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Screening mammograms have only a modest impact on breast-cancer deaths, study finds

A new study from Norway is likely to reignite the ongoing heated debate about the risks versus benefits of routine screening mammograms.

A new study from Norway is likely to reignite the ongoing heated debate about the risks versus benefits of routine screening mammograms. The study, published today in the New England Journal of Medicine, reports that the central benefit attributed to screening — saving lives — is much, much smaller than previously believed.

As Marie McCullough writes in the Philadelphia Inquirer:

Much of the mammography debate rests on now-outdated studies done from the 1960s through the 1980s in which women were randomly assigned to get mammograms or not.
The latest study, in contrast, uses data from Norway’s public health-care system, which gradually phased in breast cancer screening across the country starting in 1996. (Before that, mammograms were mostly used to diagnose palpable lumps.) Unlike U.S. health plans, Norway covers the test only for women ages 50 to 69, and only every other year.
The nine-year phase-in of the program enabled researchers to tease out how much of the reduction in deaths was due to earlier cancer detection with mammography, vs. greater awareness and better treatments. The study analyzed data from more than 40,000 women of all ages who developed breast cancer in the decades before and after screening was introduced. Of these, 4,791 died of the disease.
It turned out that overall, breast cancer mortality fell 28 percent in the decade after screening began, but it dropped even among women who weren’t getting mammograms.
The researchers concluded that 10 percent of the overall mortality reduction – about a third – was directly linked to screening, while the rest was due to better, more timely treatment, such as surgery, radiation, and chemotherapy.

But even that 10 percent reduction may not be attributable to screening, as science writer Rena Xu notes at the Now@NEJM blog. “Before enrollment in the program, each county had to establish multidisciplinary breast cancer management teams,” she points out. “As a result, the benefits of integrated, comprehensive care may have contributed to the observed relative reduction in mortality.”

In other words, this study suggests that for breast cancer screening to be most effective at saving lives, it needs to be part of a broader integrated medical care system that’s available to everybody. (Health-care policy makers, take note.)

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Xu also translates that 10 percent into hard numbers: “The current 10-year risk of breast-cancer mortality for a 50-year-old woman is estimated to be about 4 in 1,000. Assuming the benefits of screening are already accounted for, this means that screening avoids 0.4 deaths per 1,000 women.  Put another way, 2,500 women would need to be screened over 10 years for a single breast cancer death to be avoided.”

And what about the 2,499 women who underwent screening to achieve this benefit? “What happens to [them] is also relevant,” writes Dr. H. Gilbert Welch of the Dartmouth Medical School in an editorial that accompanied the new study. “Estimates of harm vary considerably. In the United States, more than 1,000 women would be expected to have at least one false positive result, a number that would be considerably lower in Europe. Less frequent but more worrisome is the problem of overdiagnosis. Somewhere between 5 and 15 women would be expected to be needlessly treated for a condition that was never going to bother them, with all the accompanying harms.”

The 10 percent benefit from screening found in this study is considerably less than the 15-23 percent mortality reduction estimated by the U.S. Preventive Services Task Force last fall, Welch also points out. The task force’s recommendations — basically, that most women don’t need screening mammograms until age 50, and even then they don’t need them more than every two years — set off a firestorm of public anger. People claimed (unfairly) that the task force was trying to ration necessary care.

But attitudes may be shifting.

“[T]he public widely perceives screening mammography to be one of the most important services provided by modern medicine,” writes Welch. “The perception is largely the product of well-crafted public health messaging, such as the American Cancer Society’s print campaign in the 1980s that featured the headline ‘If you haven’t had a mammogram, you need more than your breasts examined.’ Given current data, such messaging must become more balanced.”

It’ll be interesting to follow the response to this latest study.

For a detailed analysis of the study, I suggest John Gever’s summary at MedPage Today or Gina Kolata’s article in the New York Times.