Primary anorectal melanoma-a rare entity: case report

Abstract

Anorectal melanoma is a rare disorder. There have only been a few cases reported and there is no consensus of treatment. We report a case of 50-year-old Caucasian man presenting intermittent prolapse of an anorectal mass during 6 months with occasional bleeding. Biopsies came up with the diagnosis of malignant melanoma. No distant metastases were found. He underwent an abdominoperineal resection due to internal sphincter invasion. A second tumour was diagnosed in final histologic examination: a dysplastic rectal polyp invaded by the melanoma (collision tumour). At 12 months of follow up he presented loco-regional recurrence (a single pelvic lymph node) and hepatic metastases. He was included in a study protocol comparing new medical treatments (nivolumab versus ipilimumab or both).

Figure 1:

(A) T2-weighted axial, coronal and transverse MRI images, demonstrating the polypoid 3 cm anorectal mass. (B) PET–CT showed an intensely hypermetabolic lesion centered on the lower rectum.

Figure 2:

Histologic illustration of the collision tumour.