Cancer of the anus is quite rare. There are fewer than 400 new cases in the United Kingdom each year.

The cause of anal cancer is not always clear but they are often associated with infection by a virus called the human papilloma virus (HPV).

There are many different types of HPV. Some types of HPV cause warts in the genital area but many others do not. Usually, if we are infected with HPV our body's defence against infection, the immune system, clears the virus within a few months. But, in some cases, our body fails to clear the virus.

The virus can cause changes in the cells that line the anus. For a small number of people these changes may, over a number of years, lead to anal cancer.

Anal cancer is slightly more common in women than in men and is more common in gay men and in people with a higher than average number of sexual partners. HPV infection may be a factor in this.

Our immune system is important in clearing HPV infection. So, people who have a lowered immunity, because of medical conditions such as HIV, are at increased risk of anal cancer. Cigarette smoking also lowers our immune system and can increase the risk of anal cancer.

Reference

• Daling et al. Human papillomavirus, smoking and sexual practices in the etiology of anal cancer. Cancer 2004:101(2);270 - 280.