The incidence of anal squamous cell carcinoma is higher in gay men, presumably because receptive anal intercourse increases exposure to and infection by human papillomavirus (HPV). There are multiple types of HPV. The early changes in the rectal mucosa (the cells lining the rectum) that sometimes develop into cancer are more common in cases of infection with multiple types of HPV -- especially types 6, 16 and 18.

A key question is the extent, if any, to which HIV makes HPV infection more likely after a given level of exposure, and/or makes the consequence of HPV infection worse if it occurs. Cancer epidemiology studies suggest the association between receptive anal intercourse and anal squamous cell carcinoma has more to do with receptive anal intercourse than with HIV. HIV-uninfected gay men seem to be at almost the same risk of anal squamous cell carcinoma as HIV-infected men. The modest increased risk in HIV-infected gay men over HIV-uninfected gay men may occur because more sexually active gay men are at increased risk of both HIV and HPV infection. The matter has not been resolved, however, because we cannot randomize HIV infection.

One way of getting at this issue is to compare the incidence of anal squamous cell carcinoma in HIV-infected persons before and after the introduction of HAART. If the incidence were to decrease, it would suggest that HIV-mediated immunodeficiency plays a part in the problem.

Details of This Study

Bower et al report on a cohort of 8,640 HIV-infected persons seen at London's Chelsea & Westminster Hospital, comprising 40,126 person-years of follow-up.

The incidence of anal squamous cell carcinoma in the pre-HAART era (1984-1995) was 35 cases per 100,000 patient-years. In the post-HAART era (1996-2003) the incidence increased to 92 cases per 100,000 patient-years. These rates are not statistically different, meaning they are close enough to have arisen by chance. In both eras, however, the rates were much higher (P<.001), after age and gender matching, than those in a regional cancer registry.

Significance for Patients/Clinicians

These results provide additional support for the belief that the high rates of anal squamous cell carcinoma seen in gay men have more to do with rectal exposure to HPV than with HIV infection. They also reinforce the available information that anal squamous cell carcinoma is an important problem for anyone (male or female) who has been the receptive partner in anal intercourse. Whatever policies we adopt to deal with this problem should be applied to both HIV-uninfected and HIV-infected persons.