Cellular neurothekeoma. A distinctive variant of neurothekeoma mimicking nevomelanocytic tumors.

We describe the clinical, histopathologic, and immunohistochemical characteristics of five examples of a distinctive subtype of neurothekeoma we term "cellular neurothekeoma" (CNT). These lesions are nondescript papules or nodules primarily involving the head and neck areas of young adults. Histopathologically, CNT are fairly well-defined proliferations involving the reticular dermis; they consist of fascicles of polygonal and spindle cells with eosinophilic or pale-staining cytoplasm and neuroid characteristics. Low-grade cytologic atypia and mitotic activity are common. All immunohistochemical markers--including S-100 protein, myelin basic protein, epithelial membrane antigen, and histiocytic antigens--have failed to show positivity in our laboratory. Separation from myxomatous variants of neurothekeoma is based on greater cellularity, less myxomatous change, and less pronounced plexiform compartmentalization by fibrous septae, which resemble perineurium. The differential diagnosis usually includes spindle and epithelioid cell (Spitz) nevus, malignant melanoma (particularly desmoplastic-neurotropic melanoma), cellular blue nevus, and fibrohistiocytic proliferations. The recognition of CNT and its differentiation from melanoma are important so that overly aggressive therapy is avoided.