RESUMO
Malignant melanoma arising from the mucosal surface is a rare, virulent malignant neoplasm frequently associated with an extremely poor prognosis. The sites of initial presentation include the nose, oral cavity, larynx, vulva, vagina, anal canal and rectum. Primary melanoma of the rectum is extremely rare and its histogenesis is controversial. We present a case of primary malignant melanoma arising from the rectum in a 77-year-old woman who presented with intermittent anal bleeding for 2 months. Colonoscopic examination of the rectum revealed a 3.5 3.0 2.7 cm exophytic tumor with a granular surface in the rectum, 5cm from the anal verge. Black colored pigmentation was detected. Distant metastasis to liver was detected on abdominal CT scan. Light microscopy of tumor revealed malignant melanocytes surrounded by normal mucosa. The tumor cells reacted positively for immunohistochemical staining with S-100 protein and HMB-45. Clinical and laboratory examination excluded the presence of melanoma at sites other than rectum. Local excision of the tumor was performed.