Editor—Olsen et al found a 25% reduction in breast cancer mortality in Copenhagen compared with what they would expect to find in the absence of screening.1 They did an observational study, which is not considered to be a reliable method for evaluating mortality reductions with screening.
The full mortality reduction was seen after only three years of follow-up, where it nearly reached significance, and it stayed at that level for the next seven years. However, both randomised trials and cohort studies have shown clearly that a positive effect of screening does not come that quickly. It therefore seems likely that the study has provided an exaggerated mortality benefit.
Olsen et al did not provide data on harms but refer to another of their studies when they claim that the introduction of mammography screening in Copenhagen did not lead to an increase in the incidence of breast cancer apart from the expected prevalence peak. Their conclusion in that study is even stronger since they say that mammography screening can operate without over-diagnosis. However, their data do not support the conclusion,2 and much larger studies, both randomised and non-randomised, have shown that the level of over-diagnosis is about 30%, or even more.3-5
Proper evaluation of findings about any medical intervention, in particular cancer screening of healthy people, requires an honest, balanced, and unbiased presentation of major benefits and harms.
Competing interests: PCG was involved in a systematic review that questioned the value of screening.
References
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