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Mammography screening doesn’t change overall death rates, study update finds

The journal BMJ has published a new update from the long-running Canadian National Breast Screening Study.

More breast tumors were found among the women who had annual mammograms, but the rates at which the women died from the disease were statistically the same for both groups.
REUTERS/Enrique Castro-Mendivil

The controversy over mammography screening heated up again this week with the publication in the journal BMJ of the latest update from the long-running Canadian National Breast Screening Study.

The study has now tracked 89,835 women for 25 years. As it found at two earlier points in the study, women who underwent both an annual physical breast exam by a trained nurse and an annual mammogram between the ages of 40 to 59 did not have a lower risk of dying from breast cancer than women who had received only the annual physical breast exam.

More breast tumors were found among the women who had annual mammograms, but the rates at which the women died from the disease were statistically the same for both groups. Among the 44,925 women in the mammography group, 3,250 were diagnosed with breast cancer and 500 died from the disease over the 25 years of the study. Among the 44,910 women in the control group, 3,133 were diagnosed with breast cancer and 505 died from the disease.

The problem of overdiagnosis

The women in the mammography-screening group did experience more harm, however. One in five of those women (22 percent) were overdiagnosed with breast cancer during the course of the study. In other words, the mammography found a cancer that would never have caused symptoms or death if it had remained undetected. Most of the women in the study underwent surgery to remove the lesions.

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In recent years, scientists have come to understand that some breast cancers grow very slowly or even stop growing, while others grow exceedingly fast or metastasize (spread to other parts of the body) long before they can be detected by any currently-in-use screening technology.

“It is true that if you find cancer early it could be at a more treatable stage,” Dr. Anthony B. Miller, a physician and epidemiologist at the University of Toronto and the lead author of the study, told Medical News Today reporter David McNamee. “But there is no evidence that early detection affects the inherent biology of the cancer. Indeed, it is possible that finding the cancer at an earlier stage will result in undertreatment. There is some evidence that is so.”

Miller and his colleagues conclude in their study that “annual mammography does not result in a reduction in breast cancer specific mortality for women aged 40-59 beyond that of physical examination alone or usual care in the community. The data suggest that the value of mammography screening should be reassessed.”

“Nevertheless,” they add, “education, early diagnosis, and excellent clinical care should continue to be provided to women to ensure that as many breast tumours as possible are diagnosed at or less than 2 cm.”

‘Deeply flawed’

The American College of Radiology and Society of Breast Imaging immediately criticized the study, calling it “deeply flawed and widely discredited.” They also said that using the findings to develop breast-cancer-screening policy “would place a great many women at increased risk of dying unnecessarily from breast cancer.”

The radiologists claim, for example, that the mammography equipment originally used in the Canadian study is now out of date and that there were problems in how the women were randomized to the study’s two groups — charges the authors of the study anticipated and addressed in their paper.

“The greatest resistance [to the study’s findings] will come from radiologists, who are convinced that finding cancers results in benefit, and who also benefit financially from that belief,” Miller told Medical News Today. “We have demonstrated that is not so. Others whose advice resulted in the establishment of breast screening programs will also feel threatened.”

“It is unfortunate that although we maintain we should practice evidence-based medicine, when that evidence is produced, there will be many who will seek the means to discredit that evidence because it is against their vested interests,” he added.

Furor less fierce than before

So far, though, the furor over this study’s 25-year findings has not been as virulent as that over the 2009 recommendation by the U.S. Preventive Services Task Force (USPSTF) that women not at high risk for breast cancer wait until age 50 to begin screening mammograms, and that even then, getting a mammogram only every other year is fine. The USPSTF came to that conclusion after an exhaustive review of all existing research on the topic.

Perhaps women are starting to understand that, as one expert told New York Times reporter Gina Kolata, “The decision to have a mammogram should not be a slam dunk.”

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In other words, just as USPSTF emphasized when it issued its 2009 recommendations, women need to weigh the risks and benefits of mammography screening and then determine for themselves what kind of screening schedule works best for them.

“I think there’s growing realization that all is not well with mammography,” Dr. H. Gilbert Welch, an epidemiologist and biostatistics professor at Dartmouth College and the co-author of the book “Overdiagnosed: Making People Sick in the Pursuit of Health,” told L.A. Times reporter Monte Morin. “People in the cancer community and the cancer surgery community are aware of the problem of overdiagnosis. They’re aware that mammography was oversold, that its benefits were exaggerated and its harms were kind of downplayed.”

Attitudes and practices take a long time to change in medicine, however. But, as many breast-cancer advocates have been saying for years, women do not have the luxury of waiting for those changes. It’s time — long past time — to realize the limits of early detection and to start focusing more resources on preventing and curing the disease.