S3735 When Routine Fails: Rapid Onset of Anorectal Melanoma Despite Recent Colonoscopy

Introduction: Anorectal melanoma (AM) is a rare and aggressive diagnosis, representing just 1.3% of all melanomas. Patients often experience delayed diagnosis due to multiple factors, including nonspecific symptoms, misdiagnosis, and late presentation. Five-year survival rates are estimated to be between 10% to 20%, a reflection of the challenges associated with an aggressive malignancy. In this case report, we describe a presentation of AM with rapid growth and metastasis. Case Description/Methods: A 77-year-old woman with a past medical history of heart failure and muscular dystrophy presented to the emergency department with abdominal pain. She also reported constipation, nausea, diaphoresis, a 25-pound weight loss, and generalized abdominal pain over the course of the last year. A computed tomography (CT) scan of her abdomen from 1 year prior showed no masses, while colonoscopy from 6 months prior found 3 polyps: 2 tubular adenomas (6 mm and 10 mm) and 1 sessile serrated polyp (12 mm). On admission, a CT abdomen pelvis with IV contrast revealed a large, irregular mass within the distal rectum at the anal verge measuring 6.6 cm by 4.0 cm with numerous large hepatic lesions, suggestive of a primary colonic neoplasm with hepatic metastasis. The patient then underwent diagnostic colonoscopy which showed a fungating and ulcerated non-obstructing large mass in the rectum just proximal to the anal verge. Biopsies were taken and returned positive for anorectal melanoma. General surgery and medical oncology were consulted for further management recommendations, however the patient opted for hospice care given her poor prognosis. Discussion: The natural history of development of colorectal adenocarcinoma from a precursor adenoma is estimated at around 10 to 15 years, making it highly amenable to routine screening with colonoscopy. In contrast, AM follows a much more aggressive course. Limited data exists, however, to highlight how rapidly the malignancy may grow. In our case, the patient had been reporting new and non-specific symptoms of constipation and abdominal pain for 1 year, prompting a colonoscopy just 6 months prior to presentation with overall benign findings. Despite this, the patient’s symptoms persisted, culminating in her presentation to the Emergency Department with abdominal pain and finding of advanced disease. This case highlights an aggressive progression of AM in order to show the limitations of colonoscopy in rapidly-growing colorectal cancers.