Anal Cancer: Introduction

Approved by the Cancer.Net Editorial Board, 03/2021

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. Use the menu to see other pages. Think of that menu as a roadmap for this entire guide.

About the anus

Anal cancer begins in the anus. The anus is part of the body's gastrointestinal tract, also called the digestive system. It is the opening at the end of the large intestine, below the rectum, where bowel movements leave the body.

About precancer and anal cancer

Anal cancer starts when healthy cells in or on the anus change and grow out of control, forming a mass called a tumor.

A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body. A benign tumor means the tumor can grow but will not spread.

Often, before cancer develops, there are changes in cells that are abnormal. Researchers believe that some of these abnormal changes are the first step in a series of slow changes that can lead to cancer. Sometimes abnormal cells go away without treatment. But these abnormal cells can also become cancerous.

This phase of the disease with abnormal cell growth is called dysplasia. Dysplasia in the anus is called anal intraepithelial neoplasia (AIN) or anal squamous intraepithelial lesions (SILs).

Growths, like polyps or warts, can also occur in or around the anus. These growths can become cancerous over time. In some cases, the precancerous tissue should be removed to keep cancer from developing.

Types of anal cancer

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 25% of all anal cancers. Cloacogenic cell cancer likely starts from cells that are similar to squamous cell cancer, and it is treated similarly.

  • Adenocarcinoma begins in 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 color found in the skin or anal lining. Learn more about melanoma.

The next section in this guide is Statistics. It helps explain the number of people who are diagnosed with anal cancer and general survival rates. Use the menu to choose a different section to read in this guide.