Incidence

Epidermoid cancer of the anus includes the anal canal and perianal skin, i.e., tumors within a radius of approximately 5 cm from the anal orifice. It is a relatively rare disease. In the US, approximately 2000 new cases are registered annually. It comprises about 3% of all colorectal malignancies. There are two peak incidences; one at 40–50 years and another at 70–80 years. Industrialized countries appear to show an age adjusted incidence of < 1 per 100 000. Cancers of the anal canal are approximately three times more common in females than in males. In contrast, carcinomas in the area of the anal margin are slightly more common in males. There is a slight female preponderance overall. A viral etiology has been suggested since 50% of cancers show evidence of human papilloma virus type 16 and 18, similar to carcinoma of the cervix. For this reason receptive homosexual intercourse is accepted as a risk factor and the incidence of anal intraepithelial neoplasia (AIN) is also higher in homosexual activity. Even before the AIDS epidemic an excess risk of 40–50 times was observed in the homosexual population for anal cancer. Immunosuppression is a further important risk factor, and anal cancer is not uncommon in renal and cardiac transplant recipients. Other recognized risk factors include previous pelvic radiotherapy and cigarette smoking.

Carcinoma of the anus usually runs an indolent course with a low rate of distant metastases.