Abstract
Melanoma is a rare tumor in Saudi population. Therefore, diagnosing melanoma among the Saudi population in unexpected location is difficult without a high index of suspicion and the help of laboratory investigations, including ancillary immunohistochemical (IHC) studies. Here, we present the first reported case of anorectal melanoma in the central region of Saudi Arabia and the second reported case in the whole country. This is a report of a 59-year-old Saudi female patient who showed an abdominal large rectal mass. Urgent colonoscopy examination revealed a large fungating rectal mass with the ulcerated surface. The microscopic examination of the colonoscopy biopsies showed colonic mucosa with a lamina propria infiltrated by pleomorphic atypical round/oval neoplastic cells arranged in sheets, clusters, and trabecular patterns. The IHC stains showed that the neoplastic cells were positive for S100 and Melan-A, but negative for CKpan, CK7, CK20, CDX2, CD45, DOG1, chromogranin, and synaptophysin. The overall workup revealed a diagnosis of the primary anorectal melanoma.