(Radiographics. 1999;19:1253-1280.)
© RSNA, 1999
From the Archives of the AFIP 1
Imaging of Musculoskeletal Neurogenic Tumors: Radiologic-Pathologic Correlation
Mark D. Murphey, MD ,
W. Sean Smith, MD, 2,
Stacy E. Smith, MD ,
Mark J. Kransdorf, MD and
H. Thomas Temple, M, 3
1 From the Department of Radiologic Pathology, Armed Forces Institute of Pathology, 6825 16th Street NW, Bldg 54, Room M-133A, Washington, DC 20306 (M.D.M., W.S.S., S.E.S.); Departments of Radiology and Nuclear Medicine (M.D.M., W.S.S.) and Surgery (H.T.T.), Uniformed Services University of the Health Sciences, Bethesda, Md; Department of Radiology, University of Maryland School of Medicine, Baltimore (M.D.M., S.E.S.); Department of Surgery, Orthopedic Service, Walter Reed Army Medical Center, Washington, DC (H.T.T.); Department of Radiology, St Mary's Hospital, Richmond, Va (M.J.K.). Received April 8, 1999; revision requested April 26 and received May 28; accepted June 1. Presented as a scientific exhibit at the 1995 RSNA scientific assembly. Address reprint requests to M.D.M.
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Abstract
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Numerous neurogenic tumors can affect the musculoskeletal system, including traumatic neuroma, Morton neuroma, neural fibrolipoma, nerve sheath ganglion, neurilemoma, neurofibroma, and malignant peripheral nerve sheath tumors (PNSTs). The diagnosis of neurogenic tumors can be suggested from their imaging appearances, including lesion shape and intrinsic imaging characteristics. It is also important to establish lesion location along a typical nerve distribution (eg, plantar digital nerve in Morton neuroma, median nerve in neural fibrolipoma, large nerve trunk in benign and malignant PNSTs). Traumatic and Morton neuromas are commonly related to an amputation stump or are located in the intermetatarsal space, respectively. Neural fibrolipomas show fat interspersed between nerve fascicles and are often associated with macrodactyly. Nerve sheath ganglion has a cystic appearance and commonly occurs about the knee. Radiologic characteristics of neurilemoma, neurofibroma, and malignant PNST at computed tomography (CT), ultrasonography, and magnetic resonance imaging include fusiform shape, identification of entering and exiting nerve, low attenuation at CT, target sign, fascicular sign, split-fat sign, and associated muscle atrophy. Although differentiation of neurilemoma from neurofibroma and of benign from malignant PNST is problematic, recognition of the radiologic appearances of neurogenic tumors often allows prospective diagnosis and improves clinical management of patients.
Index Terms: Nervous system, neoplasms, 4.315, 4.325, 4.364 • Neurofibromatosis, 4.1444, 4.1831 • Neuroma, 4.315, 4.364
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INTRODUCTION
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Benign neurogenic tumors include traumatic neuroma; Morton neuroma; neural fibrolipoma; nerve sheath ganglion; and the benign peripheral nerve sheath tumors (PNSTs), neurilemoma (schwannoma) and neurofibroma. Neurogenic neoplasms represent approximately 10%–12% of all benign soft-tissue neoplasms (1,2).
Malignant neurogenic neoplasms are called malignant PNSTs and account for 7%–8% of all malignant soft-tissue neoplasms (3,4). Neurofibroma, rarely neurilemoma, and malignant PNST may arise in association with neurofibromatosis type 1 (NF1). NF1 is a common genetic disorder representing a mesodermal dysplasia associated with multiple neurogenic neoplasms and skeletal abnormalities.
The evaluation of neurogenic neoplasms often reveals distinctive features, either lesion location, clinical history, or radiologic appearance. In this article, we review the development and histologic features of normal peripheral nerves; discuss the clinical characteristics, pathologic features, radiologic appearances, treatment, and prognosis for various musculoskeletal neurogenic tumors; and review the imaging signs of PNSTs.
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DEVELOPMENT AND HISTOLOGIC CHARACTERISTICS OF NORMAL PERIPHERAL NERVES
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We believe many of the imaging features of neurogenic tumors are a reflection of their similarity to the normal neurogenic tissue from which they are derived. Because of this relationship, an understanding of basic nerve anatomy and histologic characteristics is important.
Peripheral nerves are derived embryologically from neural crest tissue and migrating axons from the primitive neural tube (5). The two predominant supporting elements are the connective tissue stroma and Schwann cells, which encase all peripheral nerve axons to varying degrees. A myelinated fiber results if only one axon is encased by one Schwann cell. Unmyelinated fibers result if a Schwann cell encases many axons.
Each peripheral nerve is surrounded by a thick connective tissue sheath called the epineurium. Within the nerve, groups of axons are surrounded and divided by a fibrous stroma called the perineurium, which creates multiple bundles of fibers or fascicles (Fig 1) (5). This gross appearance of a normal nerve can be recognized at ultrasonography (US) and magnetic resonance (MR) imaging, particularly in large nerve trunks such as the sciatic nerve, and has been described as a fascicular appearance (Fig 2) (6–8). Vascular supply to the peripheral nerves is relatively profuse, arises from adjacent vessels, and forms longitudinally oriented channels along the nerve.
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Figure 1. Normal nerve structure. Photomicrograph (original magnification, approximately x20; Bielschowsky silver stain) of an axial section of normal sural nerve. Nerve is surrounded by epineurium (straight arrows). Bundles of nerve fibers (*) are surrounded by perineurium (curved arrows), creating a fascicular appearance. Adipose tissue (black arrowheads) and blood vessels (white arrowheads) are seen about the nerve.
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Figure 2. Normal nerve appearance at MR imaging. Axial T1-weighted (repetition time msec/echo time msec = 500/20) MR image of the upper thigh shows the normal sciatic nerve (arrows) with small circular low-signal-intensity areas (arrowheads) surrounded by a background of mildly higher-signal-intensity areas representing the fascicular structure of the normal nerve.
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BENIGN NEUROGENIC TUMORS
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Traumatic Neuroma
Traumatic neuromas develop from a nonneoplastic proliferation of the proximal end of a severed, partially transected, or injured nerve as a result of trauma or surgery (2,9–17). Pain is the most common clinical symptom and is often reproduced with palpation or tapping on the lesion (Tinel sign). A firm soft-tissue mass at a focal pressure site may be apparent. The most common location for traumatic neuromas is the lower extremity after amputation, followed by the head and neck (frequently in the oral cavity, because more than 50% of these lesions are related to tooth extraction) (4–21). Other sites include the radial nerve and brachial plexus (22–25).
Traumatic neuromas have been divided into two major categories based on the anatomic location of the tangled, multidirectional, regenerating axonal mass with respect to the proximal nerve end (2,18). Spindle neuromas are internal, focal, fusiform swellings secondary to chronic friction or irritation to a nondisrupted, injured but intact nerve trunk. Lateral or terminal neuromas are the result of severe trauma with partial avulsion, disruption, or total transection of a nerve (2,10,11).
These lesions have a "bulbous-end" morphology in continuity with the normal nerve proximally, arise 1–12 months after transection or injury, and vary in size with no malignant potential (2,26). At histologic analysis, traumatic neuromas are nonneoplastic, nonencapsulated tangled masses of axons, Schwann cells, endoneurial cells, and perineurial cells in a dense collagenous matrix with surrounding fibroblasts (2,9,17). The disorganization of the neurogenic tissue (caused by multidirectional proliferation of cells in an abortive attempt to repair the injured nerve) allows this lesion to be distinguished from neurofibroma, although discrete bundles or fascicles are still recognized.
There are only limited reports of the computed tomographic (CT) or MR imaging appearance of traumatic neuromas (25–29). Typically, a fusiform mass or focal enlargement with an entering and exiting nerve (spindle type) or only an entering nerve terminating in a bulbous shape (lateral or terminal type) is identified (Fig 3a) (9,26–28). Lesions of small nerves may not be detected radiologically or may be seen as a nonspecific soft-tissue mass without an entering nerve (Fig 3a) (26). Lesion margins are often well defined, although some irregularity (likely related to multidirectional cell proliferation) has been seen with US. These characteristics should allow traumatic neuromas to be distinguished from other causes of amputation stump pain, including recurrent malignant tumor, osteomyelitis, abscess, bursitis, cellulitis, hematoma, heterotopic bone, foreign bodies, atrophied stump muscles, and cicatrization (9,26,27).
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Figure 3a. Traumatic neuromas that developed after a below-the-knee amputation in a 33-year-old man. (a) Coronal T1-weighted (500/16) MR image shows two masses. The proximal neuroma (terminal type) has an entering tubular tibial nerve (arrowhead) ending in a bulbous expansion (*). Spindle neuroma resulting from chronic irritation of a small superficial nerve (not visible as a distinct structure) at the prosthesis attachment is seen as a nonspecific mass (arrows). (b) On an axial T2-weighted (4,300/126) image, the proximal lesion has high signal intensity with a fascicular pattern (arrow).
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Figure 3b. Traumatic neuromas that developed after a below-the-knee amputation in a 33-year-old man. (a) Coronal T1-weighted (500/16) MR image shows two masses. The proximal neuroma (terminal type) has an entering tubular tibial nerve (arrowhead) ending in a bulbous expansion (*). Spindle neuroma resulting from chronic irritation of a small superficial nerve (not visible as a distinct structure) at the prosthesis attachment is seen as a nonspecific mass (arrows). (b) On an axial T2-weighted (4,300/126) image, the proximal lesion has high signal intensity with a fascicular pattern (arrow).
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Traumatic neuromas typically have intermediate signal intensity (similar to that of muscle) on T1-weighted MR images and intermediate to high signal intensity on T2-weighted images (Fig 3) (9,26,29). Their signal intensity is often heterogeneous, with a ringlike pattern ("fascicular sign"), which we believe correlates with the histologic morphology of nerve fascicles and which is best seen on T2-weighted images (Fig 3b) (9). Intrinsic US and CT characteristics of these lesions are nonspecific (ie, they appear hypoechoic and with attenuation similar to that of muscle) (7,27,28).
Prevention of traumatic neuroma involves approximating the two severed nerve ends so that nerve repair and regeneration results (11,30,31). Multiple surgical techniques are available to remove the proximal nerve stump from the area of scar, which limits the potential both for lesion development and recurrence (11,30,31). Initial conservative therapy, including acupuncture, cortisone injection, transcutaneous and direct nerve stimulation, and physical therapy, has been successful in up to 50% of patients (28,30,31). Surgical resection is reserved for patients in whom conservative treatment fails.
Morton Neuroma
Morton neuroma, originally described by Thomas Morton in 1876, is a nonneoplastic lesion representing perineural fibrosis of the plantar digital nerve (3). The nerve is usually affected at the level of the metatarsal head and is frequently associated with a surrounding inflammatory response. These lesions most commonly occur between the third and fourth metatarsals, and then between the second and third (32–35). Morton neuromas are uncommon between the first and second metatarsals and rare between the fourth and fifth (32,34). Because they have a marked female predilection (as high as 18:1), some suggest that these lesions are caused by nerve compression against the intermetatarsal ligament from foot position in high-heeled shoes (2,9,33–35). Although this pathogenesis intuitively seems correct, it has not been substantiated biomechanically, raising the question of
other causes including ischemia (36).
Patients may experience exercise-provoked pain that may radiate into the toes or leg and is relieved by rest. Compressing the intermetatarsal space may also elicit pain (32–35). Morton neuroma is usually not palpable, although associated synovial cysts may be clinically evident. Asymptomatic lesions may be relatively common, with a prevalence of 30% reported in one series of 70 volunteers (36). These asymptomatic lesions, unlike symptomatic Morton neuromas, had no significant gender predilection and were also statistically smaller than symptomatic lesions (mean transverse diameter of 4.5 vs 5.6 mm, respectively) (36).
A Morton neuroma appears pathologically as fusiform enlargement of the plantar digital nerve at its bifurcation, with thickening of the epineural fascicles, perineural fibrosis with high collagen content (Renaut bodies), and loss of the myelinated fibers (36–38).
Radiography invariably shows normal findings and is most useful for excluding other causes of pain (39,40). US and MR imaging are superior to CT for identifying Morton neuroma (40). At US, this lesion appears as a round or ovoid, well-defined, hypoechoic mass located just proximal to the metatarsal heads in the intermetatarsal space (38,39). Small lesions (<5 mm in size) can be difficult to evaluate with US. In a series of 45 surgically treated patients, Kaminsky et al (39) reported false-negative results in two cases related to small lesion size. Power Doppler US may be a valuable adjunct for identifying these lesions and often shows increased vascularity (Fig 4c). In one study, MR imaging had an accuracy of 90%, positive predictive value of 100%, and negative predictive value of 60% in identification of Morton neuroma (37). In our experience and that of others, these lesions are most evident on coronal, small field-of-view, T1-weighted images (Fig 4) (36,37,41–44). Zanetti et al (36) suggested three MR imaging criteria for diagnosis of Morton neuroma: (a) the lesion is centered in the neurovascular bundle, within the intermetatarsal space, and on the plantar side of the transverse metatarsal ligament (Figs 4, 5); (b) the lesion is well demarcated (excluding partial volume artifact from the adjacent joint capsule); and (c) the signal intensity of the lesion is similar to that of skeletal muscle on T1-weighted images and less than that of fat on T2-weighted images (likely reflecting high collagen content fibrosis). Erikson et al (42) reported intermetatarsal bursal fluid as an associated finding proximal to Morton neuroma in 67% of their cases. More recently, Zanetti et al (36) reported a small amount of bursal fluid in the first three intermetatarsal spaces in 67% of asymptomatic patients. A large amount of intermetatarsal bursal fluid (>3 mm in transverse diameter) or fluid in the fourth intermetatarsal space should suggest an associated Morton neuroma (36).
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Figure 4a. Morton neuroma in a 45-year-old woman. (a) Short axis T1-weighted (750/15) MR image shows a 6-mm mass (*) in the interspace between the third and fourth metatarsals at the level of the metatarsal head. (b, c) Short axis fat-suppressed contrast material-enhanced T1-weighted (700/15) MR (b) and power Doppler US (c) images show marked enhancement and increased vascularity of the lesion (*). (d) Photograph of the resected specimen shows the entering plantar digital nerve (arrowheads) and the mass (arrows) distally representing perineural fibrosis.
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Figure 4b. Morton neuroma in a 45-year-old woman. (a) Short axis T1-weighted (750/15) MR image shows a 6-mm mass (*) in the interspace between the third and fourth metatarsals at the level of the metatarsal head. (b, c) Short axis fat-suppressed contrast material-enhanced T1-weighted (700/15) MR (b) and power Doppler US (c) images show marked enhancement and increased vascularity of the lesion (*). (d) Photograph of the resected specimen shows the entering plantar digital nerve (arrowheads) and the mass (arrows) distally representing perineural fibrosis.
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Figure 4c. Morton neuroma in a 45-year-old woman. (a) Short axis T1-weighted (750/15) MR image shows a 6-mm mass (*) in the interspace between the third and fourth metatarsals at the level of the metatarsal head. (b, c) Short axis fat-suppressed contrast material-enhanced T1-weighted (700/15) MR (b) and power Doppler US (c) images show marked enhancement and increased vascularity of the lesion (*). (d) Photograph of the resected specimen shows the entering plantar digital nerve (arrowheads) and the mass (arrows) distally representing perineural fibrosis.
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Figure 4d. Morton neuroma in a 45-year-old woman. (a) Short axis T1-weighted (750/15) MR image shows a 6-mm mass (*) in the interspace between the third and fourth metatarsals at the level of the metatarsal head. (b, c) Short axis fat-suppressed contrast material-enhanced T1-weighted (700/15) MR (b) and power Doppler US (c) images show marked enhancement and increased vascularity of the lesion (*). (d) Photograph of the resected specimen shows the entering plantar digital nerve (arrowheads) and the mass (arrows) distally representing perineural fibrosis.
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Figure 5. Morton neuroma in a 31-year-old man. T2-weighted (2,500/90) MR image shows a 4-mm mass (*) between the third and fourth metatarsal heads. Conspicuity of the lesion is low (cf Fig 4a) due to minimal differences in signal intensity of surrounding fat and the lesion.
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Morton neuromas are markedly less conspicuous on T2-weighted MR images, making differentiation from surrounding muscle and fat difficult (Fig 5). Use of fat-suppressed T2-weighted sequences may allow better delineation. In our experience, the lesions often but not invariably enhance with intravenously administered contrast material (Fig 4b) (45,46). In a study of six patients with Morton neuromas, Terk et al (41) found fat-suppressed contrast-enhanced MR imaging to be superior for depicting lesions and used it to identify neuromas in two patients in whom other MR imaging sequences (including fat-suppressed T2 weighted) failed. However, Williams et al (44) reported that only four of 11 lesions were visible at enhanced MR imaging.
Initial treatment of Morton neuroma is directed at modifying patient footwear. When conservative management fails, other modes of therapy are used, including neurolysis, steroid injection, ultrasound therapy, and surgical release of the transverse metatarsal ligament for decompression. Surgical resection of the neuroma and involved nerve segment is the most successful treatment.
Neural Fibrolipoma
Mason (47) initially described neural fibrolipoma in the English literature in 1953. Additional names for this lesion have included fibrolipomatous hamartoma of nerve, perineural lipoma, fatty infiltration of the nerve, and intraneural lipoma (47,48). Neural fibrolipoma is the preferred term, as it best describes the underlying disorder (47). The cause of this disorder is unknown, although it may be related to hypertrophy of mature fat and fibroblasts in the epineurium (48).
Patients typically present before 30 years of age, often at birth or early childhood, with a soft, slowly enlarging mass in the volar aspect of the hand, wrist, or forearm (47). Males and females are equally affected, and there is no familial predisposition (48). The upper extremity is involved in 78%–96% of cases with a marked predilection for the median nerve (85% of cases in the Silverman and Enzinger series [48]) (Fig 6) (48,49). Other sites include the lower extremity (4%–22% of cases), ulnar nerve, radial nerve, and brachial plexus (48,49). Patients present with swelling, with or without accompanying pain and neurologic symptoms including carpal tunnel syndrome (51). In 27%–67% of cases, neural fibrolipoma is associated with macrodactyly, which is referred to as macrodystrophia lipomatosa (Fig 6) (48,49). This entity usually involves the second and third digits of the hand or foot, but multiple digits may be affected. Other causes of macrodactyly include angiomatosis, neurofibromatosis, Klippel-Trénaunay-Weber syndrome, and Proteus syndrome (49–57).
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Figure 6a. Neural fibrolipoma in a 34-year-old man with macrodactyly. (a, b) Photograph (a) and anteroposterior radiograph (b) of the hand show soft-tissue and osseous enlargement (in length and width) of the second and third digits with ulnar deviation and secondary degenerative changes (arrowhead in b). (c, d) Axial T1-weighted (500/20) MR images of the wrist (c) and phalanges (d) reveal adipose tissue causing thickened nerve, surrounding fascicles (arrowheads) both proximally (median nerve) and distally (interdigital nerve branches), and overgrowth of the second and third digits with predominance of fat (arrow). (e) Longitudinal US scan of the wrist shows multiple elongated, enlarged median nerve fascicles (arrowheads). (f) Intraoperative photograph of wrist dissection shows a diffusely thickened, yellow median nerve resulting from the neural fibrolipoma (arrow). (g, h) Photograph of a coronally sectioned whole-mount specimen (hematoxylin-eosin stain) (g) of the second digit and photomicrograph (h) (original magnification, x200; hematoxylin-eosin stain) demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*) surrounding nerve fascicles (arrow).
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Figure 6b. Neural fibrolipoma in a 34-year-old man with macrodactyly. (a, b) Photograph (a) and anteroposterior radiograph (b) of the hand show soft-tissue and osseous enlargement (in length and width) of the second and third digits with ulnar deviation and secondary degenerative changes (arrowhead in b). (c, d) Axial T1-weighted (500/20) MR images of the wrist (c) and phalanges (d) reveal adipose tissue causing thickened nerve, surrounding fascicles (arrowheads) both proximally (median nerve) and distally (interdigital nerve branches), and overgrowth of the second and third digits with predominance of fat (arrow). (e) Longitudinal US scan of the wrist shows multiple elongated, enlarged median nerve fascicles (arrowheads). (f) Intraoperative photograph of wrist dissection shows a diffusely thickened, yellow median nerve resulting from the neural fibrolipoma (arrow). (g, h) Photograph of a coronally sectioned whole-mount specimen (hematoxylin-eosin stain) (g) of the second digit and photomicrograph (h) (original magnification, x200; hematoxylin-eosin stain) demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*) surrounding nerve fascicles (arrow).
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Figure 6c. Neural fibrolipoma in a 34-year-old man with macrodactyly. (a, b) Photograph (a) and anteroposterior radiograph (b) of the hand show soft-tissue and osseous enlargement (in length and width) of the second and third digits with ulnar deviation and secondary degenerative changes (arrowhead in b). (c, d) Axial T1-weighted (500/20) MR images of the wrist (c) and phalanges (d) reveal adipose tissue causing thickened nerve, surrounding fascicles (arrowheads) both proximally (median nerve) and distally (interdigital nerve branches), and overgrowth of the second and third digits with predominance of fat (arrow). (e) Longitudinal US scan of the wrist shows multiple elongated, enlarged median nerve fascicles (arrowheads). (f) Intraoperative photograph of wrist dissection shows a diffusely thickened, yellow median nerve resulting from the neural fibrolipoma (arrow). (g, h) Photograph of a coronally sectioned whole-mount specimen (hematoxylin-eosin stain) (g) of the second digit and photomicrograph (h) (original magnification, x200; hematoxylin-eosin stain) demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*) surrounding nerve fascicles (arrow).
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Figure 6d. Neural fibrolipoma in a 34-year-old man with macrodactyly. (a, b) Photograph (a) and anteroposterior radiograph (b) of the hand show soft-tissue and osseous enlargement (in length and width) of the second and third digits with ulnar deviation and secondary degenerative changes (arrowhead in b). (c, d) Axial T1-weighted (500/20) MR images of the wrist (c) and phalanges (d) reveal adipose tissue causing thickened nerve, surrounding fascicles (arrowheads) both proximally (median nerve) and distally (interdigital nerve branches), and overgrowth of the second and third digits with predominance of fat (arrow). (e) Longitudinal US scan of the wrist shows multiple elongated, enlarged median nerve fascicles (arrowheads). (f) Intraoperative photograph of wrist dissection shows a diffusely thickened, yellow median nerve resulting from the neural fibrolipoma (arrow). (g, h) Photograph of a coronally sectioned whole-mount specimen (hematoxylin-eosin stain) (g) of the second digit and photomicrograph (h) (original magnification, x200; hematoxylin-eosin stain) demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*) surrounding nerve fascicles (arrow).
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Figure 6e. Neural fibrolipoma in a 34-year-old man with macrodactyly. (a, b) Photograph (a) and anteroposterior radiograph (b) of the hand show soft-tissue and osseous enlargement (in length and width) of the second and third digits with ulnar deviation and secondary degenerative changes (arrowhead in b). (c, d) Axial T1-weighted (500/20) MR images of the wrist (c) and phalanges (d) reveal adipose tissue causing thickened nerve, surrounding fascicles (arrowheads) both proximally (median nerve) and distally (interdigital nerve branches), and overgrowth of the second and third digits with predominance of fat (arrow). (e) Longitudinal US scan of the wrist shows multiple elongated, enlarged median nerve fascicles (arrowheads). (f) Intraoperative photograph of wrist dissection shows a diffusely thickened, yellow median nerve resulting from the neural fibrolipoma (arrow). (g, h) Photograph of a coronally sectioned whole-mount specimen (hematoxylin-eosin stain) (g) of the second digit and photomicrograph (h) (original magnification, x200; hematoxylin-eosin stain) demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*) surrounding nerve fascicles (arrow).
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Figure 6f. Neural fibrolipoma in a 34-year-old man with macrodactyly. (a, b) Photograph (a) and anteroposterior radiograph (b) of the hand show soft-tissue and osseous enlargement (in length and width) of the second and third digits with ulnar deviation and secondary degenerative changes (arrowhead in b). (c, d) Axial T1-weighted (500/20) MR images of the wrist (c) and phalanges (d) reveal adipose tissue causing thickened nerve, surrounding fascicles (arrowheads) both proximally (median nerve) and distally (interdigital nerve branches), and overgrowth of the second and third digits with predominance of fat (arrow). (e) Longitudinal US scan of the wrist shows multiple elongated, enlarged median nerve fascicles (arrowheads). (f) Intraoperative photograph of wrist dissection shows a diffusely thickened, yellow median nerve resulting from the neural fibrolipoma (arrow). (g, h) Photograph of a coronally sectioned whole-mount specimen (hematoxylin-eosin stain) (g) of the second digit and photomicrograph (h) (original magnification, x200; hematoxylin-eosin stain) demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*) surrounding nerve fascicles (arrow).
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Figure 6g. Neural fibrolipoma in a 34-year-old man with macrodactyly. (a, b) Photograph (a) and anteroposterior radiograph (b) of the hand show soft-tissue and osseous enlargement (in length and width) of the second and third digits with ulnar deviation and secondary degenerative changes (arrowhead in b). (c, d) Axial T1-weighted (500/20) MR images of the wrist (c) and phalanges (d) reveal adipose tissue causing thickened nerve, surrounding fascicles (arrowheads) both proximally (median nerve) and distally (interdigital nerve branches), and overgrowth of the second and third digits with predominance of fat (arrow). (e) Longitudinal US scan of the wrist shows multiple elongated, enlarged median nerve fascicles (arrowheads). (f) Intraoperative photograph of wrist dissection shows a diffusely thickened, yellow median nerve resulting from the neural fibrolipoma (arrow). (g, h) Photograph of a coronally sectioned whole-mount specimen (hematoxylin-eosin stain) (g) of the second digit and photomicrograph (h) (original magnification, x200; hematoxylin-eosin stain) demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*) surrounding nerve fascicles (arrow).
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Figure 6h. Neural fibrolipoma in a 34-year-old man with macrodactyly. (a, b) Photograph (a) and anteroposterior radiograph (b) of the hand show soft-tissue and osseous enlargement (in length and width) of the second and third digits with ulnar deviation and secondary degenerative changes (arrowhead in b). (c, d) Axial T1-weighted (500/20) MR images of the wrist (c) and phalanges (d) reveal adipose tissue causing thickened nerve, surrounding fascicles (arrowheads) both proximally (median nerve) and distally (interdigital nerve branches), and overgrowth of the second and third digits with predominance of fat (arrow). (e) Longitudinal US scan of the wrist shows multiple elongated, enlarged median nerve fascicles (arrowheads). (f) Intraoperative photograph of wrist dissection shows a diffusely thickened, yellow median nerve resulting from the neural fibrolipoma (arrow). (g, h) Photograph of a coronally sectioned whole-mount specimen (hematoxylin-eosin stain) (g) of the second digit and photomicrograph (h) (original magnification, x200; hematoxylin-eosin stain) demonstrate osseous and soft-tissue hypertrophy with predominance of fat (*) surrounding nerve fascicles (arrow).
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At gross examination, the affected nerve is diffusely enlarged (Fig 6f). The lesion appears as a tan-yellow mass within the nerve sheath, related to infiltration of the epineurium and perineurium by fibroadipose tissue (48–50). The adipose tissue surrounds and separates the usually normal-appearing nerve fascicles (Fig 6h) (48). The pathologic appearance of the nerve is identical, regardless of the presence or absence of macrodactyly. Patients with macrodystrophia lipomatosa have a diffuse, disproportionate increase in fibroadipose tissue in the affected digit (Fig 6g).
Radiographs in patients with neural fibrolipoma without macrodactyly often appear normal or may show a focal soft-tissue mass. In patients with macrodystrophia lipomatosa, osseous and soft-tissue overgrowth are seen and often affect both the length and width of the digit (Fig 6b). The phalanges are long and broad and often splayed at their distal ends. The osseous overgrowth, usually more marked volarly and distally resulting in bowing, ceases at puberty but may lead to premature osteoarthritis. Increased radiolucent fat is often apparent in the soft tissues of the affected digit.
US of neural fibrolipoma may show alternating hyperechoic and hypoechoic bands (cablelike appearance) (Fig 6e). The MR imaging appearance is pathognomonic, consisting of longitudinally oriented cylindric foci (about 3 mm in diameter) of low signal intensity surrounded by fatty signal intensity representing nerve fascicles (Fig 6c, 6d) (50,52,53).
Treatment is difficult and controversial, depending on the extent of nerve involvement and presence of macrodactyly. Motor and sensory deficits have been reported following attempted resection (50,51).
Nerve Sheath Ganglion
Ganglions occurring within nerve sheaths (ie, intraneural ganglions) have recently been reported and most frequently involve the large nerves about the knee (popliteal, peroneal, or tibial) at the level of fibular head (Fig 7) (58–63). The origin of the lesions is disputed, and many of them may be extensions of ganglions related to the tibiofibular joint with secondary nerve involvement, rather than primarily arising in the nerve sheath.
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Figure 7a. Nerve sheath ganglion in a 25-year-old man with peroneal nerve distribution symptoms. (a, b) Axial T1-weighted (600/20) (a) and fat-suppressed T2-weighted (4,666/90) (b) MR images show a fluid-appearing mass (arrowheads) adjacent to the anterolateral proximal fibula and peroneal nerve (arrow). (c, d) Intraoperative photograph (c) and photograph of the sectioned gross specimen (d) show gelatinous and myxoid consistency (*), septations (straight arrows), and the fibrous wall of the ganglion (curved arrows).
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Figure 7b. Nerve sheath ganglion in a 25-year-old man with peroneal nerve distribution symptoms. (a, b) Axial T1-weighted (600/20) (a) and fat-suppressed T2-weighted (4,666/90) (b) MR images show a fluid-appearing mass (arrowheads) adjacent to the anterolateral proximal fibula and peroneal nerve (arrow). (c, d) Intraoperative photograph (c) and photograph of the sectioned gross specimen (d) show gelatinous and myxoid consistency (*), septations (straight arrows), and the fibrous wall of the ganglion (curved arrows).
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Figure 7c. Nerve sheath ganglion in a 25-year-old man with peroneal nerve distribution symptoms. (a, b) Axial T1-weighted (600/20) (a) and fat-suppressed T2-weighted (4,666/90) (b) MR images show a fluid-appearing mass (arrowheads) adjacent to the anterolateral proximal fibula and peroneal nerve (arrow). (c, d) Intraoperative photograph (c) and photograph of the sectioned gross specimen (d) show gelatinous and myxoid consistency (*), septations (straight arrows), and the fibrous wall of the ganglion (curved arrows).
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Figure 7d. Nerve sheath ganglion in a 25-year-old man with peroneal nerve distribution symptoms. (a, b) Axial T1-weighted (600/20) (a) and fat-suppressed T2-weighted (4,666/90) (b) MR images show a fluid-appearing mass (arrowheads) adjacent to the anterolateral proximal fibula and peroneal nerve (arrow). (c, d) Intraoperative photograph (c) and photograph of the sectioned gross specimen (d) show gelatinous and myxoid consistency (*), septations (straight arrows), and the fibrous wall of the ganglion (curved arrows).
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Patients present with a palpable mass or neurologic symptoms resulting from nerve compression. These lesions show myxoid change surrounded by a fibrous lining and often occur in the connective tissue between the nerve sheath and the nerve (Fig 7d), which often results in displacement of the adjacent nerve. These changes suggest a degenerative process as the cause.
At CT, US, and MR imaging, a nerve sheath ganglion appears as a cystic mass that may contain septations (Fig 7a, 7b) (58–63). Attenuation of the mass on CT and signal intensity on T1-weighted images may be slightly higher than expected for simple fluid and is related to its high protein content. Contrast enhancement may be seen about the rim and thin septa.
Treatment is surgical resection. In particular, if it extends from the tibiofibular joint, the lesion neck should be resected to reduce the possibility of recurrence (51,61).
Benign PNSTs
Classically, benign PNSTs have been divided into neurilemoma (schwannoma) and neurofibroma. Although these soft-tissue neoplasms are similar and contain cellular elements closely related to normal Schwann cells, multiple clinical and pathologic features usually allow distinction (3,64).
Neurilemoma (Schwannoma).—Neurilemoma most frequently affects patients 20–30 years of age and constitutes approximately 5% of all benign soft-tissue neoplasms (1,3,64). Men and women are affected equally. Commonly involved sites include the spinal and sympathetic nerve roots of the head and neck, as well as nerves in the flexor surfaces of the upper and lower extremities (particularly ulnar and peroneal nerves). The posterior mediastinum and retroperitoneum may also be affected.
Neurilemomas are usually solitary, with nonaggressive features including slow growth and small size (<5 cm) (64,65). Pain and neurologic symptoms are unusual except in large tumors. Lesions are usually freely mobile to palpation except at the point of nerve attachment. In tumors of large nerves, this restricted movement is often along the long axis of the affected nerve trunks.
Although most neurilemomas are solitary and not associated with NF1, in about 5% of multiple neurilemomas, plexiform growth or association with NF1 is apparent (64). In one series, association of multiple neurilemomas with NF1 was even greater at 18% (65). Multiple neurilemomas often occur in a cutaneous distribution, and patients may have intracranial lesions (meningioma, glioma, astrocytoma) without other stigmata of NF1.
At pathologic analysis, neurilemomas are fusiform, representing the mass with the entering and exiting nerve (Figs 8–10). When large nerves are affected, the mass is eccentric in relationship to the involved nerve, with nerve fibers splayed about the neoplasm. Similar to neurofibromas, neurilemomas of small nerves may obliterate the nerve of origin (Fig 11). Both the neurilemoma and the affected nerve are within a true capsule, the epineurium (Figs 8, 10, 11).
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Figure 8a. Peroneal nerve neurilemoma in a 49-year-old woman. (a-c) Coronal T1-weighted (435/40) (a) and axial T2-weighted (2,000/80) (b) MR images and longitudinal US scan (c) (photographically spliced together to show both ends of the lesion) reveal a well-defined fusiform mass (M) with entering and exiting nerve (*). Nerve appears central and within the capsule (large black arrowheads) of the mass, making distinction of neurilemoma from neurofibroma nearly impossible. MR images also reveal the surrounding fat (split-fat sign) (white arrowheads) and fascicular sign (small arrowheads in b). (d-f) Intraoperative photographs of lesion resection initially (d) reveal the fusiform mass (M) with entering and exiting peroneal nerve (*). Subsequently (e, f), incision of the epineurium (arrowheads) shows the lesion (curved arrow in e) separable from the nerve and resected with peroneal nerve fascicles left intact (straight arrows in f).
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Figure 8b. Peroneal nerve neurilemoma in a 49-year-old woman. (a-c) Coronal T1-weighted (435/40) (a) and axial T2-weighted (2,000/80) (b) MR images and longitudinal US scan (c) (photographically spliced together to show both ends of the lesion) reveal a well-defined fusiform mass (M) with entering and exiting nerve (*). Nerve appears central and within the capsule (large black arrowheads) of the mass, making distinction of neurilemoma from neurofibroma nearly impossible. MR images also reveal the surrounding fat (split-fat sign) (white arrowheads) and fascicular sign (small arrowheads in b). (d-f) Intraoperative photographs of lesion resection initially (d) reveal the fusiform mass (M) with entering and exiting peroneal nerve (*). Subsequently (e, f), incision of the epineurium (arrowheads) shows the lesion (curved arrow in e) separable from the nerve and resected with peroneal nerve fascicles left intact (straight arrows in f).
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Figure 8c. Peroneal nerve neurilemoma in a 49-year-old woman. (a-c) Coronal T1-weighted (435/40) (a) and axial T2-weighted (2,000/80) (b) MR images and longitudinal US scan (c) (photographically spliced together to show both ends of the lesion) reveal a well-defined fusiform mass (M) with entering and exiting nerve (*). Nerve appears central and within the capsule (large black arrowheads) of the mass, making distinction of neurilemoma from neurofibroma nearly impossible. MR images also reveal the surrounding fat (split-fat sign) (white arrowheads) and fascicular sign (small arrowheads in b). (d-f) Intraoperative photographs of lesion resection initially (d) reveal the fusiform mass (M) with entering and exiting peroneal nerve (*). Subsequently (e, f), incision of the epineurium (arrowheads) shows the lesion (curved arrow in e) separable from the nerve and resected with peroneal nerve fascicles left intact (straight arrows in f).
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Figure 8d. Peroneal nerve neurilemoma in a 49-year-old woman. (a-c) Coronal T1-weighted (435/40) (a) and axial T2-weighted (2,000/80) (b) MR images and longitudinal US scan (c) (photographically spliced together to show both ends of the lesion) reveal a well-defined fusiform mass (M) with entering and exiting nerve (*). Nerve appears central and within the capsule (large black arrowheads) of the mass, making distinction of neurilemoma from neurofibroma nearly impossible. MR images also reveal the surrounding fat (split-fat sign) (white arrowheads) and fascicular sign (small arrowheads in b). (d-f) Intraoperative photographs of lesion resection initially (d) reveal the fusiform mass (M) with entering and exiting peroneal nerve (*). Subsequently (e, f), incision of the epineurium (arrowheads) shows the lesion (curved arrow in e) separable from the nerve and resected with peroneal nerve fascicles left intact (straight arrows in f).
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Figure 8e. Peroneal nerve neurilemoma in a 49-year-old woman. (a-c) Coronal T1-weighted (435/40) (a) and axial T2-weighted (2,000/80) (b) MR images and longitudinal US scan (c) (photographically spliced together to show both ends of the lesion) reveal a well-defined fusiform mass (M) with entering and exiting nerve (*). Nerve appears central and within the capsule (large black arrowheads) of the mass, making distinction of neurilemoma from neurofibroma nearly impossible. MR images also reveal the surrounding fat (split-fat sign) (white arrowheads) and fascicular sign (small arrowheads in b). (d-f) Intraoperative photographs of lesion resection initially (d) reveal the fusiform mass (M) with entering and exiting peroneal nerve (*). Subsequently (e, f), incision of the epineurium (arrowheads) shows the lesion (curved arrow in e) separable from the nerve and resected with peroneal nerve fascicles left intact (straight arrows in f).
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Figure 8f. Peroneal nerve neurilemoma in a 49-year-old woman. (a-c) Coronal T1-weighted (435/40) (a) and axial T2-weighted (2,000/80) (b) MR images and longitudinal US scan (c) (photographically spliced together to show both ends of the lesion) reveal a well-defined fusiform mass (M) with entering and exiting nerve (*). Nerve appears central and within the capsule (large black arrowheads) of the mass, making distinction of neurilemoma from neurofibroma nearly impossible. MR images also reveal the surrounding fat (split-fat sign) (white arrowheads) and fascicular sign (small arrowheads in b). (d-f) Intraoperative photographs of lesion resection initially (d) reveal the fusiform mass (M) with entering and exiting peroneal nerve (*). Subsequently (e, f), incision of the epineurium (arrowheads) shows the lesion (curved arrow in e) separable from the nerve and resected with peroneal nerve fascicles left intact (straight arrows in f).
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Figure 9a. Tibial nerve neurilemoma in a 29-year-old man. (a) Lateral radiograph of the distal thigh shows a fusiform mass (*), exiting nerve (arrow), and surrounding fat (split-fat sign) (arrowheads). (b) Early arterial-phase angiogram reveals tortuous, corkscrew nutrient feeding vessels (curved arrows) at the superior aspect of the mass and the displaced superficial femoral artery (straight arrows).
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Figure 9b. Tibial nerve neurilemoma in a 29-year-old man. (a) Lateral radiograph of the distal thigh shows a fusiform mass (*), exiting nerve (arrow), and surrounding fat (split-fat sign) (arrowheads). (b) Early arterial-phase angiogram reveals tortuous, corkscrew nutrient feeding vessels (curved arrows) at the superior aspect of the mass and the displaced superficial femoral artery (straight arrows).
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Figure 10a. Sciatic nerve neurilemoma in a 24-year-old woman with radiating leg pain. (a) Axial T1-weighted (600/15) MR image shows the mass (*) with surrounding fat (arrows) adjacent to but separable from the sciatic nerve (arrowheads), which has a fascicular appearance, findings diagnostic of a neurilemoma. (b, c) Intraoperative photographs initially (b) reveal the lesion (solid black arrow) and entering nerve (white arrow) both within the epineurium. Nodular area represents intact sciatic nerve (open arrows). Subsequently (c), the sausage-shaped neurilemoma (*) is resected from the intact sciatic nerve (arrowhead) after incision of the epineurium (arrow).
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Figure 10b. Sciatic nerve neurilemoma in a 24-year-old woman with radiating leg pain. (a) Axial T1-weighted (600/15) MR image shows the mass (*) with surrounding fat (arrows) adjacent to but separable from the sciatic nerve (arrowheads), which has a fascicular appearance, findings diagnostic of a neurilemoma. (b, c) Intraoperative photographs initially (b) reveal the lesion (solid black arrow) and entering nerve (white arrow) both within the epineurium. Nodular area represents intact sciatic nerve (open arrows). Subsequently (c), the sausage-shaped neurilemoma (*) is resected from the intact sciatic nerve (arrowhead) after incision of the epineurium (arrow).
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Figure 10c. Sciatic nerve neurilemoma in a 24-year-old woman with radiating leg pain. (a) Axial T1-weighted (600/15) MR image shows the mass (*) with surrounding fat (arrows) adjacent to but separable from the sciatic nerve (arrowheads), which has a fascicular appearance, findings diagnostic of a neurilemoma. (b, c) Intraoperative photographs initially (b) reveal the lesion (solid black arrow) and entering nerve (white arrow) both within the epineurium. Nodular area represents intact sciatic nerve (open arrows). Subsequently (c), the sausage-shaped neurilemoma (*) is resected from the intact sciatic nerve (arrowhead) after incision of the epineurium (arrow).
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Figure 11a. Neurilemoma in a 35-year-old man with a palpable soft-tissue mass. (a) Coronal T1-weighted (500/20) MR image shows an elongated, low-signal-intensity mass (arrowheads). (b) On the axial T2-weighted (2,000/90) MR image, the mass has peripheral high signal intensity (white arrow) with low signal intensity centrally (black arrow), representing the target sign. No entering or exiting nerve is seen because the affected nerve is a small gastrocnemius intramuscular branch. (c, d) Photograph of the sectioned gross specimen (c) and photomicrograph (original magnification, x75; hematoxylin-eosin stain) (d) show corresponding more cellular Antoni A regions centrally (black *) and more myxoid Antoni B areas peripherally (white *) as well as a capsule (arrow in d).
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Figure 11b. Neurilemoma in a 35-year-old man with a palpable soft-tissue mass. (a) Coronal T1-weighted (500/20) MR image shows an elongated, low-signal-intensity mass (arrowheads). (b) On the axial T2-weighted (2,000/90) MR image, the mass has peripheral high signal intensity (white arrow) with low signal intensity centrally (black arrow), representing the target sign. No entering or exiting nerve is seen because the affected nerve is a small gastrocnemius intramuscular branch. (c, d) Photograph of the sectioned gross specimen (c) and photomicrograph (original magnification, x75; hematoxylin-eosin stain) (d) show corresponding more cellular Antoni A regions centrally (black *) and more myxoid Antoni B areas peripherally (white *) as well as a capsule (arrow in d).
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Figure 11c. Neurilemoma in a 35-year-old man with a palpable soft-tissue mass. (a) Coronal T1-weighted (500/20) MR image shows an elongated, low-signal-intensity mass (arrowheads). (b) On the axial T2-weighted (2,000/90) MR image, the mass has peripheral high signal intensity (white arrow) with low signal intensity centrally (black arrow), representing the target sign. No entering or exiting nerve is seen because the affected nerve is a small gastrocnemius intramuscular branch. (c, d) Photograph of the sectioned gross specimen (c) and photomicrograph (original magnification, x75; hematoxylin-eosin stain) (d) show corresponding more cellular Antoni A regions centrally (black *) and more myxoid Antoni B areas peripherally (white *) as well as a capsule (arrow in d).
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Figure 11d. Neurilemoma in a 35-year-old man with a palpable soft-tissue mass. (a) Coronal T1-weighted (500/20) MR image shows an elongated, low-signal-intensity mass (arrowheads). (b) On the axial T2-weighted (2,000/90) MR image, the mass has peripheral high signal intensity (white arrow) with low signal intensity centrally (black arrow), representing the target sign. No entering or exiting nerve is seen because the affected nerve is a small gastrocnemius intramuscular branch. (c, d) Photograph of the sectioned gross specimen (c) and photomicrograph (original magnification, x75; hematoxylin-eosin stain) (d) show corresponding more cellular Antoni A regions centrally (black *) and more myxoid Antoni B areas peripherally (white *) as well as a capsule (arrow in d).
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The histologic hallmark of neurilemoma is identification of Antoni A and Antoni B regions (Fig 11). Lesions are also S-100 protein positive at immunohistochemical analysis (3,64,65). Antoni A areas are more organized and composed of cellular spindle cells arranged in short bundles or interlacing fascicles. Antoni B regions are hypocellular, are less organized, and contain more myxoid loosely arranged tissue with high water content. These components are intermixed within neurilemomas and occur in varying amounts. Neurilemomas in which Antoni A areas predominate are often called cellular schwannomas. They are more frequently located in the posterior mediastinum and retroperitoneum and constitute 25% of extremity lesions (3). Large neurilemomas commonly undergo degenerative changes including cyst formation, calcification, hemorrhage, and fibrosis and are often referred to as ancient schwannomas (64,65).
Treatment of neurilemoma is usually surgical excision. The affected nerve is usually separable from the neoplasm intraoperatively after incision of the epineurium, allowing the native nerve and its function to be preserved (9). Partial resection may be performed in cases that would otherwise require nerve resection for complete removal. Recurrence is unusual, even after incomplete resection, and malignant transformation is exceedingly rare.
Neurofibroma.—Neurofibroma most commonly affects patients 20–30 years of age and has no sex predilection (13,66). These lesions constitute slightly more than 5% of all benign soft-tissue tumors (1,3). Three types of neurofibromas are classically described: localized, diffuse, and plexiform (discussed with NF1). The localized variety is the most common, representing approximately 90% of these lesions, and the vast majority are solitary and not associated with NF1 (3,9). Localized neurofibromas often affect superficial cutaneous nerves, although involvement of larger nerves also occurs, causing deep-seated lesions (Figs 12, 13). Localized neurofibromas are slow-growing lesions, usually less than 5 cm in size at presentation and painless (3,9). The diffuse neurofibroma primarily affects children and young adults (Fig 14) and most frequently involves the subcutaneous tissues of the head and neck. The majority of diffuse neurofibromas (90%) are isolated lesions not associated with NF1 (3,9). Diffuse neurofibromas demonstrate a plaquelike elevation of the skin with thickening of the entire subcutis.
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Figure 12a. Tibial nerve neurofibroma in a 30-year-old man. (a) Axial CT scan shows a low-attenuation well-defined mass (arrow) with a center of slightly increased attenuation (white arrowheads) and incomplete fat rim (black arrowheads) (target sign). (b, c) Sagittal T1-weighted (500/17) (b) and axial T2-weighted (2,000/80) (c) MR images reveal entering nerve (open arrow in b), partial fat rim (split-fat sign), and capsule (solid arrow in b) and high-signal-intensity peripheral rim (target sign) (arrowheads in c). (d) Photograph of the axially sectioned gross specimen from debulking surgery shows the capsule (white arrows), peripheral myxoid region (black arrows), and central more solid tissue (*).
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Abstract
INTRODUCTION
