Histologic spectrum of neurothekeoma and the value of immunoperoxidase staining for S-100 protein in distinguishing it from melanoma.

Neurothekeoma, a benign cutaneous lesion of probable nerve sheath origin, is divided histologically into two subtypes--myxoid and cellular. However, we believe that neurothekeoma encompasses a wider spectrum of lesions, with the myxoid and cellular subtypes falling at either end of the morphologic spectrum. Because the cellular variant of neurothekeoma sometimes resembles melanoma, it presents a difficult diagnostic problem. We report the histologic and immunohistochemical findings in 14 cases of neurothekeoma and review the findings in 35 additional cases from the literature. A detailed analysis of the histologic spectrum is also included. When examined by immunostains, only the myxoid variants of neurothekeoma stain positively for S-100 protein. We conclude that when the histological differential diagnosis is between cellular neurothekeoma and melanoma, an S-100-positive lesion should be regarded as melanoma.