Study casts doubt on mammography's ability to prevent late-stage cancers

Issue 21, Summer/Fall 2013

Review of 30 years of data suggests little impact on deadliest tumors, and overdiagnosis of early stage disease


The ongoing debate over the value of large-scale mammography screening programs heated up recently with a study suggesting that three decades of screening among U.S. women have done little to lower rates of advanced breast cancer.

The study, published late in 2012 in the New England Journal of Medicine, examined U.S. government health data from 1976 to 2008. Researchers tracked trends in the incidence of earlystage breast cancer (ductal carcinoma in situ and localized invasive disease) versus late-stage cancers that had spread beyond the breast and are typically more deadly.

The result? Since the advent of widespread mammography screening in the United States, the number of earlystage cancers detected (and typically treated) has doubled -- from 112 cases per 100,000 women in the 1970s to 234 cases per 100,000 by 2008.

At the same time, the number of late-stage cancers -- those that pose the highest level of threat to women -- barely budged, falling by just 8 cases per 100,000 over the 30-year study period.

As the authors point out, if the number of early-stage cases doubled, one would expect that that would result in a significant drop in late-stage disease. But that did not occur, suggesting that many of the early-stage cases detected -- causing women worry and intensive treatment might never have proven harmful, after all.

Thus, mammography screening may have led to an 'overdiagnosis' of early-stage breast cancers, without a significant drop in late-stage disease. This is a significant concern in the evaluation of cancer screening tests and has been observed for prostate cancer screening and lung cancer screening.

While it is true that a much smaller percentage of U.S. women died of breast cancer in 2008 than in 1976, that improvement is probably attributable to improved treatment, not early detection, the researchers said.

The findings come in the context of new recommendations on mammography issued in 2009 by a panel at the U.S. Preventive Services Task Force. The panel's recommendation -- that women in their 40s should no longer get an annual mammogram -- set off a firestorm of controversy among the breast cancer community.

This new study seems to support the USPSTF view, however. "Our data show that mammography has not largely met the first prerequisite for screening to reduce cancer-specific mortality -- a reduction in the number of women who present with late-stage cancer," wrote study co-authors Dr. Archie Bleyer of St. Charles Health System in Bend, OR, and Dr. H. Gilbert Welch of The Dartmouth Institute for Health Policy & Clinical Practice in Lebanon, NH.

Instead, they say, overdiagnosis of early-stage disease has occurred, "involving more than 1 million women in the past three decades" and more than 70,000 women in 2008 alone. They estimate that a full 31 percent of breast cancers diagnosed in women age 40 and older may have been "overdiagnosed" -- meaning these women underwent needless worry and unnecessary treatments for tumors that might have never progressed.

Many groups -- most notably the American Cancer Society -- still support annual mammograms for women aged 40 and above. And Bleyer and Welch acknowledge that the notion of breast-cancer overdiagnosis remains "a complex and sometimes contentious issue."

For now, they say, "Women should recognize that our study does not answer the question, 'Should I be screened for breast cancer?' However, they can rest assured that the question has more than one right answer."