Breast cancer is the most commonly occurring non-skin malignancy among women in the United States and the second leading cause of cancer-related death. Screening for breast cancer has been shown to reduce breast cancer-related mortality among women aged 40 to 69 and screening women aged 50 to 69 has been endorsed by most professional groups. Historically, recommendations for breast cancer screening among women aged 40 to 49 and women older than age 69 have varied due to less supporting evidence, as well as the costs and risks associated with screening.
We conducted a systematic evidence review to update previous analyses and synthesize new data on various modes of breast cancer screening to assist the U.S. Preventive Services Task Force (USPSTF) in updating its recommendations on breast cancer screening, focusing primarily on the efficacy of screening with mammography, clinical breast exam, and breast self-exam.
We identified relevant publications by searching MEDLINE, the Cochrane Controlled Trials Registry, and reference lists of reviews, editorials, and original studies.
We identified eight randomized trials and one non-randomized trial of mammography, clinical breast examination, or both. All studies were rated for quality using predefined criteria and only studies of fair or better quality were included in the meta-analysis.
Two of the authors abstracted information about each randomized controlled trial. We compiled an appendix consisting of detailed information about the patient population, design, potential flaws, missing information, and analysis conducted in each trial. For the primary endpoint of breast cancer mortality, we abstracted results for each reported length of follow-up.
We conducted a meta-analysis using a Bayesian random effects model to provide summary relative risk estimates of the effectiveness of screening with mammography, either alone or with clinical breast exam, in reducing breast cancer mortality by age group. We also reviewed two randomized controlled trials, one non-randomized controlled trial and one cohort study evaluating breast self-examination.
All of the randomized controlled trials of mammography, with or without clinical breast exam have published updates since 1996 and now have between 10.5 and 20 years of follow-up. In our meta-analysis combining all studies of fair quality, the summary relative risk reduction for breast cancer screening among women of all ages was 0.84 (95% credible interval [CrI], 0.77–0.91), and the overall number needed to screen to prevent one death from breast cancer over approximately 14 years of observation was 1,224 (95% CrI, 665–2,564). These numbers reflect not just one screening, but multiple screenings in an organized program. Six fair or better quality randomized trials provided information about the efficacy of mammography and clinical breast exam among women younger than age 50. In our meta-analysis, the summary relative risk for the six studies was 0.85 (95% CrI, 0.73–0.99), and the overall number needed to screen was 1,792 (95% CrI, 764–10,540). Analyses of the trials to determine the portion of the benefit attributable to screening before age 50 have mixed results, but suggest that a significant portion of the benefit is due to screening before age 50, especially in women who were aged 39 to 44 when they had their first mammogram. No study has compared clinical breast examination to no screening. In trials that combined mammography with clinical breast examination, the contribution of clinical breast examination to reduced mortality from breast cancer is unclear. One study compared annual clinical breast examination plus mammography to a baseline clinical breast examination in women aged 40 to 49 at study entry and found no difference in breast cancer mortality. The role of breast self-examination in reducing breast cancer mortality has been evaluated in 2 randomized controlled trials and 1 non-randomized controlled trial, and none have shown benefit.
