Plexiform schwannoma of the posterior tibial nerve: a case report

Plexiform schwannoma is one of the least common variants of schwannoma. It is usually found on the trunk, head, neck and upper extremities. Most reported cases are small tumors, less than 2cm in maximum diameter, arising from superficial nerves. Trauma and neurofibromatosis type 2 are well-recognized risk factors for plexiform schwannoma. It is important to differentiate it from plexiform neurofibroma, because the former has neither an association with von Recklinghausen's disease nor a malignant potential.

We report a case of a large plexiform schwannoma arising from the posterior tibial nerve in proximity with the medial malleolus. The patient had no history of ankle strain, fracture or neurofibromatosis type 2. Magnetic resonance imaging demonstrated a multinodular, inhomogeneous lesion, measuring 6 x 4 x 2.8 cm. Fine needle biopsy was suggestive of a benign lesion, deriving from neural elements. The mass was excised marginally. Permanent section showed that the lesion was multilobular, surrounded by a thin fibrous capsule and consisting of elongated cells, rare typical mitosis, cells with degenerative features and stained positive for S-100 protein. The patient was not evident disease at the latest follow-up 2.3 years later, with an excellent functional result. No sensory or motor deficits were encountered.

There are no reports in the literature for large plexiform schwannomas arising from the tibial nerve. Marginal excision seems to be the recommended therapy for this rare tumor.