Teaching Case

Palpable soft-tissue mass on the posterior aspect of the right thigh

An otherwise healty 38-year-old female patient presented with a slowly growing mass in the posterior aspect of her right thigh. Clinical examination revealed a mobile, moderately tender mass. Tinel’s sign (paraesthesias produced by percussing the tumour) was negative. Radiographs were normal. MRI scan was performed. An ovoid mass oriented longitudinally in the common peroneal nerve distribution was demonstrated. The lesion was iso-slightly hyperintense relative to muscle on T1-weighted images (Fig 1 a,b) and predominantly of high signal intensity with low-signal-intensity central cystic degeneration area on T2-weighted images (Fig 2 a,b). After IV administration of gadolinium chelates the mass was diffusely enhanced except for central cystic degeneration area (Fig 3 a,b). Surgical resection followed with histopathological examination allowed the diagnosis of an ancient schwannoma.

Schwannomas and neurofibromas are the most commonly seen benign peripheral nerve sheath tumors that arises from the schwann cells of the nerve sheath [1-3]. Malignant peripheral nerve sheath tumors could also be seen and both benign and malignant forms can be associated with neurofibromatosis [1]. Most of schwannomas present as a slowly growing, painless soft-tissue mass [1]. They are usually solitary, encapsulated and mobile in palpation. Schwannomas occur in the 2nd to 5th decade of life with no sex predilection [2,3]. Degeneration and cystic cavitation are much more common in schwannomas than in neurofibromas [1]. “Ancient” schwannomas refer to long-standing degenerative lesions with calcification, hyalinization, and cystic cavitation [1]. These degenerative changes can be misdiagnosed as malignant fibrous histiocytoma, malignant peripheral nerve sheath tumor, liposarcoma, synovial sarcoma or hemangiopericytoma [3]. The malignant degeneration of a schwannoma is extremely rare.

Sonography shows a well-defined, homogeneous, hypoechoic ovoid mass with a healthy nerve at the proximal and distal aspects of the mass [2,3]. CT reveals a low density well-circumscribed, inhomogeneous mass. MRI is the best imaging modality for evaluating schwannomas. The “target sign” has been reported for 52% of benign nerve sheath tumors which is composed of a peripherally hyperintense rim and a central low intensity area on T2-weighted images [3]. Although the target sign is seen more frequently in neurofibromas, it may be seen in schwannomas as well [4]. The “split-fat sign” represents a rim of fat surrounding the margins of the nerve sheath tumor in an intermuscular location. MRI also shows associated muscle atrophy and the relationship with the nerve [1]. They are isointense or slightly hyperintense relative to muscle on T1- weighted images and markedly hyperintense on T2-weighted images. Ordinary schwannomas show strong enhancement by gadolinium contrast [3]. Hence schwannomas are generally separable from the underlying nerve fibers, surgical excision can usually spare the parent nerve [1].