What is anal cancer?
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 the passage of stool (feces) during a bowel movement. It is lined with cells that are like the cells lining the bladder, vagina, urethra, and other places in the body. These cells are called squamous cells.
Many kinds of tumors can grow in the anus. Not all of these tumors are cancers -- some are benign, which means they are not cancer. There are also some growths that start off 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 and skin tags (small pieces of skin that hang loose from the body). To find out more about benign anal tumors, please see our 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, (pronounced “in SY-too”), or CIS. It may also be called Bowen disease. Some doctors think this is the early form of anal cancer and others think it is a pre-cancer but not a true cancer.
Invasive anal cancers
Squamous cell carcinomas
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 the lower part of the anus and most of the anal canal. In its earliest stages it is known as carcinoma in situ or CIS. If it has spread beyond the surface cells to the deeper layers it is called invasive.
If the tumors are found in the skin around the anus (perianal skin) they are treated like the squamous cell carcinomas of the skin found elsewhere in the body. To learn more, see our document, Skin Cancer: Basal and Squamous Cell.
Cloacogenic carcinoma is a type of squamous cell cancer that starts in an area of the anus called the cloaca. It is sometimes seen as a sub-type of squamous cell cancer, but is treated the same.
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. Paget disease is a type of adenocarcinoma that spreads through the surface layer of skin and can affect the anal area. (This should not be confused with Paget disease of the bone which is a very different disease.)
Most adenocarcinomas start in the rectum and are treated as rectal carcinomas. For more information on this, see our document, Colorectal Cancer.
A small percentage of anal cancers are basal cell carcinomas, a type of skin cancer. Another 1%-2% are malignant melanomas, another type 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
These are rare anal cancers that are more often found in the stomach or small intestine. When these 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).
Last Medical Review: 01/14/2013
Last Revised: 01/17/2013