Anal Melanoma and Basal Cell Cancer of the Perianal Region

Introduction

Anorectal melanoma is a rare disease with a poor prognosis. It can be found anywhere in the anal canal or distal rectum and can spread anywhere in the body. Standard chemotherapy and radiation options are not effective, and thus surgery is the primary treatment. Debate is ongoing about whether the treatment of choice is local excision or radical resection.

Epidemiology and Risk Factors

Anorectal melanoma represents approximately 0.5% of all anal malignancies and accounts for 0.3% of all melanomas, making it the second most common site for mucosal-based melanoma after the head and neck. However, mucosal melanomas represent only 1.2% of all melanomas, with cutaneous melanoma accounting for the majority. The average age at diagnosis of anorectal melanomas is approximately 60 to 70 years, with males presenting at a slightly younger age. Females are more commonly affected than males with a ratio of 1.7:1. It is estimated that 1 in 1.7 million Americans will be diagnosed with anorectal melanoma. Unlike in cutaneous melanomas, exposure to ultraviolet-B radiation and a Caucasian background are not risk factors. A review from 1999 reported an indirect link to human immunodeficiency virus disease. This link was inferred mainly from a spike in the disease in the 1980s and 1990s in young males from San Francisco. These intriguing results have not been followed up.

Pathophysiology

Anal melanoma develops from the melanocytes found in the anal transition zone. Mutations in KIT are more commonly seen in mucosal than in cutaneous melanoma, which might play a role in future adjuvant therapies.

The disease is most commonly seen in the anal canal at or below the dentate line. However, up to 35% of tumors are found in the distal rectal mucosa. Rectal lesions tend to be more advanced than their anal counterparts. Anorectal melanoma is aggressive and penetrates the muscularis in 79% of cases. Approximately 20% of patients present with lymphatic involvement and 7% to 25% present with metastases. Disseminated disease develops in 90% of patients with nodal disease. The most common site of metastatic involvement is the liver. Other common sites include the lungs, bone, and brain.