Risks and causes of anal cancer

Anal cancer is a rare cancer. About 930 people are diagnosed each year in England, Scotland and Wales. It is slightly more common in women than men, with rates increasing in women over the past 10 years. We don’t know the cause of anal cancer but there are several risk factors.

Anything that increases your risk of getting a disease is a risk factor. Different cancers have different risk factors. Even if you have more than one risk factor it doesn’t mean you will definitely get the disease. And just because researchers investigate a possible cause, that doesn't mean it will turn out to be a risk factor.

There are over 100 different types of human papilloma virus (HPVs). Some types are called the ‘wart virus’ or ‘genital wart virus’ as they cause genital warts. Some types of HPV are passed on from one person to another through sexual contact.

Up to 8 out of 10 people (80%) in the UK are infected with the HPV virus at some time during their lifetime. For many people, the virus causes no harm and goes away without treatment.

Around 8 out of 10 (80%) people diagnosed with anal cancer have evidence of HPV infection in the anal area. Of the different types of HPV, type 16 is the most common in anal cancer. Invasive anal cancer is thought to develop from the growth of abnormal cells (squamous intraepithelial lesions) caused by HPV infection.

If you have a history of genital warts (which are caused by the HPV virus) you have an increased risk of anal cancer. The risk is higher again if you have had anal warts.

There is some evidence that that the more sexual partners you have had, the more likely you are to develop anal cancer. This may be because more partners means more chances of contact with the HPV virus. Also, women who have anal intercourse, and men who have receptive anal intercourse have a higher risk of developing anal cancer. This may be because they are more likely to have HPV infection of the anus, or it might be due to other factors, such as inflammation of the anus.

Some studies have shown that other sexually transmitted infections, including the herpes virus, syphilis and gonorrhoea, are associated with an increased risk of anal cancer. This is probably because people who have had other sexually transmitted infections are more likely to have had contact with HPV.

Some studies show that if you have had cervical or vaginal cancer you have a higher risk of developing abnormal cells in the anus or anal cancer than the general population.The risk is also slightly increased for women with a history of abnormal cells in the cervix (cervical intraepithelial neoplasia). This is probably due to risk factors common to cervical and anal cancers, such as HPV infection and smoking. But one study found that women who have had cervical cancer only have an increased risk of anal cancer if they had radiotherapy as part of their cervical cancer treatment. We need more research to fully understand how cervical or vaginal cancer affect the risk of anal cancer.

Smoking has also been shown to increase the risk of cancer of the anus. Giving up smoking can reduce your risk of developing many cancers, including anal cancer.

If you have lowered immunity you are at greater risk of developing anal cancer. Compared to the general population, you have an increased risk if you have HIV or are taking medicines to damp down your immune system after an organ transplant (for example, a kidney transplant). The risk of anal cancer related to HIV infection is especially high in younger adults.

People who have inflammatory anal conditions may have an increased risk of anal cancer. These conditions include

• A split in the anal wall (fissure)
• A hole between the anal canal and another organ (fistula)
• A collection of pus in the anal wall (anal abscess)

Your risk of developing anal cancer increases as you get older. But the risk is still small because anal cancer is a rare cancer.