Multiple synchronous anorectal malignant melanoma coexisting with adenocarcinoma of the sigmoid colon

Primary anorectal malignant melanoma is an exceptionally rare neoplasm associated with poor prognosis. Anorectal malignant melanoma has been very rarely described with coexisting primary tumors of the colorectum. A 56-year-old female patient was admitted with a history of rectal bleeding. She had experienced increasing constipation and a sense of obstruction in the rectum for 6 months. Flexible rectosigmoidoscopy showed a large, pedinculated polypoid lesion extending from the anal canal to the rectum. She underwent a transanal local excision and was diagnosed with a melanoma of the anorectum with positive margins. Therefore, a formal abdominoperineal resection was performed. In addition to multiple synchronous anorectal malignant melanoma, we incidentally found another primary tumor in the proximal surgical margin of the resected specimen. Histopathologically, the lesion was an intramucosal adenocarcinoma of the sigmoid colon. Postoperatively, the patient received adjuvant chemotherapy of six cycles duration. At present, the patient has completed 18 months of follow-up.