Volume 62, Issue 3 , Pages 176-182, August 2011
Pictorial Essay: Imaging of Peripheral Nerve Sheath Tumours
Article Outline
- Abstract
- Résumé
- Neurofibromas
- Schwannomas
- Malignant PNSTs
- Imaging Characteristics of PNSTs
- Conclusion
- References
- Copyright
Abstract
Peripheral nerve sheath tumours (PNST) may be benign or malignant. Benign PNSTs include neurofibroma and schwannoma. Neurogenic tumours share certain characteristic imaging features, suggested by a fusiform-shaped mass with tapered ends, the “split-fat” sign, atrophy of the muscles supplied by the involved nerve, the “fascicular sign,” and the “target sign”; these imaging features are best demonstrated on magnetic resonance imaging. This pictorial essay emphasizes the characteristic signs and distinguishing features of PNSTs on imaging.
Résumé
Les tumeurs des gaines des nerfs périphériques peuvent être bénignes ou malignes. Parmis les bénignes, on compte les neurofibromes et les schannomes. Les tumeurs neurogènes partagent certaines caractéristiques à l'imagerie, soit une masse fusiforme (atténuées aux deux extrémités), le signe « split-fat » (couronne périphérique de tissus adipeux), une atrophie des muscles innervés par le nerf atteint, le signe « fasciculaire » et le signe « de la cible ». Ces caractéristiques sont mieux vues à l'imagerie par résonnance magnétique. Cet article mets en évidence les signes caractéristiques et distinctifs des tumeurs des gaines des nerfs périphériques à l'imagerie.
Key Words: Peripheral nerve sheath tumours, Neurofibroma, Schwannoma, Neurogenic tumour, Imaging, Magnetic resonance imaging
Peripheral nerve sheath tumours (PNST) are divided into 2 major categories: benign and malignant. Benign PNSTs include schwannomas and neurofibromas. Malignant PNST is now the general term used to describe malignant entities such as malignant schwannoma and neurofibrosarcoma. Malignant PNST is the preferred terminology because the cell of origin in these lesions often is unknown [1]. Both benign and malignant PNSTs may be associated with neurofibromatosis. The incidence of PNSTs is approximately 1 per 100,000 individuals per year [2]. This pictorial essay illustrates the imaging characteristics of the various types of PNSTs, with particular attention to key distinguishing features on magnetic resonance (MR) imaging.
Neurofibromas
Neurofibromas most commonly affect patients 20–30 years of age, with no sex predilection [3]. Neurofibromas represent slightly more than 5% of benign soft-tissue neoplasms [4], [5]. Three subtypes of neurofibromas have been described: localized, diffuse, and plexiform. Most lesions are solitary (localized form), which represent about 90% of these neurofibromas. Up to 10% of neurofibromas are associated with neurofibromatosis (NF). This comorbidity results in a markedly increased risk of malignant transformation. Localized neurofibromas often arise from cutaneous nerves, with occasional involvement of deep-seated larger nerves; they are usually slow growing. The diffuse subtype commonly originates from the nerves in the subcutaneous tissues of the head and neck, and primarily affects children and young adults. Unlike the more-common focal neurofibroma, diffuse neurofibromas are often ill defined and spread extensively along connective tissue septa and in-between adipose tissue. Diffuse neurofibroma typically involves the subcutaneous tissue down to the level of the fascia [6].
Neurologic symptoms are likely when a neurofibroma originates from a deep-seated nerve, whereas superficial neurofibromas are usually small painless masses. The plexiform subtype manifests as a diffuse tumour with tortuous expansion along the branches of the parent nerve, which results in a serpentine “bag of worms” appearance on gross inspection and cross-sectional imaging. Plexiform neurofibromas are pathognomonic for neurofibromatosis type 1 (NF1). On pathologic examination, neurofibromas are intimately associated with their nerve of origin and are not encapsulated, unlike schwannomas. As a result, complete excision of a neurofibroma requires removal of the nerve.
Schwannomas
Schwannomas most commonly affect patients 20–40 years of age and constitute about 5% of benign soft-tissue neoplasms [7], [8]. Males and females are affected equally. The commonly involved sites include the spinal and sympathetic roots of the head and neck, as well as nerves in the flexor surfaces of the upper and lower extremities (in particular, the ulnar and peroneal nerves). Most lesions are solitary and not associated with NF1. The typical presentation is a slow-growing, painless, soft-tissue mass. Pain and neurologic symptoms are unusual except in large tumours. About 5% of multiple schwannomas are associated with NF1. Vestibular schwannomas are the most-common and recognized feature of neurofibromatosis type 2 (NF2), and leads to significant morbidity. Dumbbell-shaped spinal cord schwannomas are also common in NF2. On pathologic examination, schwannomas are fusiform masses eccentrically located adjacent to the involved nerve, and both the schwannoma and the affected nerve are contained within a true capsule, the epineurium. Schwannomas are composed primarily of Schwann cells. Treatment of schwannoma is usually surgical resection. The affected nerve is usually surgically separable from the tumour, with sparing of the parent nerve and its function. Recurrence is unusual and malignant transformation is extremely rare.
Malignant PNSTs
Malignant PNSTs usually affect patients 20–50 years of age, with no significant sex predilection, and account for about 5%–10% of all soft-tissue sarcomas [5], [9]. A high proportion (about 50%) of malignant PNSTs occurs in association with NF1. However, only a small proportion of patients with NF1 (about 5%) develop malignant peripheral nerve sheath tumours [7], [8]. A malignant PNST generally presents as a soft-tissue mass that involves the major nerves and presents with pain and neurologic symptoms. A secondary malignant PNST may arise after radiation treatment, with a latent period that exceeds 10 years [7], [8].
Imaging Characteristics of PNSTs
Radiographs of PNSTs frequently are normal. Calcification, which may be chondroid, osteoid, or amorphous, is uncommon and mild in extent when present [10], [11]. Bone involvement from either extrinsic erosion or invasion is unusual. On computed tomography (CT), neurofibromas are seen as a well-defined, soft-tissue mass that is hypodense relative to muscle. Low attenuation is frequently observed (5–25 HU) because of the high lipid content of myelin from Schwann cells. On CT, neurofibromas usually show little or no enhancement. Ultrasonographic findings consist of a well-defined homogeneous, hypoechoic, fusiform-shaped mass that is oriented longitudinally in the nerve distribution, with tapered ends contiguous with the parent nerve. It may show posterior acoustic enhancement and simulate a cyst. An ultrasonographic target sign may be seen in neurofibromas with a hypoechoic peripheral zone and a hyperechoic central zone. Angiography of deep PNSTs may demonstrate displacement of major vascular structures around the parent nerve. Increased vascularity often is observed but variable and is typically more prominent in malignant PNSTs. An angiographic feature described to be characteristic is the presence of tortuous corkscrew nutrient feeding vessels at the upper and lower poles of the tumour [12], [13], which represent hypertrophy of nutrient nerve.
MR imaging is currently the modality of choice for evaluation of PNSTs. Classic MR imaging findings, if present, are often highly suggestive of a benign neurogenic tumour. On MR imaging, neurogenic tumours are seen as a fusiform-shaped mass with tapered ends that are low-to-intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images (Figure 1). The diagnostic confidence is increased if the mass arises from the anatomic location of a known nerve (Figure 2). The signal intensity on T2-weighted images may be either homogeneously hyperintense (Figure 3) or show a characteristic MR target sign that consists of high signal intensity in the periphery and low signal intensity in the central region of the lesion (Figure 4). This MR imaging finding corresponds with pathologic findings to fibrous tissue (with high collagen content) centrally and more myxoid tissue peripherally. Such a mass, if associated with muscular fatty atrophy that is best seen on T1-weighted images in a typical nerve distribution, is indicative of a PNST. Muscle atrophy is not commonly seen in other soft-tissue tumours.
Figure 1.
Median nerve schwannoma in a 35-year-old man, demonstrating the typical fusiform shape with tapered ends. (A) Coronal T1-weighted (W) magnetic resonance (MR) image of the wrist, showing a well-circumscribed fusiform isointense (compared with skeletal muscle) mass with tapered ends that arise from the median nerve at the level of the distal ulna (proximal to the ulna-carpal joint). (B) Coronal T2-W fat-suppressed MR image, showing the fusiform-shaped mass to be of heterogeneously hyperintense T2 signal, more clearly showing it to arise from the median nerve compared with the T1 image. (C) Coronal contrast-enhanced T1-W fat-suppressed MR image, showing heterogeneous enhancement of the mass. (D) Sagittal T2-W fat-suppressed MR image, showing the mass to be eccentric in location in relation to the median nerve, typical of a schwannoma.
Figure 2.
Ulnar nerve schwannoma in a 22-year-old woman. (A) Axial T1-weighted (W) magnetic resonance (MR) image, showing a well-circumscribed isointense (compared with skeletal muscle) mass arising from the ulnar nerve at the level of the distal ulna (proximal to the ulnar-carpal joint). (B) Axial T2-W fat-suppressed MR image, showing the lesion to be heterogeneously hyperintense. (C) Axial contrast-enhanced T1-W MR image, showing mild heterogeneous enhancement. (D) Photomicrograph (H&E, original magnification × 20) of the excised specimen confirms the diagnosis of schwannoma.
Figure 3.
Tibial neurogenic tumour in a 48-year-old man. (A) Coronal proton-density magnetic resonance (MR) image, showing a well-circumscribed mass that is slightly hyperintense compared with skeletal muscle; it arises from the tibial nerve. (B) Axial T2-weighted (W) fat-suppressed MR image, showing the lesion to be homogeneously hyperintense. (C) Coronal contrast–enhanced T1-W fat-suppressed MR image, showing a homogeneous avidly enhancing solid mass arising from the tibial nerve medial to the talo-calcaneal joint.
Figure 4.
Anterior deltoid intramuscular schwannoma in a 21-year-old man, demonstrating the classic split-fat sign and the target sign. (A) Coronal T1-weighted (W) magnetic resonance (MR) image, showing a well-circumscribed ovoid isointense mass (compared with the skeletal muscle) within the right deltoid muscle. The tapered rim of hyperintense fat is seen adjacent to the proximal and distal ends of the lesion. (B) Coronal T2-W fat-suppressed MR image, showing peripheral hyperintense T2 signal intensity, whereas the central area of the lesion is more hypointense, demonstrating the target sign. (C) Coronal contrast-enhanced T1-W fat-suppressed MR image, showing heterogeneous enhancement that is less avid peripherally. (D) Axial T2-W fat-suppressed MR image, again demonstrating the target sign, which consists of peripheral hyperintensity and central hypointensity. (E) Axial contrast-enhanced T1-W fat-suppressed MR image, showing predominantly central heterogeneous enhancement.
The split-fat sign (best appreciated on T1-weighted images) represents a rim of fat that surrounds the tumour (Figure 4, Figure 5), particularly in relation to the proximal and distal portion of the nerve. This sign is not specific for PNSTs but is suggestive that the tumour originates in the intermuscular space, in which neurogenic tumours are the most-frequent cause. This sign is more common in benign PNSTs and lesions that arise from large nerves. Malignant PNSTs less frequently demonstrate a complete fat rim because of its more aggressive and infiltrative growth pattern. Another MR imaging sign that suggests a neurogenic tumour is the fascicular sign, which manifests as multiple small hypointense foci interspersed within a hyperintense area on T2-weighted or proton-density weighted MR images (Figure 6). This sign may be seen in both superficial and deep-seated lesions, which corresponds to the fascicular bundles seen, on pathologic examination, in neurogenic tumours. Imaging findings of schwannomas and neurofibromas are often similar and, in many cases, cannot be distinguished from each other. When the parent nerve is identified, an eccentrically located tumour suggests a schwannoma (Figure 1D), whereas a centrally located soft-tissue mass suggests a neurofibroma (Figure 6). A heterogeneous appearance with degeneration and cystic changes are much more frequently seen in schwannomas than in neurofibromas.
Figure 5.
Intramuscular schwannoma in a 26-year-old man who presented with a painless mass in the posterior compartment of the left thigh, with images demonstrating a classic split-fat sign. (A) Sagittal T1-weighted (W) magnetic resonance (MR) image, showing a well-circumscribed ovoid heterogeneous mass (slightly hyperintense compared with the skeletal muscle) located within the semitendinosus muscle. The tapered rim of hyperintense fat is seen adjacent to the proximal and distal ends of the lesion, typical of the split-fat sign. (B) Sagittal TIRM MR image, showing the lesion to be heterogeneously hyperintense; the adjacent split-fat sign is present.
Figure 6.
Ulnar and median neurofibromas in a 15-year-old girl, with images demonstrating the magnetic resonance (MR) fascicular sign. (A) Sagittal T1-weighted (W) MR image, showing a well-circumscribed elongated mass arising from the median nerve at the level of the carpal bones, which is isointense to skeletal muscle. (B) Sagittal T2-W fat-suppressed MR image, showing the lesion to be heterogeneous hyperintense, with thickened hypointense strands within. (C) Axial T1-W MR image, showing 2 well-circumscribed isointense lesions, which correspond to the volar-sided nerves of the wrist; the more radial lesion is the median neurofibroma, whereas the more ulnar lesion is the ulnar neurofibroma. (D) Axial T2-W fat-suppressed image, showing the median and ulnar neurofibromas as hyperintense masses with speckled central areas of hypointense foci; these represent thickened fascicles. (E) Coronal T2-W fat-suppressed MR image, showing both the median and ulnar neurofibromas in the same plane. (F) Axial contrast-enhanced T1-W fat-suppressed MR image, showing heterogeneously enhancement of the ulnar and median neurofibromas.
Malignant PNSTs tend to be larger (>5 cm) and may demonstrate ill-defined margins, which suggest infiltration of adjacent tissues and associated oedema. Heterogeneity with central necrosis is frequently seen in malignant lesions than in benign lesions. The pattern of enhancement is variable in both benign (Figures 1C, 2C, 3C, 6E, and 6F) and malignant PNSTs. In general, increased heterogeneous enhancement, particularly peripheral nodular enhancement, suggests malignant PNSTs. Lesions that demonstrate the target sign typically enhance more avidly in the central region (Figure 4, C and E). Irregular infiltrative tumour borders and internal heterogeneity are more common in malignant PNSTs but may also be present in benign neurofibromas [14]. Unless there are distinctive characteristics, imaging is not entirely reliable in differentiating benign from malignant PNSTs, with the most-common abnormality detected on imaging of PNSTs being a nonspecific soft-tissue mass (Figure 7). If the lesion does not have these classic imaging findings, or, if there is any clinical or radiologic feature that raises the possibility of malignancy, then a carefully planned biopsy specimen should be obtained in consultation with the referring surgeon.
Figure 7.
Thenar intramuscular schwannoma in a 74-year-old man. (A) Radiograph of the left hand, showing a vague nonspecific soft-tissue mass in the first web space, with no abnormal bony changes. (B) Axial T1-weighted (W) magnetic resonance (MR) image of the left hand, showing a heterogeneously hyperintense nonspecific lobulated mass (compared with skeletal muscle) located within the thenar muscle. (C) Axial T2-W fat-suppressed MR image, showing the well-circumscribed lesion to be heterogeneously hyperintense. (D) Axial contrast-enhanced T1-W fat-suppressed MR image, showing heterogeneous enhancement of the lesion. There are no specific features to suggest a neurogenic tumour, and pathologic examination is required. Diagnosis of thenar schwannoma was confirmed, by histologic examination, after excision.
Conclusion
In summary, MR imaging is the modality of choice for evaluation of suspected neurogenic tumours. If present, then this constellation of MR imaging findings are highly suggestive of a benign neurogenic tumour: fusiform-shaped soft-tissue mass with tapered ends that are contiguous with the parent nerve, associated muscular fatty atrophy in a typical nerve distribution, complete split-fat sign, target sign, and fascicular sign. There are no clear imaging criteria to distinguish between benign and malignant PNSTs. If the characteristic findings described for benign PSNT are not present, then a malignant PNST or other soft-tissue tumours cannot be excluded, and a carefully planned biopsy specimen should be obtained.
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PII: S0846-5371(10)00111-7
doi:10.1016/j.carj.2010.04.009
© 2011 Canadian Association of Radiologists. Published by Elsevier Inc. All rights reserved.
Volume 62, Issue 3 , Pages 176-182, August 2011
