Perineurial cell differentiation in benign tumors and tumorlike proliferation of peripheral nerves.

An unusual, benign, spindle cell peripheral nerve sheath tumor (PNST) is reported that on ultrastructural examination was composed only of perineurial cells. The neoplastic cells showed positive immunoreactivity for S-100 protein and negative immunoreactivity for epithelial membrane antigen; this is not the expected immunophenotype of a perineurioma. Continued examination of benign PNSTs using electron microscopy and immunohistochemistry has the potential to reveal additional variations in the cellular composition and immunophenotype of these soft tissue tumors. PNSTs such as the one described will renew the debate about the cytogenesis of the perineurial cell and its role in PNSTs.