1543 Melanoma Coli: A Rare Case of Diffuse Metastatic Melanoma to the Colon Presenting as Diarrhea

Abstract

INTRODUCTION: An 84-year-old male with history of melanoma of his left ear removed six years ago with clear margins presented to the hospital with a one year history of worsening diarrhea and weight loss, but denied rectal bleeding. Labs showed anemia of chronic disease, elevated inflammatory markers, with infectious and chronic diarrhea workup negative. Upper endoscopy was unremarkable, however colonoscopy showed a large 3 cm fungating, ulcerated, infiltrative mass within the cecum with multiple masses throughout the colon (Figure 1a,b). Biopsies were taken from the multiple masses, which all showed colonic mucosa with infiltrating epithelioid and plasmacytoid cells with frequent mitotic figures (Figure 2c,d). Immunohistochemical staining was positive for melanocytic markers S100, HMB45 and Sox10 consistent with malignant melanoma (Figure 3e–g). Further molecular testing showed positive BRAF and NRAS mutations, which are reported in 13-25% of malignant melanomas. CASE DESCRIPTION/METHODS: Metastatic melanoma to the colon is rare due to the absence of melanocytes in the colon apart from the ano-rectal region. More than 95% are diagnosed post-mortem, which is. The average interval between diagnosis of primary melanoma and discovery of colonic involvement is between 5-7 years. The overall incidence of large bowel metastasis is 0.3% with a mean age at diagnosis of 57.8. Hematochezia is the most common symptom, followed by anemia, obstruction and perforation. Primary therapy is based on chemotherapy and immunotherapy, unless surgical intervention is required for obstructive symptoms or perforation. Overall prognosis remains poor with an average survival of 7 months and a 5-year overall survival of 25%. DISCUSSION: Here we present a rare case of metastatic melanoma to the colon as a cause of diarrhea and anemia. This should always be considered in any patient with a remote history of melanoma.