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Catching cancer early in a mammogram can be life-saving — smaller tumors are  easier to remove surgically, and therapy often has a much greater effect. But paradoxically, breast cancer screening also sometimes picks up tumors that would have caused less harm if they’d remained hidden.

These cases, known as “overdiagnoses,” may never go on to pose a threat to a patient’s health for a number of different reasons. A new study, published Monday in Annals of Internal Medicine, suggests they occur in 1 of 7 breast cancer cases detected during screening. That new estimate comes as a relief to breast cancer clinicians, who say that the study should reinforce the idea that the benefits of mammography generally outweigh its risks. Still, experts said, it doesn’t minimize the real danger of overdiagnosis or the need to effectively communicate the risks and benefits of screening to patients.

“Honestly, [the study] is reassuring,” said Michael Hassett, an oncologist at the Dana-Farber Cancer Institute who did not work on the study. “Most of the cases we’re finding are not overdiagnosis cases and most are true cases. The problem we’re left with is less about overdiagnosis and more about how do we tailor the intensity of treatment to intensity of cancer.”

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Not all cancers will advance into invasive malignancies that spread across the body, eventually killing the patient. Some tumors will halt their progression at an early stage or grow so slowly that they’ll never pose a threat during the patient’s lifetime. In some cases, overdiagnosis can occur because a patient will die of other causes, such as old age or existing health conditions, before even a progressive cancer can cause an issue.

But on a mammogram, a silent, indolent tumor looks the same as a deadly, progressive cancer that just hasn’t yet spread. If a clinician finds cancer on a mammogram, it often still triggers a slew of follow-up treatments possibly including biopsies, surgery, chemotherapy, and radiation — all unneeded and harmful interventions if the cancer was destined to remain silent.

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That presents a trade-off to screening that can only really be understood if researchers know how often overdiagnoses occur during breast cancer screening. The trouble has been that scientists haven’t been able to agree on an estimate.

“The range was from zero to 54%, so a huge range and virtually no consensus as to what the true rate [of overdiagnosis] was,” said Marc Ryser, a population health scientist at Duke University.

One of the most prominent of those estimates came from a 2012 New England Journal of Medicine paper that suggested roughly 31% of breast cancer cases were overdiagnoses. So, Ryser and colleagues at the Fred Hutchinson Cancer Research Center set out to create a new estimate, one based on the best available data and methods today.

“It was to correct the [poor] information that has been out there about breast cancer overdiagnosis for decades,” said Ruth Etzioni, a biostatistician at the Hutch and the senior author on the new study.

To create the estimate, the team drew data on 35,986 women between the ages of 50 and 74 from the Breast Cancer Surveillance Consortium that tracks breast cancer cases from screening. Of those patients, 64% were white, 19% were Asian, 12% were Black, and 11% identified as Hispanic. Using those data, the scientists built a model that estimated the fraction of tumors that never progress, the share that do progress, and the average amount of time it would take for them to become symptomatic. They also modeled the rate of death from other causes in women.

Aside from predicting that overdiagnoses occurred in about 15% of breast cancer cases detected during screening, the team also found that one-third of overdiagnoses came from indolent or non-progressive tumors. The other two-thirds were progressive cancers, but the patients died of a different cause before the cancer could cause trouble. This was generally truer for older women than younger ones in the model.

The authors hope these findings  will put to rest some of the questions about breast cancer overdiagnosis and help provide more nuanced information for women deciding whether to get screening. “We all want to live long, healthy, good lives, right? The screening decision is one of many we make about our health,” Etzioni said. “I really hope this will empower women so they know what the chances are overall.”

“The rigor and methods they used are very strong,” said Katrina Armstrong, a professor of medicine at Harvard Medical School who did not work on the study. “Hopefully this will reduce the controversy about the risk of overdiagnosis and help us move forward as a scientific community.”

While the new study estimates overdiagnoses happen less often than some had previously thought, Armstrong said that it suggests that overdiagnoses remain a real risk. By this estimate, about 25,000 women per year in the United States may be going through unneeded treatment for an overdiagnosed breast cancer, she wrote in an accompanying editorial.

“Most women over 50 these days believe that the benefits of mammography screening are worth the potential harms. That number in the study should reassure women there isn’t a need to make a big change in their individual mammography,” Armstrong said. “But it’s still a lot of women who are undergoing treatments that aren’t needed. As a medical community we have a responsibility to reduce that harm.”

That responsibility raises different and more difficult questions. Doctors still are figuring out how to tell the difference between a non-progressive tumor and one that will eventually invade the rest of the body. Breast cancers known as ductal carcinomas in situ, for example, are less likely to progress than other malignancies detected during screening. The question, then, is how to appropriately treat them.

“Perhaps overdiagnosis is not as big of a problem as we thought. Then maybe it’s helpful to detect those cancers, but know that they’re so indolent our approach should be less intensive,” said Dana Farber’s Hassett. “We need to make sure that when we do find low- and high-risk cancer based on screening, our treatments are tailored.”

He added that if clinicians do see a tumor that they think isn’t likely to become dangerous, it’s also still an open question as to how to tell the patient about it.

“Whenever I tell a woman she has breast cancer, that has an impact on her quality of life and her mental state,” Hassett said. “That’s an important question.”

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