Abstract
A 67-year-old man visited our hospital because of bloody stools and a hard mass that prolapsed from the anus. Colonoscopy revealed a protrunding lesion with ulceration, a nodule, elevated plaques, and necrotic tissue. The tumor was located close to the anal canal over the Herrmann line from the rectum. Pathological analysis of a biopsy specimen indicated a diagnosis of poorly differentiated adenocarcinoma, and the patient was admitted our hospital. Rectal cancer was diagnosed, and abdominoperineal resection was performed. Histopathological examination of the resected specimen indicated an amelanotic melanoma(positive for S-100, HMB45, and MART-1). Up to 90% of cases of malignant melanoma that occurs in the anorectum are of melanotic tumor. Therefore, malignant melanoma can be diagnosed before surgery, and immunohistochemical staining was used for diagnosis in many cases. However, an amelanotic melanoma is difficult to diagnose because of the small amount of melanin as a result of which it does not appear black in color.
