A Case of Primary Melanoma of the Anus that Spread to the Submucosa of the Rectum

Primary malignant melanoma of the anorectum is rare, representing about 1% of all colorectal carcinoma and less than 1% of all melanomas. The most common symptom of malignant melanoma of the anorectum is anal bleeding and this is often misdiagnosed as hemorrhoids. A 72-year-old female patient was admitted due to intermittent anal bleeding for 6 months. Colonoscopic examination showed a large exophytic mass with an irregularly ulcerated and greenish-brown pigmentation on the anus and the examination also simultaneously showed a submucosal tumor-like lesion in the rectum that was located 5 cm from the anal verge. Light microscopy of the tumor revealed malignant melanocytes and the tumor cells reacted positively for immunohistochemical staining with S-100 protein and HMB-45. Distant metastasis to the brain was detected on brain MRI.