A Patient with Amelanotic Malignant Melanoma in the Rectum

Abstract

A 62-year-old woman with a history of mental retardation underwent surgery for descending colon cancer 15 years previously. She was then referred to our Department of Internal Medicine for a chief complaint of blood in the stool during defecation. Colonoscopy showed a polypoid lesion with friability in the lower rectum (Rb), and rectal cancer was suspected. Abdominal CT showed a tumor with extramural invasion in the rectum, with enlargement of regional lymph nodes and lymph nodes along the left internal iliac artery. Histopathology of a biopsy specimen with HE staining suggested a poorly/undifferentiated malignant tumor. A malignant melanoma was diagnosed by immunostaining.
Abdominoperineal rectal resection and D3 lymph node dissection were performed. Histopathology showed a malignant melanoma (pT4a, pN3 (21/38), M0 pStage IIIc). Malignant melanomas of the anorectal region are relatively rare and account for only 0.38% of malignant tumors at this site. Colonoscopy in this patient showed no dark-colored areas of the tumor, and histopathology showed only a very small amount of melanin. Therefore, immunostaining was required for a confirmatory diagnosis. This case is presented along with a review of the relevant literature.