In HIV-positive men who have sex with men (MSM), the incidence of anal cancer is 60-190 times higher than in the general population. The incidence has been rising for many years, but has declined in recent years. In this population, where we assume that almost everybody had a history of contact with oncogenic Human Papillomaviruses (HPV), the most important risk factor for anal cancer is a low CD4 cell count. Little is known about the likely impact of increased antiretroviral therapy (ART) coverage and screening on anal cancer incidence.
We developed a mathematical model to estimate the incidence of anal cancer in HIV-positive MSM in Switzerland up to 2030. We considered two scenarios of future ART coverage: ‘stable’ (same cART coverage between 2016 and 2030 as between 2010 and 2015 and no screening for AIN2/3) and ‘100% cART coverage’ (all individuals diagnosed with HIV are on ART); and four screening scenarios: ‘no screening’, ‘yearly cytology’, ‘yearly anoscopy’ and ‘CD4-dependent anoscopy’ (people were screened with anoscopy five years after their CD4 cell count dropped to below 200 cells/µl). We parameterized the model with data from the Swiss HIV Cohort Study (SHCS) and the literature. We considered CD4 cell count trajectory the main predictor of anal cancer.
The median nadir CD4 cell count of 6,411 MSM in the SHCS increased from 112 cells/μl in 1980-1999 to 394 cells/μl after 2010. Predicted cancer incidence increased to a maximum of 78.7/100,000 person-years in 2010 and has since stabilized. Model estimates up to 2014 are consistent with observed anal cancer incidence in the SHCS. By 2030, incidence will decrease to 58.9/100,000 person-years in the stable scenario, and to 52/100,000 in the 100% cART coverage scenario. Treating patients with electrocautery after yearly anoscopy decreased anal cancer incidence by 37.9%, after yearly cytology by 30.9% and after CD4-dependent anoscopy by 13% (Figure 1). To prevent one anal cancer case 3817 screening tests were needed in the yearly anoscopy strategy, 4684 in the yearly cytology strategy and 242 in the CD4-dependent strategy.
Yearly anoscopy leads to the most pronounced decrease in anal cancer incidence and CD4 dependent anoscopy results in most anal cancers prevented per screening test. Expanding ART will have only a modest effect.