p16 expression differentiates between desmoplastic Spitz nevus and desmoplastic melanoma.

BACKGROUND: Loss of p16 in melanomas reflects worse outcomes for patients. It is associated with depth of invasion, ulceration, vascular invasion, lymph node metastases, metastases, recurrence of melanoma and decreased 5-year survival. Desmoplastic melanoma is an insidious malignant melanoma subtype that commonly occurs on sun-damaged skin of the head and neck area in elderly patients. The diagnostic conundrum occurs with confusion of desmoplastic melanoma with scars, hyalinizing blue nevi, desmoplastic Spitz nevi and diffuse neurofibromas. METHODS: The present study uses immunohistochemistry with a p16 antibody to differentiate desmoplastic Spitz nevi (n = 15 cases) from desmoplastic melanomas (n = 11). To date, no other studies have been published defining the expression pattern of p16 in desmoplastic melanoma. RESULTS: 81.8% of desmoplastic melanomas were negative for p16 and 18.2% were only weakly stained. In contrast, all desmoplastic Spitz nevi were moderately to strongly positive for p16. CONCLUSIONS: The staining pattern for p16 in desmoplastic melanomas and Spitz nevi in conjunction with the histopathologic features, S100 staining, Ki67 proliferation index and clinical scenario may aid in the difficult differential diagnosis between these two entities. Further confirmatory studies are indicated.