What Are the Risk Factors for Anal Cancer?

A risk factor is anything that affects your chance of getting a disease such as cancer. Different cancers have different risk factors. Some risk factors, like smoking, can be changed. Others, like a person’s age or family history, can’t.

Several factors can affect your risk of anal cancer. But having a risk factor, or even several risk factors, does not mean that you will get cancer. Many people with risk factors never develop anal cancer, while others with this disease may have few or no known risk factors.

Human papilloma virus (HPV) infection

Most squamous cell anal cancers seem to be linked to infection by the human papilloma virus (HPV), the same virus that causes cervical cancer. In fact, women with a history of cervical cancer (or pre-cancer) have an increased risk of anal cancer.

HPV is a group of more than 150 related viruses. They are called papilloma viruses because some of them cause papillomas, which are more commonly known as warts. There are many subtypes of HPV, but the one most likely to cause anal cancer is HPV-16. HPV-16, as well as HPV-18, HPV-31, HPV-33, HPV-45, and some others are considered high-risk types of HPV because they are strongly linked to cancer. They can also cause cancers of the cervix, vagina, and vulva in women, as well as cancer of the penis in men, and throat cancer in both women and men.

Other subtypes of HPV can cause warts in the genital and anal areas. The medical term for an anal or genital wart is condyloma acuminatum. The 2 types of HPV that cause most cases of anal and genital warts are HPV-6 and HPV-11. They are called low-risk types of HPV because they tend to cause warts but not cancer. HPV infection can cause anal and genital warts, but most people infected with HPV do not have genital warts or any other signs of infection.

HPV is passed from one person to another during skin-to-skin contact with an infected area of the body. HPV can be spread during sex – including vaginal, anal, and oral sex – but sex doesn’t have to occur for the infection to spread. All that is needed is for there to be skin-to-skin contact with an area of the body infected with HPV. The virus can be spread through genital-to-genital contact, or even hand-to-genital contact.

An HPV infection can also spread from one part of the body to another. For example, an HPV infection might start in the genitals and then spread to the anus.

It can be very hard to avoid being exposed to HPV. It might be possible to prevent genital HPV infection by not allowing others to have contact with your anal or genital area, but even then there could be other ways to become infected that aren’t yet clear.

Infection with HPV is common, and in most cases the body can clear the infection on its own. But in some people the infection doesn’t go away and becomes chronic. Chronic infection, especially with high-risk HPV types, can eventually cause certain cancers, including anal cancer.

HPV in men

For men, the 2 main factors influencing the risk of genital HPV infection are circumcision and the number of sexual partners.

Men who are circumcised (have had the foreskin of the penis removed) have a lower chance of becoming and staying infected with HPV. The reasons for this are unclear. It might be that the skin on the glans (tip) of the penis goes through changes after circumcision that make it more resistant to HPV infection. Another theory is that the surface of the foreskin (which is removed by circumcision) is more easily infected by HPV. Still, circumcision does not protect completely against HPV infection – men who are circumcised can still get HPV and pass it on to their partners.

The risk of being infected with HPV is also strongly linked to having many sexual partners (over a man’s lifetime).

HPV in women

In women, HPV infections occur mainly when they are younger and are less common in women over 30. The reason for this is not clear. Certain types of sexual behavior increase a woman’s risk of getting a genital HPV infection, such as having sex at an early age and having many sexual partners.

Although women who have had many sexual partners are more likely to get infected with HPV, a woman can still get infected even if she has had only one sexual partner. This is more likely if she has a partner who has had many sex partners or if her partner is an uncircumcised male.

For more information about HPV and HPV vaccines, see HPV Vaccines.

Anal warts

Anal warts (also known as condyloma acuminata) are caused by infection with certain types of HPV – usually different types from those most likely to cause anal cancer. While anal warts themselves are unlikely to develop into anal cancer, people who have had anal warts are more likely to get anal cancer. This is because people who are infected with the low-risk HPV subtypes that cause anal and genital warts are also more likely to be infected with high-risk HPV subtypes that cause anal cancer.

Having certain other cancers

Women who have had cancer of the cervix, vagina, or vulva are at increased risk of anal cancer. This is probably because these cancers are also caused by infection with HPV.

In men, it would seem likely that having had penile cancer, which is also linked to HPV infection, would increase the risk of anal cancer, but this link has not been shown in studies.

HIV infection

People infected with the human immunodeficiency virus (HIV), the virus that causes AIDS, are much more likely to get anal cancer than those not infected with this virus. For more information about HIV and AIDS, see our document HIV Infection, AIDS, and Cancer.

Sexual activity

Having multiple sex partners increases the risk of infection with HIV and HPV. It also increases the risk of anal cancer.

Receptive anal intercourse also increases the risk of anal cancer in both men and women, particularly in those younger than 30. Because of this, men who have sex with men have a high risk of this cancer.

Smoking

Smoking increases the risk of anal cancer. Current smokers are several times more likely to have cancer of the anus compared with people who do not smoke. Quitting smoking seems to reduce the risk. People who used to smoke but have quit are only slightly more likely to develop this cancer compared with people who never smoked.

Lowered immunity

Higher rates of anal cancer occur among people with reduced immunity, such as people with AIDS or people who have had an organ transplant and must take medicines that suppress their immune system.

Gender and race/ethnicity

Anal cancer is more common in women than men overall and in most racial/ethnic groups. However, in African Americans it is more common in men than in women.