Anal cancer is a type of cancer that forms in the tissues of the anus.
The anus is the end of the large intestine. It is where stool (solid waste) leaves the body. The anus is formed partly from the outer skin layers of the body and partly from the intestine. The anus is connected to the rectum by the anal canal, which is about 1 to 1½ inches long. This area is controlled by two ring-like sphincter muscles, which contract to hold stool in and relax to allow its passage out of the body.
Anal cancer can start in the lining of the anal canal, called the mucosa, or in the perianal skin, the squamous cells outside of the anus that contain hair follicles and sweat glands.
Tumors of the perianal skin that do not involve the anal sphincter are usually treated the same as anal cancers, although local therapy (treatment directed to a limited area of skin) may be used for some.
Squamous cell carcinoma is the most common type of anal cancer. In the United States, squamous cell carcinoma is the most common type of anal cancer. Studies show that human papillomavirus (HPV) infection is the main cause of this type of anal cancer.
Another type of anal cancer, called anal adenocarcinoma, is very rare and is not discussed in this summary.