Primary rectal melanoma with CD117 expression: A case report and review of the literature

Primary rectal melanoma is a rare and aggressive malignancy, often diagnosed at an advanced stage. We report a case of an 84-year-old woman presenting with rectal bleeding, found to have a lower rectal tumor extending to the anal canal with nodal involvement on imaging. Histological examination revealed an amelanotic malignant proliferation composed of epithelioid and spindle cells. Immunohistochemistry showed diffuse SOX10 and S100 expression, with focal HMB45 and Melan-A positivity. Notably, tumor cells expressed CD117, raising the differential diagnosis of gastrointestinal stromal tumor, which was excluded by negative DOG1 and CD34 staining. This case highlights a major diagnostic pitfall and underscores the importance of a comprehensive immunohistochemical panel in undifferentiated anorectal tumors.