Infections with the Human Papilloma Virus (HPV), which is a very common infection in women, is an important risk factor. Most of the infections are temporary, but can be chronic and possibly cause cancer.
Infection with HPV is confirmed in about 90% of anal cancer and its pre-stages (dysplasia). Among the almost 100 HPV subtypes, there are very different tendencies to cause cancer. HPV 16 is the most important carcinogenic subtype and is confirmed the most frequently (about 80%). HPV 18 is detected more rarely (about 10%). HPV can also be found in genital warts. Mechanisms are unknown.
Patients who have had cervical cancer due to HPV have 4-5 times higher risk for developing anal cancer.
- Men and women who practice anal sex have a considerably increased risk for anal cancer, probably through the HPV infection.
- HIV-positive patients who now live longer due to effective treatment and live long enough to develop cancer have a much higher risk.
- Immunosuppressed patients, due to organ transplantation for example, have a 10 fold increased risk compared to the rest of the population.
- High use of tobacco appears to increase the risk for developing anal cancer, but the mechanism is unknown.
A large percentage of the population is infected with HPV. Most cases of anal and perianal cancer occur in heterosexual HIV negative and non-immunosuppressed patients.