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By Maggie Fox

A new study adds to the confusing questions about mammograms and how many lives they save.

It found that while mammograms do indeed find breast tumors when they are very small and presumably the easiest to treat, widespread breast cancer screening doesn’t necessarily lower the overall death rate from breast cancer or even cut back on the number of biggest breast tumors found later.

“We found that counties with the most screening had substantially more breast cancers being diagnosed,” said Charles Harding, a Seattle data analyst who worked with a team at Harvard and Dartmouth universities on the study.

The team studied 16 million women aged 40 who were included in a cancer registry covering 547 U.S. counties. They included women who got mammograms. In the year 2000, 53,207 of the women were diagnosed with breast cancer. The researchers followed up for 10 years.

“An absolute increase of 10 percentage points in the extent of screening was accompanied by 16 percent more breast cancer diagnoses but no significant change in breast cancer deaths,” the team wrote in the Journal of the American Medical Association’s JAMA Internal Medicine.

“We did not find any significant change in breast cancer deaths."

“We did not find any significant change in breast cancer deaths,” Harding said. “This was quite surprising to me,” he added. “I think this tells us that the potential of overdiagnosis is high.”

Overdiagnosis means women were diagnosed with and almost certainly treated for tumors that never would have bothered them or caused symptoms during their lives.

“We increasingly realize that in clinical medicine, the more we look, the more we find,” said Dr. Joann Elmore of the University of Washington, who wrote a commentary on the findings.

“So this study is not surprising.”

It’s at the center of the debate over what to tell women about mammograms. The idea is, of course, to catch cancer early, when it’s easiest to cure.

But even the experts disagree on how early is too early. The American Cancer Society says women over 40 should get a mammogram every year. The U.S. Preventive Services Task Force, which advises the federal government, says women over 50 should get a mammogram every other year. It says women 40 to 49 should decide what they want, based on their health history, and it's not clear if women over 75 should bother with mammograms.

In 2014, Canadian researchers found 22 percent of the women screened got diagnosed with cancers that would never have harmed them. In 2012, a study published in the New England Journal of Medicine found that as many as a third of cancers detected through routine mammograms may not be life-threatening. It said 1 million women may have been overdiagnosed.

And a study published in 2014 found that women aged 50 to 74 who got screened every other year were no more likely to have advanced stage cancer or big tumors than women screened more frequently — even if they had so-called dense breasts, which are harder to read on an X-ray.

Frequent mammograms cost the country $4 billion a year, and there’s evidence many women who get suspicious-looking readings or who actually get diagnosed with a tumor are traumatized.

The answer is to find out which women need the most frequent screening, Harding’s team said.

“As is the case with screening in general, the balance of benefits and harms is likely to be most favorable when screening is directed to those at high risk, provided neither too frequently nor too rarely, and sometimes followed by watchful waiting instead of immediate active treatment,” they wrote.

“You do not know whether the women who were screened for breast cancer were the ones who actually were diagnosed or the ones whose lives were actually helped by breast screening."

Elmore says doctors need to get better at saying “I don’t know." “Part of informed decision making is providing all the information, even our uncertainty,” she wrote in a commentary.

Harding’s team also found more screening did not reduce the rate of larger breast tumors, defined as those 2 cm or larger. It’s impossible to tell if women who had tiny tumors removed ever got bigger tumors later, or even which women it was who got diagnosed with larger tumors.

Elmore said the way the study was conducted leaves many questions unanswered.

“You do not know whether the women who were screened for breast cancer were the ones who actually were diagnosed or the ones whose lives were actually helped by breast screening,” she said.