Repeated mammographic screening reduces breast cancer mortality along the continuum of age.

Abstract

The optimal age boundaries for breast cancer screening are still under debate. A case-referent design was used to describe the effect of mammographic screening on breast cancer mortality along the continuum of age, based on a 20 year follow up period. The population based breast cancer screening programme in Nijmegen, The Netherlands, which has biennially invited women over 35 years since 1975. Cases, defined as women who died from primary breast cancer between 1987 and 1997, were selected from the group of women who received at least one invitation to the screening programme. For 157 cases, 785 women from the same group were selected as referents. Information on the index screening (the screening examination preceding diagnosis of the case) was collected for both cases and referents. The risk of dying from breast cancer was calculated per 10 year moving age group for women who had attended the index screening versus those who had not. The youngest 10 year age group showing an effect in our study were women aged 45-54 at their index screening. Breast cancer mortality for women in this group who attended the index screening was 32% lower, although not significant, than for women who did not (odds ratio (OR) 0.68, 95% confidence interval (95% CI) 0.33 to 1.41). This reduction in risk was not explained solely by an effect in women over 50 because the OR in women aged 45-49 was 0.56 (95% CI 0.20 to 1.61). Reductions in mortality became smaller with increasing age. Nevertheless, for women over 60 at index screening, participation in screening over a maximum 4 year period before diagnosis of the case yielded protective effects at least up to an age around 80. Although our results are based on a relatively small number of cases, they suggest that even in a programme with a 2 year screening interval there may be a benefit of starting screening around age 45. Also older women who participate at least once every 4 years still have much to gain from screening.