What's the Evidence?
The anus has cells that can become infected with human papilloma virus (HPV). HPV causes changes to those cells that can lead to high-grade squamous intraepithelial lesions (HSIL).
HSIL is considered to be pre-cancerous which means that in some (but not all!) people, it can progress to cancer. This is similar to how HPV can lead to cervical HSIL and cancer. In the cervix these changes can be detected on a Pap smear, which if found leads to a procedure called colposcopy. At colposcopy the areas that shed the abnormal cells on the Pap smear are visualized and sampled (biopsied) to confirm the diagnosis. If HSIL is found in the biopsy, it is treated to reduce the risk that it will progress to cervical cancer. This is the basis of the cervical Pap smear screening that is routinely performed in women and which has cut the incidence of cervical cancer by 80 percent.
What Data are Still Missing?
Unlike screening for cervical cancer, screening for anal cancer is not yet routine. This is because it has not yet been proven that finding and treating anal HSIL reduces the risk of anal cancer as was shown for treatment of cervical HSIL in reducing the risk of cervical cancer. Before anal screening can become standard of care and done routinely for all people at high risk of anal cancer, we need to prove that it works, i.e., actually prevents anal cancer.
The best way to show that is to recruit people with HSIL into a study and assign them randomly to a treatment arm or a monitoring arm. We then follow everyone for about five years to compare the rates of cancer in both study arms. At the end of the study we’ll know whether screening and treatment of HSIL are effective strategies in preventing anal cancer. We’ll also learn a lot about HPV and other risk factors and why these progress to cancer in some people but not others.