Abstract
BackgroundIncidence-based mortality modelling comparing the risk of breast cancer death in screened and unscreened women in nine Swedish counties has suggested a 39% risk reduction in women 40 to 69 years old after introduction of mammography screening in the 1980s and 1990s.ObjectiveWe evaluated changes in breast cancer mortality in the same nine Swedish counties using a model approach based on official Swedish breast cancer mortality statistics, robust to effects of over-diagnosis and treatment changes. Using mortality data from the NordCan database from 1974 until 2003, we estimated the change in breast cancer mortality before and after introduction of mammography screening in at least the 13 years that followed screening start.ResultsBreast mortality decreased by 16% (95% CI: 9 to 22%) in women 40 to 69, and by 11% (95% CI: 2 to 20%) in women 40 to 79 years of age.DiscussionWithout individual data it is impossible to completely separate the effects of improved treatment and health service organisation from that of screening, which would bias our results in favour of screening. There will also be some contamination of post-screening mortality from breast cancer diagnosed prior to screening, beyond our attempts to adjust for delayed benefit. This would bias against screening. However, our estimates from publicly available data suggest considerably lower benefits than estimates based on comparison of screened versus non-screened women.
Highlights
Swedish randomized trials of mammography screening (MMS) have shown decreases in breast cancer mortality of 21% (RR = 0.79; 95% CI: 0.70–0.89) in women 40 years old and more, after a median follow-up time of 6.5 years [1]
Trends in breast cancer mortality using WHO data or NordCan data were sufficiently similar that they cannot be distinguished graphically, indicating similarity of data sent by the Swedish authorities to WHO and mortality data used by the NordCan project
According to our descriptive epidemiology analyses, in women 40 to 69 years of age, the introduction of MMS in nine Swedish counties was associated with a 16% reduction of breast cancer mortality
Summary
The greatest reduction of relative risk of breast cancer death, by 32% (RR = 0.68; 95% CI: 0.59–0.80), was reported in the Two-County Trial after 20 years of follow-up [2]. Attempts to correct these results for the possible effect of confounding by self-selection and temporal trends in breast cancer incidence [3] suggest a reduction of 39% (RR = 0.61; 95% CI: 0.55 to 0.68). Incidence-based mortality modelling comparing the risk of breast cancer death in screened and unscreened women in nine Swedish counties has suggested a 39% risk reduction in women 40 to 69 years old after introduction of mammography screening in the 1980s and 1990s
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