ON THIS PAGE: You will find some basic information about this disease and the parts of the body it may affect. This is the first page of Cancer.Net’s Guide to Anal Cancer. To see other pages, use the colored boxes on the right side of your screen. Think of those boxes as a roadmap to this full guide. Or, click “Next” at the bottom of each page.
The anus is part of the gastrointestinal tract. It is the opening at the end of the large intestine, below the rectum, where bowel movements leave the body. Anal cancer begins when normal cells in or on the anus change and grow uncontrollably, forming a mass called a tumor. A tumor of the anus can be benign (noncancerous) or malignant (cancerous, meaning it can spread to other areas 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.
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