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.
Long-term effects of mammography screening: updated overview of the Swedish randomised trials.
There has been much debate about the value of screening mammography. Here we update the overview of the Swedish randomised controlled trials on mammography screening up to and including 1996. The Kopparberg part of the Two-County trial was not available for the overview, but the continuation of the Malmö trial (MMST II) has been added. The article also contains basic data from the trials that have not been presented before. Methods The trials (n=247010, invited group 129750, control group 117260) have been followed up by record linkage to the Swedish Cancer and Cause of Death Registers. The relative risks (RR) for breast cancer death and mortality were calculated for the invited and the control groups. The trial-specific as well as the age-specific effects were analysed. RRs were calculated by the density method, with total person-time experience of the cohort by time interval of follow-up as a basis for estimating mortality rates. We calculated weighted RRs and 95% CI with the Mantel-Haenszel procedure.
                The median trial time-the time from randomisation until the first round was completed for the control group or if the control group was not invited, until end of follow-up-was 6.5 years (range 3.0-18.1). The median follow-up time, the time from randomisation, to the end of follow-up, was 15.8 years (5.8-20.2). There were 511 breast cancer deaths in 1864770 women-years in the invited groups and 584 breast cancer deaths in 1688440 women-years in the control groups, a significant 21% reduction in breast cancer mortality (RR=0.79, 95% CI 0.70-0.89). The reduction was greatest in the age group 60-69 years at entry (33%). Looking at 5-year age groups, there were statistically significant effects in the age groups 55-59, 60-64, and 65-69 years (RR=0.76, 0.68, and 0.69, respectively). There was a small effect in women 50-54 years at randomisation (RR=0.95). The benefit in terms of cumulative breast cancer mortality started to emerge at about 4 years after randomisation and continued to increase to about 10 years. Thereafter the benefit in absolute terms was maintained throughout the period of observation. The age-adjusted relative risk for the total mortality was 0.98 (0.96-1.00).
                The advantageous effect of breast screening on breast cancer mortality persists after long-term follow-up. The recent criticism against the Swedish randomised controlled trials is misleading and scientifically unfounded.