Benefits of Screening Mammography

Abstract

Breast cancer is a leading cause of premature death for women in their forties, and mammographic screening significantly reduces the risk of death due to breast cancer. The latter was the conclusion of the United States Preventive Services Task Force (USPSTF) meta-analysis of the randomized controlled trials (RCTs) of screening mammography1. The USPSTF concluded, however, that the absolute benefit from routine mammographic screening of women in their forties was too low to offset the harms. As a woman in her forties, and a radiologist specializing in breast imaging, I find their conclusions puzzling. RCTs underestimate the true benefits of mammographic screening2. RCTs assess mortality reduction among women randomized to the group invited to be screened rather than those who actually participated in screening. In an RCT, an invited woman who never has a mammogram and dies of breast cancer is counted as having been screened, just as a woman in the control group who decides to have a mammogram outside the trial is counted as having not been screened. In several RCTs, mammographic quality was quite poor or only a single mammographic view was obtained, both of which reduce cancer detection. Two observational studies among women who underwent screening mammography (with the current standard of two views of each breast), comprising millions of person-years of observation of both screened and unscreened populations, reported a 30 to 40% reduction in breast cancer mortality for women in their forties3, 4. However, these high-quality observational data were not considered by the USPSTF, which limited its focus to RCTs. For women aged 40 to 49 in a screening program, on average, about 1 in 460 will be diagnosed with breast cancer in any given year; during that decade of life, one in 69 women will be diagnosed with invasive breast cancer2. The incidence of breast cancer increases with age, particularly above age 40, with the rates at ages 45 to 49 virtually identical to those at ages 50 to 54, and no abrupt change at age 502. At least a third of potential years of life lost to breast cancer occur among women who died of breast cancer after being diagnosed in their forties5. Over 40,000 women die from breast cancer each year, of whom 18% were diagnosed during their forties (personal communication, Carol DeSantis, MPH, American Cancer Society, December 10, 2009); assuming a 30% reduction in breast cancer deaths due to screening, annual mammography prevents over 2000 deaths per year among women diagnosed in their forties in the United States. Mammographic screening also increases the chance of detecting breast cancer small enough to be treated by lumpectomy instead of mastectomy and decreases the need for chemotherapy. Harms of treating more advanced disease and the increased risk of recurrence for women not screened by mammography were not considered by the USPSTF.