Melanoma arises from the melanocyte, a pigmented dendritic-like cell located in the base of the epidermis, the eye, the epithelia of the nasal cavity, oropharynx, anus, vagina, and urinary tract.3 Approximately 90% of melanomas are cutaneous and associated with ultraviolet radiation exposure. The remaining are non-cutaneous melanomas consisting of uveal and mucosal melanoma.3 Mucosal melanomas arise from the mucosa of the head and neck (55%), anorectum (24%), and vagina (18%).1

Anorectal mucosal melanoma (ARMM) is an extremely rare tumor with a 5-year overall survival (OS) estimated between 10-20%.3, 4, 5, 6 Unlike cutaneous melanomas, sun exposure is not a risk factor for mucosal melanomas. Human immunodeficiency virus may be a risk factor but otherwise the risk factors for anorectal melanoma are poorly understood.1,6,9 The incidence of ARMM increases with age with over 50% of mucosal melanomas presenting in the 8th decade of life. The median age of presentation for anal melanoma is 55 years with the range being wide, 29 to 91 years old.7,8 ARMM is seen more frequently in women, however women are more likely to undergo a perineal examination potentially confounding the observed female predilection.9 Additionally, the incidence of ARMM is 1.7-fold higher in Caucasians than African Americans.10,11

The anorectum consists of the distal rectum, the transitional zone at the pectinate line, and the anal canal. Most (65%) ARMMs are found in the anal canal or transition zone, with the other 35% arising from the distal rectum.12 Patients present with bleeding, discomfort, change in bowel habits, and/or an anal mass. Diagnosis is often delayed due to location, nonspecific symptoms, and the high rate of amelanotic (up to 20%) tumors.9,13 In a study of 142 patients, 60% presented with localized disease, 19% had regional lymph node involvement, and 20% had distant disease at the time of diagnosis.4 The presence of melanin pigment, junctional changes, atypical epidermoid cells or pleomorphic spindle cells adjacent to the tumor focus aid the diagnosis.13 In scenarios of amelanotic tumors or those lacking junctional changes, histologic markers (S-100, melanoma antigen HMB-45, vimentin, cytokeratin, epithelial membrane antigen, and carcinoembryonic antigen) aid in the diagnosis.5,14, 15, 16