Primary anorectal malignant melanoma: an uncommon anorectal pathology.

Anorectal malignant melanoma (AMM) is most common primary melanoma of gastrointestinal tract, accounting for 0.05% and 1% of all colorectal and anal cancers. We reported an 85 year-old woman with no significant past medical history who presented two-month period of rectal bleeding, abdominal pain, tenesmus and 2kg weight-loss. Laboratory markers were unremarkable, although rectal examination revealed two small haemorrhoids and a firm, non-obstructing mass in the lower rectum. Colonoscopy confirmed presence of an ulcerated pigmented neoplasm arising at dental line [A,B]. No distant metastases were found on computed tomography [C] although presented metastatic regional lymph nodes on pelvic MRI [D]. Therefore, abdominoperineal resection was performed, confirming loco-regional disease. Histopathology showed malignant melanoma with positive stains in immunohistochemistry for protein S100, HMB-45 and Melan-A [E,F,G,H] and stained negative for c-Kit.