The incremental benefits of implementing effective cervical cancer screening

Dear Editor, We read with interest the article by L€onnberg et al. proposing a novel means to estimate the number of cervical cancer cases prevented by opportunisticand population-based screening in Norway over the last half century. Their projections rely on the explicit assumption that the secular decline in cervical cancer incidence is driven by screening-prevented squamous cell carcinoma (SCC), while adenocarcinoma (ADC) incidence trends are wholly unaffected by the intervention. The increasing ADC trend is therefore used as a reference to project SCC rates, assuming that equivalent increases in SCC would have occurred in the absence of screening. Although screening history has been associated with a decreasing risk of both SCC and ADC, the reduction has been reported as much smaller for ADC. Extensive sensitivity analyses were carried out by L€onnberg et al. to account for the possibility that the incidence of ADC might also have been reduced to some extent by screening. We have recently published research with a similar aim but different methodology and an expanded focus on four Nordic countries (Denmark, Finland, Norway and Sweden). Our estimated proportion of cervical cancer prevented by screening in Norway was 72% in 2010, a remarkably similar figure to the 68% proposed by L€onnberg et al. The corresponding estimates for the other Nordic countries were: 82% for Denmark; 68% for Finland, and 67% for Sweden. Assuming a factitious absence of screening activities in the Nordic countries, we estimated the number of cases averted by their implementation. Using modified age-period-cohort models, separation of the three major temporal components affecting cancer rates was made possible by including constraints on the age effects according to the underlying historical relationship between age and cervical cancer incidence. We observed opposing trends in the period and cohort effects in each country. The upward cohort effects among recent generations were considered as markers of an increasing background cervical cancer risk following changes in sexual behavior and consequent increase in high-risk human papillomavirus infection, whereas the downward period effects were considered an indicator of the beneficial impact of screening. In comparing our approach with that of L€onnberg et al., we highlight the fact that the beneficial effects of implementing a screening program accumulate with time. By computing the cumulative number of cases prevented by screening across five decades in four Nordic countries (Fig. 1), we showed that the averted proportion varied with time. In Norway, for instance, the earliest gain is visible in the mid-1970s, in line with the advent of large-scale screening. This gain, however, increases rapidly with time as the coverage and quality of screening programs improves. Our current findings (Fig. 1) accord with those reported by L€onnberg et al. (see Fig. 3) and confirm that the gap between the projected rates (assuming an absence of screening) and the observed rates for cervical cancer increased steadily over time. The other Nordic countries showed similar patterns although the beneficial effects of screening were seen earlier than in Norway, and decreased in Sweden and Finland after 1990. Our findings are a strong reminder that the benefits of national screening programs accumulate over time. The extent to which reductions in the incidence of cervical cancer via screening are observed partially depends upon the metrics