What Does the Evidence Tell Us?
I’m always pleased to see new research that can help women make informed decisions about breast cancer prevention, detection and treatment. But I disagree with some fundamental aspects of a recent study that has renewed the debate over the value of screening mammograms. My recommendation for breast cancer screening remains unchanged by this recently published study. Women at average risk for breast cancer should have an annual mammogram beginning at age 40.
The study, published in the journal The Annals of Internal Medicine, makes claims about the overdiagnosis of breast cancer in women following screening mammograms. The authors looked at the rates of cancer diagnosis among women in Denmark over a 30-year period (1980 to 2010), and drew two main conclusions:
- breast cancer screening was not associated with a reduction in the incidence of advanced cancer, and
- many breast cancers detected through mammography would never pose a health threat and therefore represent “overdiagnosis” of breast cancer.
The authors estimate overdiagnosis to occur for somewhere between 9.9 and 49 percent of women who get screening mammograms. This wide range highlights how difficult it is to estimate the amount of overdiagnosis that occurs. Another study, also from Denmark, and published in the British Medical Journal in 2013, estimated the rate of overdiagnosis to be 2.3 percent. The American Cancer Society reviewed the literature supporting screening mammography in a paper published in 2015. They concluded that assumptions required to determine estimates of overdiagnosis in published studies are unverifiable. In other words, given the current methods of analysis, it is impossible to know with certainty the rate of overdiagnosis.
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This recent paper, which received much media attention, did not evaluate the effect of screening mammography on breast cancer mortality. Multiple different types of scientific trials have demonstrated that screening mammography lowers the risk of dying from breast cancer by 25–35 percent. This is the data that should inform our decision-making regarding screening mammography. We know that overdiagnosis occurs, but at a much lower rate than reported in this paper, and it does not change the fact that screening mammography lowers your risk of dying from breast cancer.
For these reasons, I do not expect that any major guideline-setting organizations — such as the American Cancer Society and National Comprehensive Cancer Network — will make any changes to their recommendations on screening mammography based on this new study.
I hope future technologies and tools will help us to more accurately distinguish between breast cancers that should be treated and those that will never become clinically significant. For now, I will continue to recommend annual screening mammography for women starting at age 40, as supported by extensive medical evidence. This recommendation saves the most lives. This is what I advise for my patients, friends and family and what I follow myself.