Overview

Anal cancer begins when normal cells in or on the anus begin to 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). There are also growths that can occur in or around the anus, such as polyps or warts. While these growths are not cancerous, some may become cancerous over time.

The anus is part of the gastrointestinal tract and is the opening at the end of the large intestine, below the rectum, where bowel movements leave the body. 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 cancer arises between the outer part of the anus and the lower part of the rectum. Cloacogenic cell cancer likely starts from similar cells as squamous cell cancer and 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 in the skin or anal lining that darken the skin.

Find out more about basic cancer terms used in this section.