Study Finds No Advantage to Annual Mammography
When researchers in Canada compared breast cancer incidence and mortality up to 25 years in women aged 40-59 who did or did not undergo mammography screening, the results surprised nearly everybody. The ubiquitous, widely-promoted annual breast cancer screening procedure was found to be ineffective at reducing mortality outcomes in women under age 60 and led to overdiagnosis an astonishing 22% of the time.
The study was a follow-up of the Canadian National Breast Screening Study, a randomized trial analyzing 15 cancer screening centers, the study’s central office data and information from cancer registries and vital statistics databases. As reported in BMJ, between 1980 and 1985, just under 90,000 women aged 40 to 59 were randomized to five years of annual mammography or no mammography. Women aged 40-49 in the mammography arm and all women aged 50-59 in both arms received annual physical breast examinations. Women aged 40-49 in the control arm received a single examination followed by usual care in the community. The main outcome measured was deaths from breast cancer.
After up to 25 years' follow-up, there was no discernible difference between groups in breast cancer mortality. Of the screen-detected tumors, 22% were overdiagnoses—one for every 424 women in the mammography arm.
Andrew Kaunitz, an OB/GYN with NEJM Journal Watch, wrote: "This important Canadian report documents the failure of screening mammography to impact mortality from breast cancer, as well as breast cancer overdiagnosis. Based on these and other recently published data, clinicians and women should move away from starting screens among women in their 40s and from screening annually. While we reevaluate the practice of screening mammography, adopting the U.S. Preventive Services Task Force 2009 recommendations (beginning screening in average-risk women at age 50 and screening biennially) would appear prudent."
Not all organizations agree on mammogram guidelines. For instance, the U.S. Preventive Services Task Force mammogram guidelines recommend women begin screening at age 50 and repeat the test every two years. The American Cancer Society and other organizations recommend screening begin at 40 and continue annually.
Mammogram screening isn't perfect. Studies have concluded, for example, that despite more women being diagnosed with early breast cancer due to mammogram screening, the number of women diagnosed with advanced breast cancer hasn't decreased. And now the BMJ-published findings reveal significant overdiagnosis. In between are findings that some women with early breast cancer were diagnosed with cancer that may never have affected their health.
Unfortunately doctors can't distinguish dangerous breast cancers from those that are non-life-threatening. But concerns about mammogram screening for breast cancer include a significant false-positive result risk, and exposure to low levels of radiation. The unavoidable fact is that annual mammography in women aged 40-59 does not reduce mortality from breast cancer beyond that of physical examination or usual care when additional assistive therapy for breast cancer is freely available. Overall, 106 out of 484 of screen-detected invasive breast cancers were overdiagnosed, representing one overdiagnosed breast cancer for every 424 women who received mammography screening in the trial.