EP1048 - Mucosal Melanoma of the Anal Canal: A Poor Prognosis.

Purpose/Background: Skin cancer is the most commonly diagnosed neoplasm worldwide, with melanoma accounting for a significant portion of skin cancer-related mortality. Mucosal melanoma is a rare subtype of melanoma arising from the mucosal surfaces - anorectal mucosal melanoma (AMM), originating from the anal canal and rectum. Unlike cutaneous melanoma, there are no strong environmental risk factors for the development of AMM including ultraviolet rays. AMM has been found to be more aggressive than cutaneous melanoma, due to worse prognosis and higher rates of metastasis. We will discuss a 60- year-old female patient who presented with new fecal incontinence, intermittent anal bleeding and biopsy proven anal melanoma lesion to the anterior midline region of the anus.

Methods/Interventions: Biopsy with immunohistochemical staining was obtained. A colonoscopy revealed a mobile, pedunculated mass at the anterior anal verge. She was presented at multidisciplinary tumor board. She underwent PET-CT scanning without evidence of metastatic disease. Staging MRI imaging of the brain and pelvis negative for distant metastases. Operative gross tumor excision was performed; surgical pathology demonstrated invasive malignant melanoma and associated melanoma in situ involving the anal skin. Immunohistochemical staining and genetic testing was done- patient's pathology was determined to be Stage 1. Patient underwent post operative radiation therapy and was started on immunotherapy, adjuvant nivolumab. Unfortunately, follow up PET scan displayed metastatic disease to bilateral inguinal lymph nodes. Patient was started on ipilimumab 1mg/kg and nivolumab 3mg/kg times two cycles.

Results/Outcomes: Patient appears to be responding appropriately with most recent PET scan displaying reduced anal uptake and stable inguinal lymphadenopathy. Patient is to continue immunotherapy and further PET scan follow-up Fall 2025.

Conclusion/Discussion: AMM can be a difficult diagnosis to make, as presenting symptoms can be easily mistaken for benign conditions such as polyps or hemorrhoids. Current recommendations support biopsy confirmation of AMM, followed by staging MRI in addition to proctoscopy ± flexible sigmoidoscopy. Stage I is diagnosed when disease is only present locally, stage II includes regional lymph node involvement, and stage III includes distant metastatic disease. There is no consensus strategy on the management of mucosal melanoma. For AMM, surgery is the most effective treatment, but the manner of surgical approach is a topic of debate. Following surgical excision, mutation and cell surface receptor expression-targeted adjuvant chemotherapy, as well as radiotherapy (RT), can be considered for AMM, though efficacy of these interventions is debated as recurrence rates are high. Ultimately, prognosis associated with AMM is poor, with AMM having the worst prognosis of all mucosal melanomas, with a five-year survival rates estimated to range between 0-25%.