Abstract
A comparatively rare case of anorectal malignant melanoma was experienced. A 43-year-old male was admitted to the hospital because of anal bleeding and prolapse of a tumor. Preoperative biopsy specimen revealed anorectal malignant melanoma, and abdomino-perineal resection of the rectum with lymph node dissection R3 was carried out. Resected specimen showed a greying brown, partially blackish brown peduncular tumor on the dentate line. Microscopical features were drawn as follows, sm, ly0, V0, and n1 (+). The tumor cells studded with melanine granules in their cytoplasms in part. The majority of the tumor was amelanotic malignant melanoma. S-100 protein was positive in cytoplasms of both of the pigmented cells and amelanotic cells. The possible existence of malignant melanoma should be entertained in elevating anorectal tumors. Immunohistochemically S-100 protein was very useful to diagnose malignant melanoma, especially amelanotic type.
