Anal cancer is a disease in which malignant (cancer) cells form in the tissues of the anus.

The anus is the end of the large intestine, below the rectum, through which stool (solid waste) leaves the body. The anus is formed partly from the outer skin layers of the body and partly from the intestine. Two ring-like muscles, called sphincter muscles, open and close the anal opening and let stool pass out of the body. The anal canal, the part of the anus between the rectum and the anal opening, is about 1-1 ½ inches long.

Anatomy of the lower digestive system, showing the colon and other organs.

The skin around the outside of the anus is called the perianal area. Tumors in this area are skin tumors, not anal cancer.

Risk factors include the following:

• Being infected with human papillomavirus (HPV).
• Having many sexual partners.
• Having receptive anal intercourse (anal sex).
• Being older than 50 years.
• Frequent anal redness, swelling, and soreness.
• Having anal fistulas (abnormal openings).
• Smoking cigarettes.