April 11, 2014


Are Mammograms Worth It? (EMILY OSTER, 4/10/14, 538)

Putting all of these results together, things don't look good for mammography. It would be a shame to stop with this study, though, since it's only one of many large randomized controlled trials of mammography. If we want the entire picture, we need to look at all of them together.

Most of the work is already done for us by Cochrane Reviews, which published on this topic in 2013. The Cochrane Reviews are a series of summary documents on a whole host of medical questions. Their goal is to aggregate information from individual randomized controlled trials to provide evidence-based guidance on best practices.

In the case of mammograms, the review in question aggregated eight large randomized trials encompassing more than 600,000 women. All of these trials had a similar structure to the Canadian trial: They divided women into two groups, and one group got mammograms with some frequency over a period of several years while the other group got "usual care." The researchers then compared breast cancer deaths, diagnoses and treatments between the two groups.

One of the key advantages of these Cochrane Reviews is that they try to say something about the quality of each study they cover. In this case, the authors argue that three of the large trials were well-randomized and unlikely to be biased, and five were less well randomized and more likely to be biased. What it means to be "sub-optimally randomized" varies across trials, but to give one example: in a large trial in New York, which started in the 1960s, more than twice as many people with a history of breast cancer were excluded from the mammogram group than from the control group. This suggests more women with previous breast cancer were included in the control group, thus biasing the conclusions in favor of mammograms.

When we focus on the high-quality trials (the Canadian study is one of these), the Cochrane Reviews' authors found those who were screened with mammograms were only slightly less likely to die from breast cancer in the seven or 13 years following the trial. This effect was not statistically significant. And, perhaps more important, they were no less likely to die overall.2

It's not that mammograms do nothing. Women who were randomized into the mammography group were much more likely to be diagnosed and treated for breast cancer -- this was true for all the studies. And it starkly illustrates the over-diagnosis issue. In the control group, some small tumors were not detected or treated, but they were detected in the mammogram group, hence the higher diagnosis rates in the latter group. And yet women in the control group were no more likely to die of breast cancer. This suggests those tumors that were missed were often not fatal.

Posted by at April 11, 2014 1:55 PM

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