Introduction

Anorectal melanoma is an uncommon form of melanoma, accounting for 0.3–3% of all melanomas and 1–3% of anorectal tumors. It arises from the transitional zone and squamous region of the anal canal.

Endoscopy

We present the case of a 74-year-old man with a history of right hemicolectomy and hepatic resection for metastatic colon adenocarcinoma, with no recurrence on colonoscopy in 2021. In 2025, due to rectal bleeding, a new colonoscopy revealed a pigmented, ulcerated, and necrotic lesion in the rectum, involving the pectinate line and occupying 75% of the circumference. Biopsies confirmed a poorly differentiated melanoma. CT imaging showed pancreatic metastases causing obstructive jaundice, managed with biliary stent placement via ERCP. A subsequent PET-CT demonstrated rapid metastatic progression to infradiaphragmatic lymph nodes, pancreas, adrenal glands, peritoneum, muscle, lung, and bone. The patient is currently receiving immunotherapy with ipilimumab/nivolumab for stage IV rectal melanoma, BRAF wild type.

Conclusions

Anorectal melanoma is aggressive, often metastatic at diagnosis, and usually presents with nonspecific symptoms. Up to 60% of cases show dissemination to lymph nodes, liver, lungs, and the central nervous system. Its etiology remains unclear, though HPV or HIV infections may increase risk. The prognosis in advanced stages is poor; however, immunotherapy has shown promising outcomes.