Primary anorectal malignant melanoma with synchronous hepatic metastases in middle-aged patient: a case report

Anorectal malignant melanoma is a rare, notably aggressive malignancy, comprising fewer than 1% of all melanoma cases. Its early symptoms frequently resemble benign anorectal conditions, which may lead to diagnostic delays and poor outcomes. We report a case of a 42-year-old, previously healthy male who presented with a two-month history of progressive constipation, unintentional weight loss, and lower abdominal discomfort associated with melena. Imaging revealed a mass extending from the anal canal into the distal rectum, with hepatic metastases. Histopathological examination confirmed malignant melanoma with positive immunohistochemical staining for HMB-45 and Melan-A, and negative S-100. A diverting functional colostomy was performed for impending obstruction, and first-line nivolumab was initiated, but the patient deteriorated rapidly and died on best supportive care. This case highlights the silent nature of this malignancy and the importance of considering rare yet aggressive tumors when dealing with atypical or unexplained anorectal lesions.