1486 Rectal Melanoma Occurring in a Properly CRC Screened Patient

Abstract

INTRODUCTION: Rectal melanoma is a very rare, highly aggressive malignancy. Anal melanoma is the 3rd most common melanoma after the cutaneous and ocular varieties. It is the most common site of primary GI melanoma. Cutaneous melanoma metastasizes to the GI tract only 2% of the time, and only 2% of these metastases are to the rectum. CASE DESCRIPTION/METHODS: A 68-year-old male with a past medical history of HTN presented for evaluation of changed bowel movements. His stool diameter had decreased, and he noticed bright red blood mixed with his stool. He sometimes experienced fecal urgency, with no output. Rectal bleeding occurred just before or with bowel movements. He recently changed his diet, because of constipation. He recently lost 10 lbs. unintentionally. He did not use NSAIDs aside from aspirin 81 mg qd. He had a colonoscopy 5 years ago, with a 2 mm polyp removed (no abnormality seen on pathology). He had no family history of colon cancer. He was a retired chiropractor. A colonoscopy was performed, which found a suspicious lesion in the rectum (biopsied). Pathology showed pigmented melanoma. EUS staged the rectal mass as uT2uN1. Immunostains of the biopsy were performed: CK20-, CDX2-, p63-, S100 focally positive, HMB45 diffuse and strongly positive. No evidence of cutaneous melanoma was found. He was referred for surgical and oncology evaluation and treatment. He successfully underwent laparoscopic abdominal perineal resection with end colostomy. 9/10 sampled lymph nodes during surgery were positive for melanoma. He was started on nivolumab/ipilimumab dual therapy. He proceeded to undergo chemotherapy as outpatient (currently on chemotherapy cycle 2). DISCUSSION: Anorectal melanoma is a rare disease, accounting for ∼ 0.05% of all colorectal malignancies, and 1% of all anal cancers. Risk factors for anorectal mucosal melanoma are not currently known, although there is an increased risk associated with HIV infection. Survival is poor, depending on staging (Stage I – 24 month median survival; 5-year survival 26.7%, Stage II – median survival 17 months; 5-year survival 9.8%, Stage III – median survival 8 months; 5-year survival 0%). Adjuvant therapy consists of radiotherapy, however adjuvant immunotherapy with nivolumab has an established role in treating lymph node involvement. Anti-CTLA4 monoclonal antibody ipilimumab has been shown to significantly prolong survival in cutaneous melanoma, however more investigation is necessary to clarify role of ipilimumab in patients with mucosal melanoma.