Anorectal malignant melanoma: morphologic and immunohistochemical features.

We reviewed 17 cases of primary anorectal malignant melanoma. Morphologic features evaluated included junctional change, pigmentation, morphology, and mitotic rate. Immunohistochemical stains were performed for/with S-100 protein, HMB-45, MelanA, tyrosinase, vimentin, KIT, and pankeratin. Morphologic subtypes were as follows: epithelioid, 12 cases; spindle cell, 7 cases; lymphoma-like, 10 cases; and pleomorphic, 6 cases. Pigmentation was present in 9 cases. Junctional change was present in 6 cases. The mitotic rate was 3 or more per high-power field in 8 cases. S-100 protein was present in all cases, HMB-45 stained 16 of 17, MelanA was present in 14 of 15, tyrosinase in 12 of 14, vimentin in 13 of 14, and KIT in 12 of 16. Pankeratin was absent in all cases. The mean length of follow-up was 25.6 months (range, 8-96 months), and the average survival with disease was 32.3 months (range, 8-96). No morphologic or immunohistochemical features were predictive of survival. Anorectal malignant melanoma shows considerable morphologic variability. Immunohistochemical staining is similar to cutaneous melanomas. Expression of KIT was present frequently, including cases with spindle cell morphologic features, in which it may lead to confusion with gastrointestinal stromal tumors.