If we assume that screening reduces breast cancer mortality by 15% and that overdiagnosis and overtreatment is at 30%, it means that for every 2000 women invited for screening throughout 10 years, one will avoid dying of breast cancer and 10 healthy women, who would not have been diagnosed if there had not been screening, will be treated unnecessarily. Furthermore, more than 200 women will experience important psychological distress including anxiety and uncertainty for years because of false positive findings. To help ensure that the women are fully informed before they decide whether or not to attend screening, we have written an evidence-based leaflet for lay people that is available in several languages on www.cochrane.dk. Because of substantial advances in treatment and greater breast cancer awareness since the trials were carried out, it is likely that the absolute effect of screening today is smaller than in the trials. Recent observational studies show more overdiagnosis than in the trials and very little or no reduction in the incidence of advanced cancers with screening.
An analysis of the mortality effect in a breast cancer screening study.
In order to better understand the effect of breast cancer screening on mortality, we use the theory of competing risks to analyse deaths from the long-term follow-up of the HIP breast cancer screening trial. We conclude that some, but not all, of the breast cancer cases detected early as a result of screening realized a benefit in terms of elimination of the risk of breast cancer mortality. Breast cancer screening appears not to affect the rate of mortality from causes of death other than breast cancer. Methodologically, we demonstrate the importance of analysing mortality separately for the breast cancer cases as well as for the entire population in a trial.