Estimates of Screening Benefit: The Randomized Trials of Breast Cancer Screening

The benefits of mammography screening have been studied in nine randomized controlled trials comparing women randomized to screening or nonscreening groups. Randomized controlled trials are regarded as the strongest study design to determine effectiveness because they are less susceptible to bias than other study designs. The trials began between 1965 and 1991, and involved over 600,000 women in the United States, Canada, United Kingdom, and Sweden. The trials varied in their recruitment of women, the number of women enrolled, and randomization. Major limitations include comparability of screening and nonscreening groups, clinical relevance to current populations and practice, and whether the effects of screening in trials differ from the general population. Results indicate reduced breast cancer mortality with screening that varies by age. Results for ages 40-49 and 50-59 years are of borderline statistical significance and vary by how cases were accrued in trials; estimates for age 70-74 years are limited by the small number of trial participants. All-cause mortality was not reduced with screening, while advanced breast cancer was reduced for women age 50 and older, but not younger women.