Intradermal Spitz nevus differentiated from reticulohistiocytic granuloma by immunoreactivity to S-100 protein.

A firm, dark reddish, smooth surfaced nodule on the forearm of a 16-year-old boy was diagnosed as a Spitz nevus with the aid of a strong positive reactivity to S-100 protein. Histological examination revealed an intradermal epithelioid cell tumor with prominent multinucleated giant cells, suggesting the diagnosis of reticulohistiocytic granuloma. However, immunoperoxidase staining of the tumor cells showed strong positive reactivity to S-100 protein and vimentin; it was negative for lysozyme and alpha-1 antitrypsin. Although a few melanosomes in the tumor cells seen in electron microscopic examination provided crucial proof for the diagnosis of Spitz nevus, the positive reactivity to S-100 protein in ordinary formalin-fixed, paraffin-embedded tissues proved to be very useful in the differentiation of Spitz nevus from tumors of histiocytic origin, especially those of the mononuclear phagocytic system.