Effect of Mammography on Breast Cancer Mortality

Background: A variety of estimated of the benefits and harms of mammographic screening for breast cancer have been published, and national policies vary. Objectives: To assess the effect of screening for breast cancer with mammography on mortality and morbidity. Search Strategy: The authors searched PubMed (November 2008). Selection Criteria: The authors selected randomized trials comparing mammographic screening with no mammographic screening. Data Collection and Analysis: The authors independently extracted data. Study authors were contacted for additional information. Main Results: Eight eligible trials were initially identified. The authors excluded a biased trial and included 600,000 women in the analyses. Three trials with adequate randomization did not show a significant reduction in breast cancer mortality at 13 years (relative risk [RR] = 0.90; 95% confidence interval [CI], 0.79 to 1.02); four trials with suboptimal randomization showed a significant reduction in breast cancer mortality (RR = 0.75; 95% CI, 0.67 to 0.83): The RR for all seven trials combined was 0.81 (95% CI, 0.74 to 0.87). The authors found that breast cancer mortality was an unreliable outcome that was biased in favor of screening, mainly because of differential misclassification of cause of death. The trials with adequate randomization did not find an effect of screening on cancer mortality, including breast cancer, after 10 years (RR = 1.02; 95% CI, 0.95 to 1.10) or on all-cause mortality after 13 years (RR = 0.99; 95% CI, 0.95 to 1.03). Numbers of lumpectomies and mastectomies were significantly larger in the screened groups (RR = 1.31; 95% CI, 1.22 to 1.42) for the two adequately randomized trials that measured this outcome; the use of radiation therapy was similarly increased. Authors' Conclusions: Screening is likely to reduce breast cancer mortality. Because the effect was lowest in the adequately randomized trials, a reasonable estimate is a 15 percent reduction corresponding to an absolute risk reduction of 0.05 percent. Screening led to 30 percent overdiagnosis and overtreatment, or an absolute risk increase of 0.5 percent. This means that for every 2,000 women invited for screening over 10 years, one will have her life prolonged, and 10 healthy women who would not have been diagnosed if there had not been screening will be treated unnecessarily. Furthermore, more than 200 women will experience important psychological distress for many months because of false-positive findings. It is not clear whether screening does more good than harm. To help ensure that women are fully informed of benefits and harms before they decide whether to attend screening, the authors have written an evidence-based leaflet for patients, which is available at http://www.cochrane.dk/screening/mammography-leaflet.pdf.