The incidence of anal cancer in the general population is higher among women than men, and has been rising steadily since the 1970s. Although the incidence in the general population is relatively low, certain groups of women are at increased risk of anal cancer. As with algorithms designed to identify and treat cervical high-grade squamous intraepithelial lesions (HSIL) to prevent cervical cancer, anal cancer prevention programs include screening algorithms that include visual inspection of the at-risk areas, known as high resolution anoscopy (HRA). HRA requires substantial training and has limited availability, even in regions with large populations of at-risk individuals. Given the high cost and limited availability of HRA, several techniques have been used or are under investigation to screen at-risk individuals to identify those who should be referred for HRA.
The optimal approach to designing a rational anal cancer prevention program is to identify the populations that would most benefit from screening, identify optimal screening techniques, optimize treatment of anal HSIL, and document that treatment of anal HSIL reduces the incidence of anal cancer.
The populations of women that would most benefit from screening are those with the highest risk of anal cancer, and these include women with a history of cervical or vulvar HSIL or cancer and women with immunocompromise due to HIV infection or other causes such as medication to prevent transplant rejection. Women with autoimmune diseases and other forms of immunocompromise may also be at increased risk. Anal cytology is the most commonly used screening tool to identify those who need HRA. Although it has a high positive predictive value for biopsy-proven HSIL when HSIL is found on cytology, it also has substantial limitations, including low sensitivity and routinely under-calls the grade of the lesion. Other techniques such as various forms of HPV detection are under investigation, either as primary screening tools or combined with cytology.
The utility of HPV as a screening tool alone or in combination with anal cytology needs to be established. However it is likely that the utility of HPV testing will vary considerably depending on the population being screened, e.g. HIV-infected men who have sex with men versus HIV-uninfected women with a history of cervical HSIL, and each different at-risk population may need an individualized approach. The efficacy of treating HSIL to reduce the incidence of anal cancer is currently under investigation and will be key to establishing anal cancer prevention as standard of care for at-risk populations.