A comprehensive review of 50 year’s worth of international studies assessing the benefits and harms of mammography screening suggests that the benefits of the screening are often overestimated, while harms are underestimated. The authors report that the best estimate of the reduction in mortality from breast cancer due to annual screening for women overall is about 19 percent. For women in their 40s, the reduction in risk was about 15 percent, and for women in their 60s, about 32 percent. But how much a woman benefits depends on her underlying risk of breast cancer.
Background: The American Cancer Society estimates that about 40,000 U.S. women will die of breast cancer this year. In 2009, based on evidence that the benefit-risk ratio for mammography screening is higher among women over 50 and with less frequent screening, the U.S. Preventive Services Task Force (USPSTF) reversed its previous recommendation of mammography every one to two years beginning at age 40, and recommended routine screening every two years starting at age 50, the researchers noted. The recommendations remain controversial among the general public and the medical community. Recent evidence suggests that use of mammography in the U.S. has not changed following the updated recommendations.
“What I tell my patients is that the mammogram is not a perfect test,” said Nancy Keating, co-author of the report, associate professor of Health Care Policy at HMS and associate professor of medicine at Brigham and Women’s. “Some cancers will be missed, some people will die of breast cancer regardless of whether they have a mammogram, and a small number of people that might have died of breast cancer without screening will have their lives saved.”
The researchers estimated that among 10,000 women in their 40s who undergo annual mammography for 10 years, about 190 will be diagnosed with breast cancer. Of those 190, the researchers estimate that about 5 will avoid death from breast cancer due to screening. About 25 of the 190 would die of breast cancer regardless of whether they have a mammogram or not. The rest will survive, thanks largely to advances in breast cancer treatment.
However, according to Keating, the chief harm associated with mammography is the risk of overdiagnosis. This is the diagnosis of cancers that never would have become clinically evident during a woman’s lifetime, either because the cancer never grew or because the patient died first of another cause. While it is impossible with current techniques to know which cancers we could safely observe and which need to be treated, the review cites findings that roughly 19 percent of women who are diagnosed based on findings from a mammogram are overdiagnosed. That means that roughly 36 of the 190 women who received annual mammography for 10 years and were diagnosed with breast cancer would receive unnecessary surgery, chemotherapy or radiotherapy.
My Take: I suspect that this study underestimates the benefits of mammograms, given the study includes results obtained with ancient techniques from decades ago! Still, most women at average risk overestimate the benefits of mammograms. I think that in the future, we will do a much better job of individualizing screening recommendations, incorporating risk factors such as breast density, age, family history, personal history, and perhaps race. Much of the controversy regarding mammograms today has to do with the fact that we have to screen so many to save a life. But what is the “right” ratio of screened to saved? That is a more challenging societal question.
I’m Dr. Michael Hunter.
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References: 1. Lydia E. Pace, Nancy L. Keating. A Systematic Assessment of Benefits and Risks to Guide Breast Cancer Screening Decisions. JAMA, 2014; 311 (13): 1327 DOI: 10.1001/jama.2014.1398; 2. Harvard Medical School. “The mammography dilemma: 50 years of analysis.” ScienceDaily. ScienceDaily, 1 April 2014. <www.sciencedaily.com/releases/2014/04/140401162150.htm>.