Antibody to S100a6 protein is a sensitive immunohistochemical marker for neurothekeoma

BACKGROUND: Neurothekeoma is a benign tumor of putative peripheral nerve sheath origin. It occurs in a myxoid (classic) variant, cellular variant, and intermediate (mixed) variant. Cellular neurothekeoma (CNT) usually involves the head and neck or extremities of young patients. Histologically, CNT can be confused with melanocytic and fibrohistiocytic lesions. An immunohistochemical antibody panel is often necessary to confirm the histological impression and exclude melanocytic and/or fibrohistiocytic lesions. METHODS: Formalin-fixed, paraffin-embedded archival tissues were evaluated by immunohistochemistry using antibodies specific for S100A6 and PGP9.5 in 11 cases of neurothekeoma (seven cellular, four myxoid). A variety of other antibodies were evaluated by immunohistochemistry at the time of initial diagnosis. RESULTS: All 11 neurothekeoma cases were positive for S100A6 protein (four cases, weak/1+; seven cases, strong/2+), corresponding to 100% sensitivity. In contrast, eight of 11 neurothekeoma cases (73% sensitivity) were positive for PGP9.5. All seven CNT cases were negative for S100B, as expected. CONCLUSIONS: Anti-S100A6 is a highly sensitive antibody for neurothekeomas, including CNT, and, in our experience, is superior in sensitivity to PGP9.5. However, like other antibodies used in evaluating neurothekeomas, S100A6 lacks specificity, as has been demonstrated in previous studies. Nevertheless, S100A6 can be useful in an immunohistochemical antibody panel to evaluate lesions where the differential diagnosis includes CNT.