We reviewed 117 cases of anorectal melanoma to better define epidemiologic and survival characteristics of this rare neoplasm.

The National Cancer Institute Surveillance, Epidemiology, and End Results database covering the period 1973 through 1992 was used. This represents 9.5 percent of the United States population. Melanoma arising in the anorectum was identified using International Classification of Diseases for Oncology codes. Two-tailed Student's t-test, chi-squared, and Wilcoxon's tests were used for comparisons of means, proportions, and actuarial survival rates, respectively.

One hundred seventeen cases of anorectal melanoma were identified, representing 0.048 percent of all colorectal malignancies in the database. The male-to-female ratio was 1:1.72. The mean age was 66 +/- 16 years. Mean age by gender, however, was lower for males (57 years) then for females (71 years; P < 0.001). The age difference represents an increased incidence of anorectal melanoma in males younger than the age of 45 years. Furthermore, the incidence of anorectal melanoma in young males ages between 25 to 44 years tripled in the San Francisco area when compared with all other locations (14.4 vs. 4.8 per 10 million population; P = 0.06). Males have a survival advantage over females (62.8 percent vs. 51.4 percent 1-year and 40.6 percent vs. 27.7 percent 2-year; P < 0.01).

The overall incidence of anorectal melanoma continues to rise and survival rates remain poor. A new trend toward bimodal age distribution was observed. There is indirect evidence that implicates human immunodeficiency virus infection as a risk factor. Survival rate is better in young patients aged 25 to 44 years.