Perineurioma: an uncommon soft tissue neoplasm distinct from localized hypertrophic neuropathy and neurofibroma

Perineurial cells, which normally surround the nerve fascicles within a nerve, can be distinguished from Schwann cells by their immunoreactivity for epithelial membrane antigen (EMA) and lack of reactivity for S-100 protein. We report two cases of perineurioma, a tumor composed exclusively of perineurial cells and distinct from other nerve sheath tumors. The first case involved a deep, soft-tissue mass of the neck, and the second involved a tumor located in the infraclavicular subcutaneous tissue. Both tumors were well circumscribed. Histologically, they were hypocellular and composed of spindle cells possessing elongated nuclei and bipolar, wavy, slender, strikingly elongated cytoplasmic processes, disposed in a background of collagen in the form of short bundles and whorls. In the first case, there were frequent calcospherites and remnants of a small nerve at the periphery. The spindle cells stained for EMA but not S-100 protein, chromogranin, neuron-specific enolase or Leu-7. Ultrastructurally, they possessed long cytoplasmic processes with incomplete basal lamina and occasional pinocytotic vesicles. Axons were not identified. Review of the literature shows that genuine perineuriomas are rare, and most cases reported as such are merely examples of localized hypertrophic neuropathy, a mononeuropathy characterized by fusiform swelling of a nerve, usually in the extremities. The involved segment in localized hypertrophic neuropathy contains distended fascicles composed of whorls of perineurial cells and fibrous tissue entrapping residual axons, probably representing a hyperplastic reaction to nerve damage. The term perineurioma should be reserved for the neoplasm composed only of perineurial cells and presenting as a soft tissue tumor.