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.
Randomization in the Canadian National Breast Screening Study: a review for evidence of subversion.
The authors assess the randomization strategy that had been used in the Canadian National Breast Screening Study (NBSS). Document experts at a private investigation and security company were hired to assist in reviewing instances in which names of subjects were altered in the "allocation books" (the basic instrument used to assign, at random, participants to either the mammography or the usual-care arm). The review was restricted to records from 3 NBSS centres where women assigned to the mammography arm had a distinctly higher (not necessarily significant) number of deaths from breast cancer than those assigned to the usual-care arm, and to records from 2 centres where, for limited periods, administrative problems were reported. In most cases the underlying, original name could be identified. The document experts found no evidence of a deliberate attempt to conceal the alterations. A search of the NBSS database for the underlying and superimposed names revealed that only 1 of the women whose name had been deleted or superimpsed died of breast cancer. She was in the mammography arm. The authors' thorough review of ways in which the randomization could have been subverted failed to uncover credible evidence of it. They conclude that even if there had been acts of subversion, they could only have been few in number and, given that there was only 1 death from breast cancer in the group reviewed, the alterations could have had only a trivial effect on the study findings as reported in 1992.