Cross-Sectional Imaging of Peripheral Nerve Sheath Tumors

Pictorial Essay

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Cross-Sectional Imaging of Peripheral Nerve Sheath Tumors: Characteristic Signs on CT, MR Imaging, and Sonography John Lin 1 and William Martel

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eripheral nerve sheath tumors are divided into two major benign categories, neurofibroma and schwannoma, and a malignant form, malignant peripheral nerve sheath tumor. Each category can be associated with neurofibromatosis.

Although major nerve trunks are most commonly affected, virtually any peripheral nerve can represent a site of origin. Our purpose is to illustrate the spectrum of cross-sectional imaging findings for each type of peripheral nerve sheath tumor. Multi-

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technique correlation of CT, MR imaging, and sonography is shown with emphasis on characteristic signs of peripheral nerve sheath tumors including the relationship to the nerve, fusiform shape, “split-fat” sign, associated muscle atrophy, and the MR “tar-

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Fig. 1.—40-year-old woman with neurofibroma involving median nerve in forearm. A, CT scan reveals large low-attenuation lesion (arrows) in volar aspect of forearm and median nerve distribution. B, Coronal proton density–weighted fat-saturated MR image shows fusiform shape of slightly heterogeneous high-signal-intensity lesion (arrows ) with proximal and distal “tails” (arrowheads ), representing “entering and exiting” median nerve. C, Axial fast spin-echo short tau inversion-recovery MR image shows large well-defined predominantly high-signal-intensity lesion (arrows) in median nerve distribution.

C Received May 4, 2000; accepted after revision June 8, 2000. Presented at the annual meeting of the American Roentgen Ray Society, Washington, DC, May 2000. 1

Both authors: Department of Radiology, The University of Michigan Medical Center, 1500 E. Medical Center Dr., TC 2910, Ann Arbor, MI 48109-0326. Address correspondence to J. Lin.

AJR 2001;176:75–82 0361–803X/01/1761–75 © American Roentgen Ray Society

AJR:176, January 2001

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Lin and Martel Fig. 2.—30-year-old man with neurofibroma involving right C3 cervical nerve root. A, Oblique radiograph of cervical spine reveals smooth enlargement of right C2-C3 neuroforamen (arrows). B, Axial T1-weighted gadolinium-enhanced MR image shows large dumbbell-shaped enhancing lesion (open arrows) extending through right C2-C3 neuroforamen (arrowheads ).

A get” sign. We also present the concept of the sonographic target sign, which was recently described [1] as a sonographic correlate to the MR target sign.

Neurofibromas

Neurofibromas (Figs. 1–3) most frequently affect patients who are 20–30 years old and represent slightly greater than 5% of benign soft-tissue neoplasms [2, 3]. Most le-

B sions are solitary, but up to 10% are associated with neurofibromatosis. Neurofibromas in the setting of neurofibromatosis tend to be larger and have a higher incidence of malignant transformation. Lesions can be either superficial or deep, involving small cutaneous nerves or large major nerve trunks. Superficial neurofibromas are typically small painless masses, whereas deeper neurofibromas are commonly associated with neurologic symptoms.

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Pathologically, neurofibromas are intimately associated with the parent nerve, growing in a longitudinal fusiform manner with the nerve “entering and exiting” from the lesion. Neurofibromas are not encapsulated. Surgical resection requires sacrificing the parent nerve because the neurofibroma cannot be separated from the nerve fibers. CT shows a well-defined mass that is hypodense relative to muscle and enhances after contrast administration. Findings on MR

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Fig. 3.—18-year-old man with neurofibroma of common peroneal nerve and clinical history of foot drop. A, Transverse sonogram (7-12–MHz linear array transducer) of lateral leg just distal to knee reveals rounded relatively hypoechoic lesion (solid arrows ) with subtle central region of increased echogenicity (open arrows ), consistent with sonographic “target” sign, which is better appreciated on real-time imaging. B, Axial fast spin-echo short tau inversion-recovery MR image shows round lesion (arrow ) with high-signal-intensity periphery and low-signal-intensity central region, corresponding to lesion seen in A. Lesion represents MR target sign. C, Axial T1-weighted MR image shows fatty atrophy (arrows) of anterior compartment muscles (tibialis anterior, extensor hallucis longus, and extensor digitorum longus), consistent with peroneal nerve distribution.

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Cross-Sectional Imaging of Peripheral Nerve Sheath Tumors

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Fig. 4.—19-year-old man with neurofibromatosis and extensive plexiform neurofibromas. A, Coronal T2-weighted MR image of right knee posteriorly reveals conglomerate multilobulated masses (white arrows) representing plexiform neurofibromas. Individual lesions show high signal intensity peripherally and low signal intensity centrally (black arrows ), consistent with MR “target” sign. B, Longitudinal extended field-of-view sonogram (7.5-MHz–linear array transducer) of right lower extremity from posterior approach shows extensive multilobulated masses (arrowheads ) extending entire length of leg in sciatic and tibial nerve distributions. Note posterior femoral cortex and tibial cortices (arrows ). K = knee. (Reprinted with permission from [1]) C, Focused longitudinal sonogram (5-MHz–curvilinear transducer) of posterior thigh shows large conglomerated masses. Each lesion (arrows ) reveals target sign. D, Longitudinal sonogram (7.5-MHz–linear array transducer) of left elbow medially, distal to cubital tunnel, reveals solitary ovoid mass (arrows ) in contiguity with ulnar nerve (arrowheads ). Lesion shows hypoechoic echotexture, well-defined margins, and subtle distal acoustic enhancement. (Reprinted with permission from [1])


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Lin and Martel imaging reveal low-signal-intensity lesions on T1-weighted sequences, high-signal-intensity lesions on T2-weighted sequences, and avid contrast enhancement (Figs. 1 and 2). Signal on T2-weighted images can be either homogeneously hyperintense or show a characteristic target sign, consisting of a central hypointense region [4] (Fig. 3). A dumbbell-shape is characteristic for spinal nerve root neurofibromas, which may enlarge the

neuroforamen (Fig. 2). Another typical feature is a fusiform shape oriented longitudinally in the nerve distribution, often revealing tapered ends that are contiguous with the parent nerve (Fig. 1). Sonography shows a well-defined hypoechoic lesion and may show distal acoustic enhancement, simulating a cyst. A target sign can be seen in neurofibromas with a hypoechoic peripheral zone and a hyperechoic central zone [1]

(Figs. 3 and 4). Identification of a lesion and associated muscle atrophy in a typical nerve distribution may suggest a peripheral nerve sheath tumor [3] (Fig. 3).

Neurofibromatosis

Neurofibromatosis (Figs. 4–6), also known as von Recklinghausen’s disease, is a phakomatosis that displays a wide spectrum Fig. 5.—39-year-old man with neurofibromatosis and numerous localized neurofibromas in pelvis and lower extremities. A, Coronal fast spin-echo short tau inversion-recovery MR image of pelvis shows multiple ovoid homogeneous high-signal-intensity lesions (arrows ). B, Coronal T1-weighted MR image of knee reveals numerous ovoid homogeneous intermediate signal lesions (arrows ) in chainlike pattern in medial soft tissues, surrounded by fat representing “split-fat” sign (arrowheads).

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Fig. 6.—62-year-old man with neurofibromatosis and diffuse neurofibromas involving superficial tissues of left lower extremity. A, Anteroposterior ScoutView from CT scan shows marked enlargement of soft tissues of left lower extremity (arrowheads) as compared with right lower extremity, consistent with hemihypertrophy. B, CT scan of left calf shows marked thickening and enlargement of superficial soft tissues primarily posteriorly (curved arrows ), which is heterogeneous with mixed softtissue and fat attenuation. Muscle compartment (straight arrows ) is essentially uninvolved and normal. Percutaneous core biopsy needle (arrowheads ) was used to confirm diagnosis.

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Cross-Sectional Imaging of Peripheral Nerve Sheath Tumors of clinical expression with neurocutaneous abnormalities and involvement of multiple organ systems. There are two major forms, designated neurofibromatosis type 1 (NF 1) and neurofibromatosis type 2 (NF 2), which are clinically and genetically distinct. NF 1 is commonly associated with peripheral nerve sheath tumors, whereas NF 2 primarily affects the central nervous system. NF 1 is among the most common genetically transmitted disease, affecting one in 3000 births. It is autosomal dominantly inherited although up to 50% of cases may evolve from a new mutation, with advanced paternal age as a risk factor [2]. Pathologically, neurofibromas in NF 1 can be divided into three types: localized, plexiform, and diffuse [2, 5]. Localized neurofibromas are identical to solitary neurofibromas; however, in the setting of NF 1, they tend to be larger, multiple, and, more commonly, deep in location. Plexiform

Fig. 7.—14-year-old girl with schwannoma involving ulnar nerve in upper extremity. Sagittal T2-weighted MR image of right upper extremity shows large ovoid lesion (arrowheads ) eccentrically located in distribution of ulnar nerve (white arrows ). Lesion is predominantly high signal intensity with low-signal-intensity central region (black arrows ), representing MR “target” sign.

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Fig. 8.—37-year-old man with degenerating schwannoma of sacrum and pelvis. A, Transverse sonogram (3.5-MHz–curvilinear transducer) of pelvis from anterior approach shows large heterogeneous relatively hypoechoic lesion (arrowheads ) in presacral region. Note central anechoic areas (arrows), consistent with cystic cavities. B, CT scan of pelvis shows large soft-tissue mass (white arrowheads ) arising from left anterior sacrum (black arrowheads ) and displacing pelvic structures. Note central and peripheral fluid attenuation areas (arrows ), corresponding to cystic cavities seen on sonogram (A). C, Axial T2-weighted MR image shows pelvic lesion (black arrows ) with cystic cavities (arrowheads ) and sacral origin (white arrows ) as seen on CT scan (B).

C AJR:176, January 2001

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Lin and Martel Fig. 9.—76-year-old man with “ancient” schwannoma of sacrum and pelvis. A, CT-guided biopsy was performed in left lateral decubitus position (image is rotated 90° counterclockwise) from posterior approach. Large presacral lesion (white arrowheads ) originating from sacrum with extensive dystrophic calcifications (black arrowheads ) primarily in peripheral pattern is shown. Note core biopsy needle (arrows ) with tip in sacral mass. B, Proton density–weighted fat-saturated MR image shows presacral lesion (white arrows) originating from sacrum (arrowheads ) and central cystic region with low-signal-intensity focus (black arrow ) in nondependent location, representing gas introduced at biopsy.

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Fig. 10.—60-year-old man with malignant peripheral nerve sheath tumor involving sciatic nerve. A, CT scan with IV contrast material and patient in prone position shows right posterior thigh lesion (arrowheads ), which enhances heterogeneously, in sciatic nerve distribution. CT-guided biopsy was performed from posterior approach; note core biopsy needle (arrow ) introduced percutaneously. B, Axial T2-weighted fat-saturated MR image shows large lesion (arrows ) with marked heterogeneity and associated surrounding edema. C, Sagittal T1-weighted fat-saturated gadolinium-enhanced MR image shows fusiform posterior thigh lesion (arrow ) and heterogeneous contrast enhancement.

Fig. 11.—35-year-old man with secondary malignant peripheral nerve sheath tumor of right flank after radiation therapy for Wilms’ tumor as child. A, Coronal fast spin-echo short tau inversion-recovery MR image reveals large lobulated highsignal-intensity mass (arrow ) in right flank superficially. B, Axial T1-weighted fat-saturated gadolinium-enhanced MR image reveals heterogeneous enhancement of lesion (arrow ) located primarily in subcutaneous tissues. Absence of right kidney was consistent with prior nephrectomy (not shown).

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Cross-Sectional Imaging of Peripheral Nerve Sheath Tumors Fig. 12.—53-year-old man with malignant intraosseous peripheral nerve sheath tumor of tibia. (Reprinted with permission from [7]) A, Anteroposterior radiograph shows large lobulated intraosseous lesion, which has extended into medial soft tissues. B, Sagittal gradient-recalled echo MR image shows intraosseous and soft-tissue components of lesion (arrows ) with homogeneous increased signal intensity. C, Gross pathologic specimen shows lesion extending superficially with possible infiltration into subcutaneous tissues.

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neurofibromas are pathognomonic for NF 1, usually involving a long segment of a major nerve trunk and extending into the nerve branches, and they result in the so-called “bag of worms” appearance on gross inspection and cross-sectional imaging. Diffuse neurofibromas most commonly occur in children and young adults and are typically localized to the subcutaneous tissues [2, 5]. In view of the diffuse involvement, patients are generally treated conservatively, with surgical resection reserved for intolerable lesions. CT of plexiform neurofibromas shows large multilobulated low-attenuation masses, usually within a major nerve distribution. MR imaging reveals large conglomerate masses consisting of innumerable neurofibromas, diffusely thickening the involved nerve and often extending into nerve branches. Both sonography and MR imaging may show the target sign [1, 4] (Fig. 4). Localized neurofibromas in neurofibromatosis are radiologically identical to solitary neurofibromas, but multiple lesions are often present and may be seen with plexiform neurofibromas (Figs. 4 and 5). The split-fat sign represents a rim of fat surrounding the tumor, which originates from the nerve in an intermuscular location (Fig. 5). The diffuse form of neurofibroma is ill-defined and extensively reticulates through the subcutaneous tissue [5]. Severe thickening of the subcutaneous tissues can result in marked enlargement of the affected extremity (Fig. 6).


Schwannomas

Schwannomas (Figs. 7–9), also known as neurilemmomas, most frequently affect patients who are 20–40 years old and represent approximately 5% of benign soft-tissue neoplasms [2, 3]. Most lesions are solitary and present as a slowly growing painless soft-tissue mass. Symptoms are unusual, unless the mass has become large enough to compress the adjacent nerve. Infrequently, these tumors can be associated with NF 1, and in such cases, they are invariably plexiform or multiple lesions. Pathologically, schwannomas are fusiform masses that are eccentrically located in relation to the involved nerve and are contained within a capsule, the epineurium. They are composed primarily of Schwann’s cells. Surgical excision can usually spare the parent nerve because the schwannoma is generally separable from the underlying nerve fibers. Imaging findings of schwannomas are similar to those seen with neurofibromas and, in many cases, cannot be distinguished. The fusiform shape, split-fat sign, and target sign can be seen in either lesion. However, some features can help differentiate these two lesions. When the parent nerve is identified, an eccentrically positioned lesion (in relation to the nerve) suggests a schwannoma (Fig. 7), whereas a centrally located mass suggests a neurofibroma (Fig. 1). Heterogeneous appearance with degeneration and cystic cavitation are much more common in schwannomas than in neurofibromas (Fig. 8). “Ancient” schwan-

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nomas refer to long-standing lesions with advanced degeneration exhibiting calcification, hyalinization, and cystic cavitation, findings that can be identified on imaging [6] (Fig. 9). Malignant Peripheral Nerve Sheath Tumors

Malignant peripheral nerve sheath tumors (Figs. 10–12) have also been referred to as malignant schwannomas, neurogenic sarcomas, malignant neurilemmomas, and neurofibrosarcomas. They most frequently affect patients who are 20–50 years old and represent 5–10% of soft-tissue sarcomas [2, 3]. A high proportion of malignant peripheral nerve sheath tumors (approximately 50%) occurs in association with NF 1. On the other hand, only a small fraction of patients with NF 1 (approximately 5%) develop malignant peripheral nerve sheath tumors [2, 3, 7]. Malignant peripheral nerve sheath tumors generally involve the major nerve trunks and present with pain and neurologic symptoms, as well as a possible soft-tissue mass. Secondary malignant peripheral nerve sheath tumors can arise from prior radiation treatment, with a latency period of longer than 10 years [2, 3]. Primary intraosseous origin of malignant peripheral nerve sheath tumors is exceedingly rare, particularly involving a long bone; such lesions most commonly arise from the mandible [7, 8]. Pathologically, malignant peripheral nerve sheath tumors are usually large fusiform masses, frequently with extensive central necrosis and

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Lin and Martel hemorrhage. Malignant peripheral nerve sheath tumors are usually high-grade sarcomas and treatment typically involves a combination of surgical resection, adjuvant chemotherapy, and radiotherapy. Local recurrence and metastatic disease are common complications. Although malignant and benign lesions cannot be reliably distinguished by imaging criteria, certain findings should raise the suspicion of a malignant tumor. Malignant peripheral nerve sheath tumors tend to be larger (>5 cm). They may exhibit ill-defined margins suggesting infiltration of adjacent tissues and associated edema [3] (Fig. 10). Heterogeneity with central necrosis on crosssectional imaging is common in malignant lesions (Figs. 10 and 11) although benign lesions with degeneration can also have a heterogeneous appearance (Figs. 8 and 9). Similarly, calcification, more commonly associated with malignant lesions, can also be present in ancient schwannomas (Fig. 9). Malignant peripheral nerve sheath tumors

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can arise within a previous radiation field (Fig. 11). Primary intraosseous malignant peripheral nerve sheath tumors show a lytic osseous lesion with cortical destruction and soft-tissue extension. T1-weighted MR images reveal low-to-intermediate signal intensity, and T2-weighted MR images show high signal intensity in the lesion (Fig. 12). In general, radiologic findings are nonspecific; however, given an aggressive osseous lesion in the setting of neurofibromatosis, radiologists should consider an intraosseous malignant peripheral nerve sheath tumor [7, 8]. In conclusion, knowledge and recognition of the characteristic signs for peripheral nerve sheath tumors can aid in the proper diagnosis and treatment of these lesions.

Acknowledgments

We thank Curtis W. Hayes and N. Reed Dunnick for their contributions of cases to our article.

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