Research in context
Evidence before this study
Several previous randomised trials of mammographic screening included women aged younger than 50 years. A meta-analysis including these studies done for the US Preventive Services Task Force (USPSTF), published in 2009, identified a relative risk reduction in breast cancer mortality of 15% in women, aged 39–49 at randomisation, invited for screening, similar to that for older women, but a lower absolute reduction and greater number needed to invite. This meta-analysis included the first mortality results of the UK Age trial, and also the results of the Canadian trial (NBSS-1), the only other trial designed to study women younger than 50 years. A Cochrane review published in 2013 identified a 13% reduction in mortality in an analysis of only three of the eight trials included in the USPSTF meta-analysis, and a 16% reduction including all eight trials at 13 years of follow-up. Evidence from some service screening programmes supports a benefit of mammography in women younger than 50 years.
Estimates of overdiagnosis as a result of mammographic screening vary widely, largely because of differences in the methods used. Particularly, failure to allow for adequate follow-up will lead to an overestimate of overdiagnosis because of lead time. As a result, little reliable evidence exists about the extent of overdiagnosis in this age group.
Added value of this study
The UK Age trial is the only trial designed specifically to study the effect of mammographic screening starting at age 40 years. This study reports breast cancer mortality and incidence at a median of 17·7 years of follow-up, an increase of 7 years from the previous publication.
Implications of all the available evidence
The evidence supports a reduction in breast cancer mortality as a result of mammographic screening in women younger than 50 years at least in the first 10 years of follow-up. Further analysis of all the trials might clarify the long-term effects of early screening. No evidence for an increased amount of overdiagnosis in this age group was noted.