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Why mammograms haven’t cut cancer deaths, explained in 500 words

October is Breast Cancer Awareness month, and with the flurry of pink ribbons comes new studies about the harms and benefits of mammograms for breast-cancer screening. Every year, confusion ensues.

The whole idea behind cancer screening is that it can help find the disease earlier — even before symptoms appear. With early breast-cancer detection, doctors should, in theory, be more likely to successfully treat the disease and control any spread.

New England Journal of Medicine

But as this chart, from the latest issue of the New England Journal of Medicine, shows, the introduction of mass mammography screening — for all women over the age of 40 — has failed to deliver on this promise.

Despite widespread screening programs, incidences of metastatic cancers (or advanced-stage cancers that have already spread and are therefore more deadly) have remained stubbornly stable since 1975.

Meanwhile, the rate of finding small ("invasive") tumors has increased by 30 percent.

This gets at the heart of the controversy about breast-cancer screening. If mass mammography was a total public-health success, we’d see the red line for metastatic cancer drop at the same time the blue line for small cancers rises.

Instead, researchers increasingly worry, mammography has failed to reduce illness and death in women with advanced-stage disease. Conversely, more women with tumors that may have never advanced to anything serious are undergoing difficult, costly procedures like surgery and chemotherapy.

This is far from the first time researchers have questioned the effects of mass mammography. A 2015 study, published in JAMA Internal Medicine, found more breast cancer screening didn't actually save lives. It did however lead to more "overdiagnosis" — detecting cancers that would not have been fatal or even harmful.

In more than 500 US counties, that study found, mammography screening programs were linked with more diagnoses of smaller breast cancers — but not with fewer deaths or with fewer larger breast cancers (which theoretically would be prevented by finding the disease early through screening). So, again, more patients and treatments, but not fewer deaths.

breast cancer
How to think about the benefits and harms of mammography.
Harding Center for Risk Literacy

While this means mass mammography programs targeting all women may be delivering more harms than benefits and should therefore be reconsidered, it doesn’t mean no one should get screened.

Right now, health groups offer different advice about when women should start annual screening. The American College of Obstetricians and Gynecologists has some of the most aggressive guidance, recommending age 40 as the start time. Meanwhile, the US Preventive Services Task Force — an evidence-based group that governments and insurers usually follow — suggests screening every two years starting at age 50.

So the decision is really left to women and their individual values, preferences, and risks. Those with a family history of the disease, the BRCA1 and BRCA2 gene mutations, or habits like smoking may consider initiating screening sooner. Those who don’t may consider waiting.

Doctors can also help patients by more often explaining both the harms and risks of mammography and factors that contribute to their individual breast-cancer risk profiles.