[Pathological and immunohistochemical study on anorectal melanoma].

Forty two cases of anorectal melanoma were studied which constituted 1.19% of all the anorectal malignancies collected in this series. 90% of the tumors located near by the dentate line. Grossly, the masses were nodular, fungiform or ulcerated. Histologically, pleomorphic tumor cells mixed together giving a feature somewhat like either carcinomatoid or sarcomatoid, with a predominance of one kind of the malignant cells. Histopathologic observation supported the idea that this tumor developed from the melanocytes and nevus cells located at the basal layer of the epithelium. Immunohistochemistry proved that melanocytes originated from the neural crest. S-100 protein was marker with high sensitivity but the specificity not ideal, so that, varied markers are recommended in order to improve the accuracy of diagnosis.