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 parts 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 and how it works.
The anus is the opening at the lower end of the intestines. It’s where the end of the intestines connect to the outside of the body.
As food is digested, it passes from the stomach to the small intestine. It then moves 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’s left after going through the colon is known as feces or stool. Stool is stored in the last part of the large intestine, called the rectum. From there, stool is passed out of the body through the anus as a bowel movement. The anal sphincter (SFINK-ter) muscles control the passing of stool. These are ring-shaped muscles around the anus that keep stool from coming out until they are relaxed during a bowel movement.
The anal opening is connected to the rectum by the anal canal. The anal canal is about 1-1/2 inches long. It goes from the rectum to the anal verge. This is where the canal connects to 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 2 sphincter muscles that keep the anus closed and prevent stool leakage.
Many types of tumors can develop in the anus. Not all of these tumors are cancers – some are benign (not cancer).
Polyps are small, bumpy, or mushroom-like growths that form in the mucosa or just under it. There are many kinds.
- Inflammatory polyps start because of irritation from injury or infection.
- Lymphoid polyps are caused by an overgrowth of lymph tissue (which is part of the immune system). Lymph tissue under the anal inner lining (mucosa) is normal, but these overgrowths are not.
- 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 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’re not the same.
Anal warts (also called condylomas) are growths that form just outside the anus and in the lower anal canal below the dentate line. Sometimes they can be found just above the dentate line. They’re caused by infection with human papilloma virus (HPV). People who have or had anal warts are more likely to get anal cancer. (See “Potentially pre-cancerous anal conditions” below and 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 at first, but might later develop into a 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.
Dysplasia in cells of the anus is also called anal intraepithelial neoplasia (AIN)or 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 look like 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, with time, could become cancer. It needs to be watched closely. Some cases of high-grade AIN need to be treated.
Types of anal cancer
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 is known as carcinoma in situ, (pronounced in SY-too), or CIS. Another name for this is Bowen disease.
Some doctors see this as the earliest form of anal cancer. Others consider it the most advanced type of AIN, which is a pre-cancer (see above), but not a true cancer.
Invasive anal cancers
These are the different types of cancer that can start in the anal region:
Squamous cell carcinomas
Most anal cancers in the United States are squamous cell carcinomas. (Nearly 9 out of 10 cases.) These tumors start in 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 which is only in the surface cells).
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 much like squamous cell carcinomas of the skin elsewhere. For more on this, see Skin Cancer: Basal and Squamous Cell.
A small number of anal cancers are known as adenocarcinomas. These start in cells that line the upper part of the anus near the rectum. They can also start in the glands under the anal mucosa that release secretions into the anal canal. Most anal adenocarcinomas are treated the same as rectal carcinomas. For more information, see 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 should not be confused with Paget’s disease of the bone , which is not cancer and 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 very few anal cancers. They are often treated with surgery to remove the cancer. For more information, see Skin Cancer: Basal and Squamous Cell.
These cancers start in 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 on this, see 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 Gastrointestinal Stromal Tumor (GIST).
The anal cancer information on our web site focuses mainly on anal squamous cell carcinoma, which is, by far, the most common type of anal cancer.
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