Women offered breast cancer screening have a 20% lower relative risk of dying from breast cancer than women not invited to participate in screening, a epidemiologic study in the Lancet from the United Kingdom has found.
The study also found that women invited to screening are also more likely to be overdiagnosed, meaning that they are treated surgically for tumors that, if not for screening, would have caused no illness and remained undetected. The researchers cautioned, however, that their findings on overdiagnosis were based on lower-quality evidence than that used to determine relative risk reduction.
For their research, published online Oct. 29, epidemiologist Dr. Michael Marmot of University College London and his colleagues conducted a meta-analysis of data from 11 randomized controlled trials (Lancet 2012 [http://dx.doi.org/10.1016/S0140-6736(12)61611-0]).
Altogether, the included randomized trials enrolled about 674,000 women in Canada, Sweden, the United States, and the United Kingdom between 1963 and 1991. Women in the studies were invited for mammographic screening at intervals ranging from 12 to 33 months; their ages ranged from 38 to 75 years.
The relative risk of breast cancer mortality for women invited to screening, compared with controls, was 0.80 (95% confidence interval, 0.73-0.89), Dr. Marmot and his colleagues found. This corresponds to one breast cancer death prevented for every 235 women invited to screening, they said, and to one death averted for every 180 women screened.
Dr. Marmot and his colleagues acknowledged that the trials used in their meta-analysis were heterogeneous in design and populations enrolled, and reflected both older screening techniques and breast-cancer treatments; however, they noted, the mortality benefit arrived at was largely in keeping with that seen in other studies, including recent observational studies.
Dr. Marmot’s research group was convened as an independent panel to review evidence for the benefits and harms of breast screening in the United Kingdom, where, as in the United States, Canada and other countries, there have been fierce debates about the benefits and harms of government breast-cancer screening recommendations. None of the panel’s members – who included epidemiologists, clinicians, and statisticians – had previously published on breast cancer screening.
Women in the United Kingdom aged 50-70 years are invited to have screenings once every 3 years. Therefore, the investigators estimated, for every 10,000 U.K. women invited to screening between the ages of 50 and 70 years, some 681 cancers will be found; 129 of them will represent overdiagnosis and 43 deaths from breast cancer will be prevented.
Thus, among women invited to screening, about 11% of the cancers diagnosed during their lifetimes are considered overdiagnosis, the investigators estimated, and about 19% of the cancers diagnosed during the period while women are being screened are overdiagnosis.
Dr. Marmot and his colleagues emphasized, however, that these overdiagnosis figures represent "best estimates from inadequate data." Only 3 of the 11 trials included in the study, along with some observational studies, provided information relevant to overdiagnosis.
In an unsigned editorial accompanying the article, the editors of the Lancet called Dr. Marmot and his colleagues’ report, "the latest and best available systematic review" to date on breast cancer screening (2012 [http://dx.doi.org/10.1016/ S0140-6736(12)61775-9]). The report, they said, "shows that the U.K. breast-screening program extends lives and that, overall, the benefits outweigh the harms. Dissemination of these findings is now imperative in the media, the NHS screening program, and between doctors and their patients."
Dr. Marmot and his colleagues’ study was jointly commissioned by the National Cancer Director of England and Cancer Research UK, and funded by the U.K. Department of Health. None of the panel members declared conflicts of interest.