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Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer?

Anal cancer starts in the anus. To understand anal cancer, it helps to know about the anus.

The anus

The anus is the body’s opening at the lower end of the intestines. The anus is about an inch and a half long and connects the lower part of the large intestine to the outside of the body. It opens to allow stool (feces) to leave the body during a bowel movement.

Much of the inner wall of the anus is lined with cells called squamous cells. Most anal cancers start in these cells. But there are other types of cells here as well, and sometimes cancers start in these other cells.

Many kinds of tumors can grow in the anus.

Benign anal tumors (not cancer)

Not all anal tumors are cancers – some are benign, which means they are not cancer. There are also some growths that start as benign but over time can change into cancer. These are called pre-cancerous conditions. A common term for these potentially pre-cancerous conditions is dysplasia. Some warts, for example, contain areas of dysplasia that can develop into cancer. Benign tumors include some kinds of warts, polyps, skin tags (small pieces of skin that hang loose from the body), and others. To find out more about benign anal tumors, see our detailed document Anal Cancer.

Anal tumors that are cancer

Carcinoma in situ

Sometimes cells on the surface layer of the anus look like cancer cells but have not grown into any of the deeper layers. This is known as carcinoma in situ (CIS). It may also be called Bowen disease. Some doctors think this is the earliest form of anal cancer and others think it is a pre-cancer but not a true cancer.

Invasive anal cancers

Different types of cancer can start in the anal region:

Squamous cell cancers: These are the most common type of anal cancer and are the focus of this document. The tumors begin in the squamous cells that line much of the inside of the anus.

If the tumors are found in the skin around the anus (perianal skin) they are treated like the squamous cell cancers of the skin found elsewhere in the body. To learn more, see our document Skin Cancer: Basal and Squamous Cell.

Adenocarcinomas: A small number of anal cancers start in cells that line the upper part of the anus near the rectum or the glands found in the anal area. These cancers are called adenocarcinomas.

Most anal adenocarcinomas are treated the same way as rectal carcinomas. For more information on this, see our document Colorectal Cancer.

Skin cancers: A small percentage of anal cancers are basal cell cancers or melanomas, which are other types of skin cancer. Melanomas are far more common on parts of the body that are exposed to the sun. Most anal melanomas are hard to see and are found at a late stage. To learn more, please see our documents Skin Cancer: Basal and Squamous Cell and Melanoma Skin Cancer.

Gastrointestinal stromal tumors (GISTs): These cancers are more often found in the stomach or small intestine, but rarely, they can start in the anus. When these tumors are found at an early stage, they are removed with surgery. If they have spread beyond the anus, they can be treated with drugs. To learn more, see our document Gastrointestinal Stromal Tumor (GIST).

The rest of this document focuses mainly on squamous cell cancers, which account for most anal cancers.

Last Medical Review: 06/10/2014
Last Revised: 01/20/2016