S4097 Primary Anorectal Melanoma, an Unusual Presentation With Unfavorable Prognosis: Case Report

Introduction: Melanomas can originate from cutaneous or mucosal tissues, both types arising from melanocytes. In the rectum, melanocytes are primarily located in the anal transition zone and squamous zone. Anorectal malignant melanoma is a rare cancer with poorly defined presentation that tends to follow an aggressive course. It typically occurs in the fifth-sixth decade of life, with female predominance. Common symptoms are nonspecific and include rectal bleeding, which is frequently mistaken for hemorrhoids. Due to submucosal drainage spread of the tumor, more than 50% of patients are found to have distant metastases at time of diagnosis. Case Description/Methods: We report a case of an 86-year-old woman with a medical history of hypertension who presented with a 1-month history of constipation and diffuse abdominal pain, with worsening symptoms and the development of rectal bleeding in the past 1-week. Denied weight loss, anal pruritus, rectal mass, rectal pain, nor family history of colon cancer. Laboratory findings were consistent with normocytic normochromic anemia. Abdominopelvic computed tomography scan performed with findings of an intraluminal mass in the rectum with suspected liver metastases. Endoscopic examination revealed an ulcerating, black-colored polypoid lesion primarily located at the anorectal junction, with 2 cm proximal extension. A second similar lesion was identified approximately 6 cm from anal verge. Biopsy specimens revealed pleomorphic lesions displaying high mitotic rate, prominent nucleoli and positive immunohistochemical staining with markers HMB45, SOX100 and melA. Discussion: To our knowledge, this case adds to the few reported instances of synchronous anorectal mucosal melanomas. It’s rarity, non-specific symptoms and increase in incidence underscores the need for heightened clinical awareness. Limited understanding of its causes and late presentation in older patients pose diagnostic and treatment challenges. Increasing awareness in the gastroenterology community is essential for early detection, better outcomes, and avoiding misdiagnosis of this aggressive malignancy.