Mammograms don't reduce the number of women dying from breast cancer, according to a large and long-term Canadian study. It's the latest chunk of data to raise questions in an increasingly partisan debate about the use of mammograms to screen for cancer.

Improvements in breast cancer treatment since the study began in 1980 have reduced the importance of mammograms in saving lives, according to the study, which was published Tuesday in the British journal BMJ. The study involving almost 90,000 women found no difference in death rates between those who had mammograms and those who had annual physical exams performed by trained nurses.

There's "too much mammography," declares an editorial accompanying the study by a Norwegian cancer researcher.

That editorial notes that most of the randomized controlled trials showing benefits for screening mammography were done in the 1960s, when women were much less aware of breast cancer risks, and before the advent of tamoxifen and other treatments that have done a lot to reduce breast cancer mortality.

The study also suggested that breast cancer is being overdiagnosed, with about 1 in 5 cancers that are found through screening mammography turning out to be a kind that would have posed no lethal threat if the cases hadn't been found.

But because there's currently no way of knowing for sure which breast cancers will become life-threatening, all the cancers end up being treated. Other studies have come up with similar estimates for overdiagnosis.

"In technically advanced countries our results support the views of some commentators that the rationale for screening by mammography should be urgently reassessed by policy makers," the study authors wrote.

But not everybody agrees that there's too much mammography, that's for sure.

The American College of Radiology and Society for Breast Imaging called the Canadian results an "incredibly misleading analysis" based on a "deeply flawed and widely discredited" study. They point to the fact that the Canadian study put women who had breast lumps or enlarged lymph nodes at the start of the study in the group to get mammograms, violating a key tenet of a randomized controlled trial. And many of the mammograms were of poor quality, the radiologists say.

But this study has been used to craft mammography guidelines by key groups in the United States, including the American Cancer Society and the U.S. Preventive Services Task Force.

"When the task force reviewed screening mammography in 2009, we had eight studies for women in their 40s, and this was one of those," says Dr. Michael LeFevre, a professor of medicine at the University of Missouri School of Medicine and a member of the task force, an independent panel that makes recommendations for medical practice guidelines in the United States.

People had speculated that even though the Canadian study didn't show a benefit for mammograms in earlier years, it would prove their value in the long term. "This says, well, it wasn't that," LeFevre told Shots.

LeFevre was part of that 2009 panel. It recommended that women in their 50s and 60s should get mammograms only every other year. It also said that women in their 40s should decide whether or not they want a mammogram based on their personal risk. That guideline sparked fierce criticism from breast cancer advocacy groups and the American Cancer Society, which recommends that all women start annual mammograms at age 40.

In 2009, LeFevre says, people were unwilling to question the "hard-won belief that mammography is critically important." Now, he says, more people are willing to look at it.

Because the Canadian study ran for 25 years, it gives a much better sense of the long-term value of mammography, and also of the true risks of overdiagnosis, LeFevre says.

And though eight studies on screening mammography may sound like a skimpy basis for national guidelines, there are even fewer studies on the risks and benefits for women in their 50s and 60s, LeFevre notes. "After age 70, there's basically no data at all."

That's unfortunate, because breast cancer remains largely a disease of age. The older a woman is, the more likely she is to get breast cancer, and to die from it. LeFevre says he fears that women in their 60s will become less likely to get screened, at a time when they really could benefit. "I worry that women are tired of screening by then."

Women should be getting some new perspective on the question soon.

Both the U.S. task force and the American Cancer Society have launched reviews of the evidence to update their mammography guidelines, with the society's panel expected to complete its review later this year. The U.S. Preventive Services Task Force review will take 12 to 18 months, LeFevre says, and should be completed in 2015.

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