10% of benign soft tissue neoplasms
Derived from Schwann cells
Most common types - Schwannomas (neurilemmomas) and neurofibromas
Others include Traumatic neuroma (stump neuroma), Mortans neuroma, Neural fibrolipoma, Nerve sheath ganglion, Intraneural perineuroma, Malignant peripheral nerve sheath tumors(7-8%)
Difficult to distinguish them with imaging
Rarely show malignant transformation
Common sites - plantar digital nerve in Morton neuroma, median nerve in neural fibrolipoma (associated with macrodactyly), large nerve trunk in benign and malignant PNSTs, nerve sheath ganglion commonly occurs about the knee (cystic appearance)
The most common lesion in Neurofibromatosis 1 is neurofibroma, although neurilemmoma and malignant PNST may be seen
Neurofibroma:
Solitary; multiple as in NF
20-30 years
1. Localised neurofibroma: most common type (90%), usually solitary, not associated with NF1, 2. Usually superficial cutaneous nerves, slow growing, usually less than 5cm
Diffuse neurofibroma: Children and young adult, subcutaneous tissue of head and neck, not associated with NF1
3. Plexiform neurofibroma = NF1
Schwannona = Neurilemmoma:
Benign slow-growing encapsulated tumors of nerve sheath
Most commonly seen in extremities
Highly ordered cellularity of Antoni type A and less cellular areas with myxoid matrix of Antoni type B
20-30 years
Commonly involves spinal and sympathetic nerve roots of head and neck, flexor surfaces of upper and lower limbs (particularly ulnar and peroneal nerves), posterior mediastinum and retroperitoneum
Single, less than 5cm; if multiple, usually associated with NF1
Fusiform in shape, usually well defined
Treated surgically
Theriotically neurilemoma is eccentric and separable from normal nerve. Neurofibroma is intimately related and indistinguishable from normal nerve
Ultrasound:
Most are homogeneous (may be heterogenous) and hypoechoic
Continuity along the nerve is seen in most; may be eccentric to nerve axis
May show posterior acoustic enhancement, target appearance (hyperechoic center and hypoechoic periphery), pseudocystic appearance
Increased flow on Doppler
Several simulate ganglion cysts, but presence of flow on Doppler excludes uncomplicated ganglion cyst
Ancient Schwannoma may calcify
CT:
Well defined non-homogeneous low density mass
May also show target sign, split fat sign
MRI:
Fusiform appearance in periphery, dumbell haped in paraspinal region
Low-signal (similar to muscle) on T1WI
High signal (higher than fat) on T2WI, diffuse neurofibroma is low on T2
Target sign = peripheral high signal rim with central low-to-int signal on T2 (usually not seen in malignanct PNST), represents central fibrous tissue and peripheral myxioid tissue
Fascicular signon T2 and PD - multiple small ringlike structures with peripheral high signal intensity
Split fat sign on T1
Muscle atrophy on T1
May have fluid -fluid level due to hemorrhage
Strongly enhance on Gd
References:
1. Reynolds JR et al.Sonographic Characteristics of Peripheral Nerve Sheath Tumors . AJR 2004; 182:741-744
2. Murphey MD et al. Imaging of Musculoskeletal Neurogenic Tumors: Radiologic-Pathologic Correlation . Radiographics. 1999;19:1253-1280.
