Objective: To systematically review the observational evidence concerning the effect of screening on breast cancer mortality in actual populations of women ages 50-69 years.
Methods: We searched MEDLINE and multiple reference lists for relevant cohort and ecologic studies. At least 2 authors reviewed abstracts and full texts of studies meeting eligibility criteria. We rated each accepted study on standard quality criteria and developed a Summary Evidence Table.
Results: Seventeen studies met eligibility criteria. Five studies found no to small effect of screening (0-12% relative risk reduction [RRR] in breast cancer mortality), 4 found a large effect (greater than 33% RRR), and 8 found a moderate effect (13% to 33% reduction). The authors found concerns about quality in all studies. There was insufficient evidence to determine whether the effectiveness of screening is decreasing over time.
Conclusions: Current observational evidence shows that breast cancer screening in actual populations of women ages 50 to 69 reduces breast cancer mortality; the magnitude of the effect is probably smaller than predicted in the randomized controlled trials. Because the magnitude may change (either increase or decrease) in the future, further ecologic studies are needed. The methodology and infrastructure for these studies should be improved.
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