Anal Cancer

Most (nearly 9 out of 10 cases) anal cancers in the United States are squamous cell cancer. These tumors start in the squamous cells that line most of the anal canal and the anal margin.

Squamous cell cancers in the anal canal have grown beyond the surface and into the deeper layers of the lining.

Squamous cell cancers of the anal margin (perianal skin) can be treated as an anal cancer or like squamous cell cancer of the skin. It is important to see a specialist if you are diagnosed with perianal cancer to determine the best course of treatment.

The information on these pages focuses mainly on anal squamous cell carcinoma since it is the most common type of anal cancer.

Adenocarcinoma

A small number of anal cancers are adenocarcinomas. These cancers 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 disease is a type of apocrine gland carcinoma that spreads through the surface layer of the skin. Paget 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 disease of the bone, which is a different disease and is not cancer.

Basal cell carcinoma

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. Very few anal cancers are basal cell carcinomas. They are often treated with surgery to remove the cancer. For more information, see Skin Cancer: Basal and Squamous Cell.

Melanoma

These cancers start in cells in the skin or anal lining that make the brown pigment called melanin. Only a very small number of anal cancers are melanomas. Melanomas are far more common on the skin of 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. Sometimes, an abdominoperineal resection (APR) might be recommended.  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 tumor (GIST)

These cancers are much more common in the stomach or small intestine and rarely 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).

Lymphoma

Anal lymphoma is a very rare cancer. It starts in the lymph tissue of the anus and is a type of non-Hodgkin lymphomas, which are cancers of the immune system.  People with weak immune systems, such as those with HIV, have a higher risk of getting anal lymphoma. It can look like an infection near the anus, so a tissue sample (biopsy) needs to be taken to make a diagnosis. Treatment usually includes chemotherapy and sometimes radiation.

Some changes in the anal mucosa are harmless at first but later might turn into cancer. These are called precancers. Precancers might also be called dysplasia. Some warts, for example, contain areas of dysplasia that can develop into cancer.

Dysplasia in cells of the anus is called anal intraepithelial neoplasia (AIN) or anal squamous intraepithelial lesions (SILs). AIN is linked to the human papillomavirus (HPV). It is found more often in people with high risk for acquiring HPV, such people with HIV, men who have sex with men, and people who have undergone an organ transplant. 

Depending on how the cells look, AIN or anal SIL can be divided into 2 groups:

• Low-grade SIL (or grade 1 AIN): The cells in low-grade SIL look like normal cells. Low-grade SIL often goes away without treatment and has a small chance of turning into cancer.
• High-grade SIL (or grade 2 AIN or grade 3 AIN): The cells in high-grade SIL look abnormal. High-grade SIL is less likely to go away without treatment and, over time, could become cancer. It needs to be watched closely and some cases of high-grade SIL need to be treated.

Many types of tumors can develop in the anus. Not all of them are cancers. Some are benign (not cancer).

Polyps

Polyps are small, bumpy, or mushroom-like growths that form in the mucosa or just under it. There are many kinds including inflammatory polyps, lymphoid polyps and 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're not the same.

Anal warts

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 papillomavirus (HPV). People who have or had anal warts are more likely to get anal cancer. (See “Anal pre-cancers” above and Risk Factors for Anal Cancer.)

Other benign tumors

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

• 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)
• Lipomas: Benign tumors that start from fat cells