be most useful by recognizing entities that are highly unlikely to represent malignancy and by confidently reporting them as benign, thereby sparing the patient an unnecessary trip to the tumor specialist.

	Footnotes

 
See the commentary by Bancroft following this article.

	References

Top Abstract LEARNING OBJECTIVES FOR TEST... Introduction Marrow Pitfalls Fibroxanthoma (Nonossifying... Articular and Juxta-articular... Soft-Tissue Pitfalls Summary References

 

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