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 normal structure and function of the anus.
The anus is the opening at the lower end of the intestines.
As food is digested, it passes from the stomach to the small intestine. It then travels from the small intestine into the main part of the large intestine (called the colon). The colon absorbs water and salt 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 lower part of the large intestine, called the rectum, which is 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 opening is connected to the rectum by the anal canal. The anal canal is about an inch and a half long. It goes from the rectum to the anal verge (where the canal meets the outside skin at the anus).
The inner lining of the anal canal is the mucosa. Most anal cancers start from cells in the mucosa.
Glands and ducts (tubes leading from the glands) are found under the mucosa. The glands make mucus, which acts as a lubricating fluid. Anal cancers that start from cells in the glands are called adenocarcinomas.
The anal canal changes as it goes from the rectum to the anal verge:
- Cells above the anal canal (in the rectum) and in the part of the anal canal close to the rectum are shaped like tiny columns.
- Most cells near the middle of the anal canal are shaped like cubes and are called transitional cells. This area is called the transitional zone.
- About midway down the anal canal is the dentate line, which is where most of the anal glands empty into the anus.
- Below the dentate line are flat (squamous) cells.
- At the anal verge, the squamous cells of the lower anal canal merge with the skin just outside the anus. This skin around the anal verge (called the perianal skin or the anal margin) is also made up of squamous cells, but it also contains sweat glands and hair follicles, which are not found in the lining of the lower anal canal.
Anal cancers are often divided into 2 groups, which are sometimes treated differently:
- Cancers of the anal canal (above the anal verge)
- Cancers of the anal margin (below the anal verge)
Sometimes anal cancers extend from one area into the other, so it’s hard to know exactly where they started.
The anal canal is surrounded by a sphincter, which is a circular muscle that keeps feces from coming out until it relaxes during a bowel movement.
Many types of tumors can develop in the anus. Not all of these tumors are cancers – some are benign (non-cancerous). Some growths 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.
- Inflammatory polyps start 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 enlarged 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 (swollen veins inside the anus or rectum), but they are not truly hemorrhoids.
Anal warts: Anal warts (also called condylomas) are growths that occur just outside the anus and in the lower anal canal below the dentate line. Occasionally they can be found just above the dentate line. They are caused by infection with the human papilloma virus (HPV). Anal warts are unlikely to develop into anal cancer, but people who have had anal warts are more likely to get anal cancer (see “Potentially pre-cancerous anal conditions” below and the section “What are the risk factors for anal cancer?”).
Other benign 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: Benign tumors that develop from smooth muscle cells
- Granular cell tumors: Tumors that develop from nerve cells and are composed of cells that contain lots of tiny spots (granules)
- Hemangiomas: Tumors that start in the cells lining blood vessels
- Lipomas: Benign tumors that start from fat cells
- Schwannomas: Tumors that develop from cells that cover nerves
Potentially pre-cancerous anal conditions
Some changes in the anal mucosa are harmless in their early stages but might 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). Depending on how the cells look, AIN (or anal SIL) can be divided into 2 groups:
Low-grade AIN (sometimes called AIN1 or low-grade anal SIL): The cells in low-grade AIN resemble normal cells in many ways. Low-grade AIN often goes away without treatment. It has a low chance of turning into cancer.
High-grade AIN (sometimes called AIN2 or AIN3, or high-grade anal SIL): The cells in high-grade AIN look much more abnormal. High-grade AIN is less likely to go away without treatment and could eventually become cancer, so it needs to be watched closely. Some cases of high-grade AIN need to be treated.
Carcinoma in situ
Sometimes abnormal cells on the inner surface layer of the anus look like cancer cells but have not grown into any of the deeper layers. This condition is known as carcinoma in situ, (pronounced in SY-too), or CIS. Another name for this condition is Bowen 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 a pre-cancer but not a true cancer.
Invasive anal cancers
Different types of cancer can start in the anal region:
Squamous cell carcinomas: Most anal cancers in the United States are squamous cell carcinomas. These tumors come from the squamous cells that line most of the anal canal and the anal margin.
Squamous cell carcinomas in the anal canal have grown beyond the surface and into the deeper layers of the lining (as opposed to carcinoma in situ).
Cloacogenic carcinomas (also called basaloid or transitional cell carcinomas) are a type of squamous cell cancer. They develop in the transitional zone, also called the cloaca. These cancers look slightly different under a microscope, but they behave and are treated like other squamous cell carcinomas of the anal canal.
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 Skin Cancer: Basal and Squamous Cell.
Adenocarcinomas: A small number of anal cancers are known as adenocarcinomas. These can develop in cells that line the upper part of the anus near the rectum, or in the glands under the anal mucosa that release their secretions into the anal canal. Most anal adenocarcinomas are treated the same way as rectal carcinomas. For more information, see our document Colorectal Cancer.
Adenocarcinomas can also start in apocrine glands (a type of sweat gland of the perianal skin). 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 exposed to the 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 Skin Cancer: Basal and Squamous Cell.
Melanomas: These cancers develop from cells in the skin or anal lining that make the brown pigment called melanin. Only a very small portion of anal cancers are melanomas. Melanomas are far more common on the skin in other parts of the body. 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 anal melanomas are hard to see, most 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 (GISTs): These cancers are much more common in the stomach or small intestine, but rarely they can start in the anal region. 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 drug therapy. For more information, see our document Gastrointestinal Stromal Tumor (GIST).
The rest of this document focuses mainly on anal squamous cell carcinomas, which account for the vast majority of anal cancers.
Last Revised: 01/20/2016