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. For example, exposure to strong sunlight is a risk factor for skin cancer. Smoking is a risk factor for cancer of the lung and many other cancers. But risk factors don't tell us everything. Having a risk factor, or even several risk factors, does not mean that you will get cancer. Also, people without risk factors can still get cancer.
Human papilloma virus 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 100 related viruses. They are called papilloma viruses because some of them cause papillomas, which are more commonly known as warts. There are several subtypes of the virus, but the one most likely to cause anal cancer is called HPV-16. HPV-16, as well as HPV 18, HPV 31, HPV 33, and HPV 45 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 these warts 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 intercourse, anal intercourse, 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. It is even possible for a genital infection to spread through hand-to-genital contact.
An HPV infection also seems to be able to be spread from one part of the body to another. This means than an HPV infection may 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 is able to clear the infection on its own. But in some cases the infection does not go away and becomes chronic. Chronic infection, especially when it is with high-risk HPV types, can eventually cause certain cancers, including anal cancer.
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 risk of being infected with HPV is also strongly linked to having many sexual partners (over a man's lifetime).
In women, HPV infections occur mainly at younger ages 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 who has had only one sexual partner can still get infected. This is more likely if she has a partner who has had many sex partners or if her partner is an uncircumcised male.
In a study that looked at risk factors for anal HPV infection in women, risk was increased in younger women and in those who had more than 5 sexual partners in their lifetime. Ever having anal sex also increased risk.
Circumcision and HPV: Men who have not been circumcised are more likely to be infected with HPV and pass it on to their partners. The reasons for this are unclear. It may be that the skin on the glans of the penis goes through changes 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 completely protect against HPV infection -- men who are circumcised can still get HPV and pass it on to their partners.
Condoms and HPV: Condoms can provide some protection against HPV, but they do not completely prevent infection. One study found that when condoms are used correctly they can lower the genital HPV infection rate in women by about 70% - but they need to be used every time sex occurs. This study did not look at the effect of condom use on anal HPV infection. In another study, men who used condoms less than half of the time had a higher risk of HPV infection. Condoms cannot protect completely because they don't cover every possible HPV-infected area of the body, such as skin of the genital or anal area. Still, condoms provide some protection against HPV, and they also protect against HIV and some other sexually transmitted diseases. Condoms (when used by the male partner) also seem to help genital HPV infections clear (go away) faster in both women and men.
For more information about HPV and HPV vaccines, see Human Papilloma Virus and HPV Vaccines FAQ.
Ever having cancer of the cervix, vagina, or vulva is linked to an increased risk of anal cancer. This is likely because these cancers are also caused by infection with HPV. Although it is likely that having penile cancer, which is also linked to HPV infection, would increase the risk of anal cancer, this link has not been shown in studies.
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.
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 the age of 30. Because of this, men who have sex with men have a high risk of this cancer.
Smoking also 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 will 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.
Higher rates of anal cancer occur among people with reduced immunity, such as people who have had an organ transplant and must take medicines that suppress their immune system.
Race and gender
Anal cancer is more common in African-Americans than in whites. Overall, it is more common in women than men, but in African Americans it is more common in men than in women.
Last Medical Review: 01/02/2013
Last Revised: 01/02/2013