The exact cause of anal cancer is not known, but most anal cancers seem to be linked to infection with the human papilloma virus (HPV). While HPV infection seems to be important in the development of anal cancer, the vast majority of people with HPV infections do not get anal cancer.

A great deal of research is now under way to learn how HPV might cause anal cancer. There is good evidence that HPV causes many anal squamous cell carcinomas. But the role of this virus in causing anal adenocarcinomas is less certain. More than 100 subtypes of HPV have been found. The subtype known as HPV-16 is often found in squamous cell carcinoma and is also found in some anal warts. Another type, HPV-18, is found less often. Most anal warts are caused by HPV-6 and HPV-11. Warts containing HPV-6 or HPV-11 are much less likely to become cancerous than those containing HPV-16.

HPV makes proteins called E6 and E7 that can shut down two important tumor suppressor proteins in normal cells. These proteins -- p53 and Rb -- normally work to keep cells from growing out of control. When they are not active, cells can become cancerous.

When the body is less able to fight off infections, viruses such as HPV may become more active and trigger the development of anal cancer. HIV, the virus that causes AIDS, weakens the body's immune system, as can medicines used to prevent rejection in patients with kidney, heart, liver, or other organ transplants.

Most people know that smoking is the main cause of lung cancer. But few realize that the cancer-causing chemicals in tobacco smoke can travel from the lungs to the rest of the body. Many studies have noted an increased rate of anal cancer in smokers, and the effect of smoking is especially important in people with other risk factors for anal cancer.

It is important to remember that some patients with anal cancers do not have any known risk factors and the causes of their cancers are not known.

Last Medical Review: 08/17/2009
Last Revised: 08/17/2009