The anus

The anus is the body's opening at the lower end of the intestines. The anal canal is the tube that connects the lower part of the large intestine (rectum) to the anus and the outside of the body. As food is digested, it passes from the stomach to the small intestine. It then travels from the small intestine into the large intestine (colon). The colon absorbs water and liquid from the digested food. The waste matter that is left after going through the colon is known as feces or stool. Feces are stored in the rectum, the final 6 inches of the digestive system. From there, they pass out of the body through the anus as a bowel movement.

The anal canal is about an inch and a half long. Its inner lining (called the mucosa) is made up of several different kinds of cells. Learning a little about these cells is helpful in understanding the kinds of cancer that develop in various parts of the anal canal. Glands and ducts (tubes leading from the glands) are found under the mucosa. These glands make mucus, which acts as a lubricating fluid.

The anal canal goes from the rectum to the anal margin (where the canal meets the outside skin at the anus). About midway down the anal canal is the dentate line, which is where most of these anal glands empty into the anus.

Cells above the anal canal (in the rectum) are mainly shaped like tiny columns, while most of those in the upper anal canal just above the dentate line are shaped like cubes and are called transitional cells. This area is called the transitional zone. Below the dentate line are flat (squamous) cells. The anal margin (also called the anal verge) is lined by squamous cells that merge with the skin just outside the anus (called perianal skin). The perianal skin is also made up of squamous cells, but it also contains sweat glands and hair follicles, while the lining of the anal canal does not. The place where the perianal skin (which has squamous cells plus hair follicles and sweat glands) meets the anal canal (which has squamous cells without hair follicles and sweat glands) is called the anal margin or anal verge. Cancers of the anal canal (above the anal verge) and cancers of the anal margin (below the anal verge) are treated very differently.

The anal canal is surrounded by a sphincter, which is a circular muscle that keeps feces from coming out until it is relaxed during a bowel movement.

Anal tumors

Many types of tumors can develop in the anus. Not all of these tumors are cancers -- some are benign (non-cancerous). There are also some growths that start off as benign but over time can develop into cancer. These are called pre-cancerous conditions. This section discusses all of these types of abnormal growths.

Benign (non-cancerous) anal tumors

Polyps: Polyps are small, bumpy, or mushroom-like growths that develop in the mucosa or just under it. There are several kinds, depending on their cause and location.

• inflammatory polyps arise because of inflammation from injury or infection.
• lymphoid polyps are caused by an overgrowth of lymph tissue (which is part of the immune system). Small nodules of lymph tissue are normally present under the anal inner lining.
• hypertrophied anal papillae are benign growths of connective tissue that are covered by squamous cells. They are simply an enlargement of the normal papillae, which are small folds of mucosa found at the dentate line. Hypertrophied anal papillae are also called fibroepithelial polyps.

Skin tags: Skin tags are benign growths of connective tissue that are covered by squamous cells. Skin tags are often mistaken for hemorrhoids but they are not truly hemorrhoids.

Condylomas: Condylomas (also called warts) are growths that occur just outside the anus and in the lower anal canal below the dentate line. Occasionally condylomas can be found just above the dentate line. They are caused by infection with the human papilloma virus (HPV). People who have had condylomas are more likely to develop anal cancer (see "Potentially pre-cancerous anal conditions" and the section "What are the risk factors for anal cancer?").

Other tumors: In rare cases, benign tumors can grow in other tissues of the anus. These include:

• adnexal tumors -- usually benign growths that start in hair follicles or sweat glands of the skin just outside of the anus. These tumors stay in the perianal skin area and do not grow into the anal region.

• leiomyomas -- develop from smooth muscle cells

• granular cell tumors -- develop from nerve cells and are composed of cells that contain lots of tiny spots (granules)

• hemangiomas -- start in the lining cells of blood vessels

• lipomas -- start from fat cells

• schwannomas -- develop from cells that cover nerves

Potentially pre-cancerous anal conditions

Some changes in the anal mucosa are harmless in their early stages but may later develop into a cancer. A common term for these potentially pre-cancerous conditions is dysplasia. Some warts, for example, contain areas of dysplasia that can develop into cancer.

Dysplasia occurring in the anus is also known as anal intraepithelial neoplasia (AIN) and as anal squamous intraepithelial lesions (SILs). AIN (or anal SIL) is divided into low-grade and high-grade depending on how the cells look under the microscope. The cells in low-grade AIN resemble normal cells while the cells in high-grade AIN look much more abnormal. Low-grade AIN often goes away without treatment. It has a low chance of turning into cancer. High-grade AIN is less likely to go away without treatment. Left untreated, high-grade AIN may eventually become cancer, and therefore high-grade AIN requires very close observation. Some cases of high-grade AIN need to be treated.

Carcinoma in situ

Sometimes abnormal cells on the surface layer of the anus look like cancer cells but have not grown into any of the deeper layers. This situation is known as carcinoma in situ, (pronounced "in SY-too"), or CIS. Another name for this condition is Bowen's disease. Some doctors view this as the earliest form of anal cancer and others consider it the most advanced type of AIN, which is considered precancerous but not a true cancer.

Invasive anal cancers

Squamous cell carcinomas: Most anal cancers in the United States are squamous cell carcinomas. These tumors come from the squamous cells that line the anal margin and most of the anal canal.

Cells of invasive squamous cell carcinomas have already spread beyond the surface to the deeper layers of the lining. Squamous cell carcinomas of the anal canal are discussed in detail in this document. Squamous cell carcinomas of the anal margin (perianal skin) are treated similarly to squamous cell carcinomas of the skin elsewhere in the body. For more information, see our document, Basal and Squamous Cell Skin Cancer.

Cloacogenic carcinomas (also called basaloid or transitional cell carcinomas) are sometimes listed as a subclass of squamous cell cancers. They develop in the transitional zone, also called the cloaca. While they look slightly different under the microscope, these cancers behave and are treated like other squamous cell carcinomas of the anal canal.

Adenocarcinomas: A small number of anal cancers are known as adenocarcinomas, which develop in glands located under the anal mucosa that release their secretions into the anal canal. They can also start in apocrine glands (a type of sweat gland of the perianal skin). Most often they arise from the rectum and are treated as rectal carcinomas. For more information, see our document, Colorectal Cancer.

Paget's disease is a type of apocrine gland carcinoma that spreads through the surface layer of the skin. Paget's disease can affect skin anywhere in the body but most often affects skin of the perianal area, vulva, or breast. This condition should not be confused with Paget's disease of the bone, which is a different disease.

Basal cell carcinomas: Basal cell carcinomas are a type of skin cancer that can develop in the perianal skin. These tumors are much more common in areas of skin that are exposed to sun, such as the face and hands, and account for only a small number of anal cancers. They are often treated with surgery to remove the cancer. For more information, see our document, Basal and Squamous Cell Skin Cancer.

Malignant melanoma: This cancer develops from cells in the skin or anal lining that make the brown pigment called melanin. Only about 1% to 2% of anal cancers are melanomas. Melanomas are far more common on parts of the body that are exposed to sun. If melanomas are found at an early stage (before they have grown deeply into the skin or spread to lymph nodes) they can be removed with surgery and the outlook for long-term survival is very good. But because they are hard to see, most anal melanomas are found at a later stage. If possible, the entire tumor is removed with surgery. If all of the tumor can be removed, a cure is possible. If the melanoma has spread too far to be removed completely, other treatments may be given. For more information, see our document, Melanoma Skin Cancer.

Gastrointestinal stromal tumors: These are rare anal cancers that are much more commonly 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 drug therapy. For more information, see our document, Gastrointestinal Stromal Tumor.

Last Medical Review: 08/17/2009
Last Revised: 08/17/2009