Roughly a third of all tumors discovered in routine mammography screenings are unlikely to result in illness, according to a new study that says 30 years of the breast cancer exams have resulted in the over-diagnosis of 1.3 million American women.
The report, published today in the New England Journal of Medicine, argues that the increase in breast cancer survival rates over the last few decades is due mostly to improved therapies and not screenings, which are intended to flag tumors when they are small and most susceptible to treatment. Instead, the widespread use of mammograms now results in the over-diagnosis of breast cancer in roughly 70,000 patients each year, needlessly exposing those women to the cost and trauma of treatment, the authors wrote.
“Our study raises serious questions about the value of screening mammography,” wrote Dr. H. Gilbert Welch, an epidemiology and biostatistics professor at Dartmouth College’s Geisel School of Medicine. “It clarifies that the benefit of mortality reduction is probably smaller, and the harm of over-diagnosis probably larger, than has been previously recognized.”
The study adds to a long-running controversy over screening mammography. Its conclusions are based upon an increasing recognition that sophisticated screening can detect ever-smaller groups of cancer-like cells that would never become dangerous if left alone.
Some radiologists and other proponents of routine annual screenings denounced the study as harmful to women and said it was part of a coordinated campaign to cut back on the tests to reduce health care expenses.
“This is simply malicious nonsense,” said Dr. Daniel B. Kopans, a senior breast imager at Massachusetts General Hospital in Boston. “It is time to stop blaming mammography screening for ‘over-diagnosis’ and ‘over-treatment’ in an effort to deny women access to screening.”
Welch and Dr. Archie Bleyer, an oncologist in Bend, Ore., analyzed screening data collected by the National Cancer Institute and the Centers for Disease Control and Prevention. They found that between 1976 and 2008, diagnosis of early-stage cancers had more than doubled from 112 to 234 cases per 100,000 women. Meanwhile, the number of late-stage cancers fell 8 percent, from 102 cases to 94 cases per 100,000 women.
The authors reasoned that if screening were to help patients, it must not only find more early-stage cancers – which it has – it must also reduce the incidence of late-stage cancers, because the tumors would have been eliminated when they were small.
But the data found that this was not the case. For every 122 additional breast cancers detected early, the number of late-stage cancers fell by only eight. To the authors, this suggested something other than screening was largely responsible for the drop in breast cancer deaths.
The idea that breast tumors may resolve on their own has been advanced by European doctors for several years. Welch and others suspect that screening technology has allowed doctors to find cells that may resemble cancer, but whose true nature is only beginning to be understood.
“Our ability to detect things is far ahead of our wisdom of knowing what they really mean,” he said.
The study was roundly criticized by radiologists who specialize in breast imaging, who questioned its methodology and the suggestion that some cancer-like growths should be ignored.
“It’s kind of unbelievable that they’re telling us we’re finding too many early-stage cancers,” said Dr. Stamatia Destounis, a breast imager in Rochester, N.Y. “Isn’t that the point?”
Dr. Stephen Feig, a radiologist at the University of California, Irvine, and president of the American Society of Breast Disease, called the study scientifically weak and worried that it would dissuade women from undergoing screening. “This is actually harming women,” he said.