What Causes Anal Cancer?

In general, cancer begins when a genetic mutation (change in certain genes) turns normal healthy cells into abnormal cells. Healthy cells grow and multiply at a set rate and eventually die at a set time. Abnormal cells grow and multiply out of control, and they do not die. The abnormal cells clump together to create a mass (tumor). Cancer cells invade nearby tissues and can separate from an initial tumor to spread to other parts of the body (metastasize). Doctors do not know what causes the genetic mutation that triggers the development of anal cancer.

The exact cause of most anal cancers is not known, but many doctors believe that infection with the human papillomavirus (HPV) causes many cases of anal cancer. Good evidence shows that HPV causes many squamous cell anal carcinomas and cloacogenic anal carcinomas. But researchers are less sure that this virus causes anal adenocarcinomas.

Scientists have found more than 100 types of HPV. One type, HPV-16, is often found in anal squamous cell carcinoma and some anal warts.

Some scientists also believe that human immunodeficiency virus (HIV), which causes AIDS, plays a role in anal cancer. This virus weakens the body's immune system, which normally helps fight infections. In people with HIV, viruses such as HPV can become more active and trigger the development of anal cancer.

Smoking seems to play a role in the development of anal cancer. The lungs absorb the cancer-causing chemicals in tobacco smoke and can spread these chemicals through the bloodstream to the rest of the body. Many studies have found an increased rate of anal cancer in smokers. 

Risk Factors for Anal Cancer 

Doctors and researchers believe that certain factors, known as risk factors, increase a person’s chances of getting anal cancer. However, not everyone with a risk factor will develop anal cancer and people without any risk factors can develop the disease.

Risk factors for anal cancer include: 

• Age - Most cases of anal cancer are diagnosed in people between the ages of 50 and 80.
• Anal fistula - An anal fistula is an abnormal tunnel between the anal canal and the outer skin of the anus. This tunnel often drains pus or liquid, which can soil or stain your clothing. An anal fistula can irritate the outer tissues of the anus or make you uncomfortable. It can increase the risk of developing anal cancer. 
• Anal irritation - Frequent redness, swelling, and soreness around the anus can increase the risk of developing anal cancer. 
• Gender - Women have a higher risk of anal cancer than men.
• Immune suppression - People with low immunity, such as people with HIV (the virus that causes AIDS) and those who take drugs to suppress their immune systems (such as people who have had an organ transplant), have an increased risk of anal cancer. Treatment for HIV does not seem to reduce the risk of anal cancer in people infected with HIV. 
• Human papillomavirus (HPV) infection - Most doctors believe that HPV causes squamous cell anal cancer and cervical cancer. There are several types of HPV; the type that is most likely to cause anal cancer is called HPV-16. This virus is usually passed from person to person by sexual contact. It is more common in people who have had many sex partners or who have had anal sex. Some of the many types or strains of HPV are more strongly associated with anal cancer than others. One sign of HPV infection is genital or venereal warts (condylomas), which are caused by HPV. HPV can make cells in the anus look abnormal (anal squamous intraepithelial lesions, or ASIL). The abnormal cells associated with ASIL are not cancerous but they can turn into anal cancer. However, some people with ASIL never get anal cancer. 
• Race - White people in the United States are much more likely than African Americans to be diagnosed with anal cancer. 
• Sexual activity - People who have had many sexual partners over their lifetime have a higher risk of anal cancer. In addition, men and women who have had anal intercourse, especially if they were younger than 30, are more likely to get anal cancer. 
• Smoking - People who smoke are about four times more likely to get anal cancer than people who do not smoke. The risk of anal cancer increases with the number of cigarettes smoked per day and the number of years that a person has been smoking. Quitting smoking can reduce the risk; people who used to smoke but have quit are only slightly more likely to develop anal cancer than people who have never smoked.

This content has been reviewed and approved by Myo Thant, MD.