Colorectal Cancer Association of Canada - CCAC

Anal Cancer > What is Anal Cancer  

Understanding Anal Cancer

The Anus

The anus is the opening at the end of the large intestine through which stool (solid waste) exits the body. It is formed partly from the outer skin layers of the body and partly from the intestine. Two ring-like muscles, called sphincter muscles, open and close the anal opening to let stool pass out of the body. The anal canal connects the rectum to the anal opening and the outside of the body.

The anal canal and structures. Image courtesy of the Canadian Cancer Society (

Anal Cancer

Anal cancer is a disease in which malignant (cancer) cells form in the tissues of the anus. Following a diagnosis, tests are performed to determine if cancer cells have spread within the anus or to other parts of the body.

At first, the changes in a cell are abnormal, not cancerous. Researchers believe, however, that some of these abnormal changes are the first step in a series of slow changes that can lead to cancer. Some of the abnormal cells go away without treatment, but others can become cancerous. This phase of the disease is called dysplasia (an abnormal growth of cells). Dysplasia in the anus is called anal intraepithelial neoplasia (AIN) or anal squamous intraepithelial lesions (SILs). Growths—such as polyps or warts—that are not cancerous can also occur in or around the anus; some may become cancerous over time. In some cases, the precancerous tissue needs to be removed to keep cancer from developing. The anus is made up of different types of cells, and each type can become cancerous. There are several different types of anal cancer based on the type of cell where the cancer began:

  • Squamous cell carcinoma is the most common type of anal cancer. This cancer begins in the outer lining of the anal canal.
  • Cloacogenic carcinoma accounts for about one-quarter of all anal cancers. This type of cancer arises between the outer part of the anus and the lower part of the rectum. Cloacogenic cell cancer likely starts from cells that are similar to squamous cell cancer, and it is treated similarly.
  • Adenocarcinoma arises from the glands that make mucous located under the anal lining.
  • Basal cell carcinoma is a type of skin cancer that can appear in the perianal (around the anus) skin.
  • Melanoma begins in cells that produce pigment (color), found in the skin or anal lining.
  • Gastrointestinal stromal tumors 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.

Staging Anal Cancer

Staging describes the extent of the cancer based on how much of the anus is affected, and if there is spread to other organs. It is important to know the stage of the disease in order to plan the most appropriate treatment.

The following are used for the staging of squamous cell anal cancer:

Stage 0 (Carcinoma in Situ) - abnormal cells are found in the innermost lining of the anus. These abnormal cells may become cancer cells and spread into nearby normal tissue.

Stage I - cancer has formed. Tumour is 2 centimeters or smaller in size.

Stage II - tumour is larger than 2 centimeters.

Stage IIIA - tumour can be any size, and has spread to either the lymph nodes near the rectum, or into nearby organs, such as the vagina, urethra, and bladder.

Stage IIIB - tumour can be any size, and has spread 1) into nearby organs and to lymph nodes near the rectum, or 2) to lymph nodes on one side of the pelvis and/or groin, and may have spread to nearby organs, or 3) to lymph nodes near the rectum and in the groin, and/or to lymph nodes on both sides of the pelvis and/or groin, and may have spread into nearby organs.

Stage IV - tumour can be any size and cancer may have spread to lymph nodes or nearby organs, and has spread to more distant organs or tissues.

Pea, peanut, walnut, and lime show tumor sizes. Image courtesy of the National Cancer Institute ( ).

Source: National Cancer Institute ( American Cancer Society Cancer Net  Download Adobe Reader- PDF File Reader - Free