Malignant melanoma is most commonly found on the skin and rarely occurs in the rectal region. This case illustrates that rectal melanoma can be misdiagnosed as hemorrhoids. It also aims to add knowledge to possible treatment options for rectal melanoma.

We report a case of a 77-year-old Filipino adult presenting with rectal bleeding for three weeks. He underwent sigmoidoscopy that showed thrombosed hemorrhoids; however, subsequent surgical excision biopsy histopathology and immunohistochemistry revealed features compatible with malignant melanoma (HMB45, Melan A, and Cytokeratin positive; CDX2 negative). Staging workup done, including abdominal magnetic resonance imaging (MRI) with IV contrast and chest computed tomography (CT), showed distant metastases. He was then started on pembrolizumab but follow up imaging showed recurrence of the rectal melanoma and progression of metastases. Molecular testing done revealed c KIT/ CD117 positive results, hence, treatment was shifted to imatinib.

It was seen that rectal melanoma is an aggressive disease; therefore, multidisciplinary management is crucial to yield the best possible outcome, despite its poor prognosis. Such as in this case, using immunotherapy (Pembrolizumab) and targeted therapy (Imatinib) still have inconsistent outcomes, thus, further studies should be pursued. In this patient, both pembrolizumab and imatinib post-surgery resulted to recurrence of the rectal tumor and progression of hepatic and osseous metastases.