The routine use of mammography to screen healthy women for breast cancer is leading to the widespread detection and treatment of tumours that would never have caused symptoms, a study in the New England Journal of Medicine asserts today1, injecting an additional dose of controversy into an area already rife with it.

The study examined screening mammography’s effects on breast cancer incidence in American women over 40 years old between 1976 and 2008. The authors concluded that more than a million women diagnosed with the disease would never have developed symptoms and would have died of other causes. In 2008 alone, they write, more than 70,000 women had such tumors diagnosed, accounting for 31% of all breast cancers diagnosed in women over 40.

“Our study raises serious questions about the value of screening mammography,” the paper concludes. “The harm of overdiagnosis [is] probably larger than has been previously recognized.” Women, the authors add, have undergone surgery, radiation, hormone therapy and chemotherapy “for abnormalities that otherwise would not have caused illness”.

The study does not address the situation of women with an inherited genetic predisposition to breast cancer, who tend to get aggressive disease at much younger ages and should be actively followed with mammography.

The findings come amidst ongoing controversy over breast cancer screening. In 2009 the US government’s Preventive Services Task Force caused an uproar when it recommended against routine screening for women in their 40s. Last month a review commissioned by the UK Department of Health found that there was substantial overdiagnosis, but concluded that the country's national screening programme was still worthwhile2.

The study has already sparked both applause and harsh criticism. The American College of Radiology (ACR) in Reston, Virginia, which represents the physicians who interpret mammograms, called it “deeply flawed and misleading”. But Fran Visco, the president of the National Breast Cancer Coalition in Washington, DC, says that the benefits of mammography have long been overstated. Today’s study, she says, adds to “more and more data that show that mammography screening is not what we have been led to believe”.

The authors, Archie Bleyer, a consultant oncologist at the St. Charles Regional Cancer Center in Bend, Oregon and H. Gilbert Welch, a professor of medicine at Dartmouth’s Geisel School of Medicine in Hanover, New Hampshire, examined the change in incidence of early-stage and late-stage breast cancer after screening mammograms began to be used by large numbers of women in the mid-1980s.

As expected with the advent of a widespread screening programme, diagnoses of early-stage disease more than doubled during the three decades, with an increase of 122 cases for every 100,000 women. The authors argue that if those cancers were indeed being stopped from progressing to later, more harmful disease — in other words, if the screening was working as intended — a roughly equivalent decrease in late diagnoses would be expected. Instead, they found that cancers diagnosed late fell off by only 8 cases per 100,000 women. Their inference: many of the early cases being detected through screening mammography would not have gone on to cause symptomatic disease. They also conclude that mammography’s contribution to the sharp decline in breast cancer mortality rates during the same period must therefore be small.

Colin Begg, Chairman of the Department of Epidemiology and Biostatistics at Memorial Sloan-Kettering Cancer Center in New York City, who was not involved in the study, says that there is unquestionably a large number of excess diagnoses due to screening. But, he adds, the study is on “much shakier ground” in minimizing mammography’s contribution to declining mortality.

“This is a critical issue, since I suspect women contemplating screening are much more concerned about preventing death from breast cancer than they are concerned about the impact of a false positive diagnosis,” says Begg. He adds: “The [positive] impact of screening on mortality has been studied in numerous randomized screening trials, and they provide much more compelling evidence on this issue than this descriptive, retrospective analysis.”

The paper “is going to lead women to think that mammography is terrible,” said Daniel Kopans, a professor of radiology at Harvard Medical School in Boston who helped draft the ACR’s statement. “Some women will avoid screening as a result, maybe a lot of women. And they will die.”

“We are not against screening,” Bleyer retorts. “It does save lives. But a need to be more concerned about the harms becomes more apparent from the results of our study.”

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