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The Pedunculated Pretender: A Case of Invasive Anorectal Mucosal Melanoma

A 57-year-old black male presented to clinic with the primary complaint of rectal bleeding and rectal prolapse for the past year’s duration. His past medical history was significant for congestive heart failure, atrial fibrillation (on Coumadin), and prior cerebral vascular accident. He reported bright red blood per rectum and prolapse with every bowel movement, five to six times per day, and had been using over the counter hemorrhoid cream without any notable improvement. A colonoscopy 16 months prior had reported a 2-mm rectal polyp that was resected, with pathology revealing colonic mucosa with lymphoid aggregates, and additionally mentioned a large, nonbleeding hemorrhoid in the anal canal. On examination in the office, a firm, approximately 4-cm mass was easily prolapsed. The patient was taken to the operating room for an examination under anesthesia and transanal resection. Intraoperatively, a large mass was noted to prolapse easily from the anal canal and was connected to the rectum by a short stalk (Fig. 1). The stalk was transected near the base using electrocautery and the specimen was delivered to pathology. Pathology revealed an invasive mucosal melanoma, with maximum tumor thickness of 40 mm. The margins were positive. Ulceration was present and mitotic rate was 14 mm. There was no definite evidence of lymphovascular invasion but multiple foci of angiotropism were noted. Pathologic stage was determined to be T4b (Fig. 2). Given this result, a referral was made to Medical Oncology. Positron emission tomography scan and brain MRI were obtained and did not reveal metastasis. Bilateral inguinal lymph node ultrasounds were negative and the patient was taken back to the OR for re-excision to obtain a negative margin. Radiotracer was injected intraoperatively without migration, thus, sentinel lymph node biopsy was not performed. A residual area of hyperpigmentation and scar from prior resection was noted, and this area was excised with a 1-cm margin. Pathology revealed residual melanoma in situ with a negative margin. The patient then started adjuvant radiation for a dose of 3000 cGy given over five treatments, and following the completion of radiation began additional adjuvant chemotherapy with a checkpoint inhibitor (nivolumab). Anorectal mucosal melanomas account for less than 2 per cent of all melanomas and less than 1 per cent of anorectal malignancies. They can present as pigmented lesions, or amelanotic masses, making the diagnosis challenging. In addition, common presenting symptoms are similar to those of hemorrhoids or other benign anorectal diseases and include rectal bleeding, sensation of prolapse, and