S4783 A Rare Presentation of Anorectal Melanoma Presenting as Hemorrhoidal Discomfort and Rectal Bleeding

Introduction: Anorectal melanoma is uncommon and aggressive. It accounts for 1.3% of melanomas and 16.5% of mucosal melanomas. Incidence increases with age and is 1.6-2.3 times higher in women. Initial symptoms are non-specific and include anal bleeding and painful anal masses. It is often misdiagnosed as hemorrhoids or rectal polyps in two thirds of patients. Case Description/Methods: A 46-year-old woman presented with intermittent left periumbilical abdominal pain, constipation, hemorrhoidal irritation, and rectal bleeding. She expressed concerns for rectal prolapse and intentional 45 pound weight loss. Physical exam and labs were within normal limits. computed tomography of the abdomen and pelvis showed L diverticulosis and fatty infiltration of the liver, and was otherwise normal. Colonoscopy revealed sigmoid diverticulosis and a 4 cm non-circumferential ulcerated mass from the anus extending into the posterior bowel wall of the rectum. Anorectal mass biopsy showed a poorly differentiated tumor with immunochemical stains of tumor cells reactive with Mel-C and SOX-10, consistent with melanoma. An oncomine mutation was detected. The patient established care with Oncology and Colorectal surgery. Computed tomography of Chest showed non-specific pulmonary nodules and magnetic resonance imaging (MRI) showed no evidence of metastases. Patient was started on ipi 1/nivo3 with optional surgery if there was a minimal response. Patient developed myositis/myocarditis during therapy. Therapy was discontinued and patient was treated with IV solumedrol. Subsequently, patient was treated with imatinib but it was discontinued due to the development of a severe erythrodemic rash. Repeat MRI showed interval increase in size of the anal tumor and nodal metastases. The patient was referred to surgery for evaluation for APR ± post operative radiation. Discussion: Melanocytes can migrate through the umbilical-mesenteric canal and undergo neoplastic transformation. The ileum is the most common site of primary melanoma of the GI tract. Melanocytes are important for supporting the physical barrier of the GI tract through mucin production in the goblet cells and defensin production in the Paneth cells. Patients who survive more than 5 years have a tumor thickness less than 2 mm. There is a 30% survival rate at 5 years for a mass without nodal involvement. Survival in recurrent or metastatic disease is less than 10 months. Anorectal mucosa is vascular and has great lymphatic supply, causing fast growth and metastatic spread. Prognosis is worse with delayed diagnosis.