A radiologist checks the results of a mammogram. (Damian Dovarganes, Associated…)
About 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 overdiagnosis of 1.3 million American women.
The report, published Thursday 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 overdiagnosis 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 overdiagnosis 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 healthcare 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 'overdiagnosis' and 'overtreatment' 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%, 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.
Welch said the gap was even more apparent when they factored in data for women under the age of 40, who are generally too young to get regular mammograms and therefore served as a control group of sorts. The rate of breast cancer deaths for women 40 and over declined by 28% over the 32 years, while deaths for younger women fell by 42%.
"There was a larger relative reduction in mortality among women who were not exposed to screening mammography than among those who were exposed," the authors wrote. "We are left to conclude, as others have, that the good news in breast cancer -- decreasing mortality -- must largely be the result of improved treatment, not screening."
Welch, an expert on screening and overdiagnosis for many forms of cancer, said the disparity between early- and late-stage diagnosis trends was probably the result of tumors being detected that were too small to ever cause clinical symptoms. He said it was likely that many of the suspicious cells revealed by screening mammography would have regressed, or never progressed to malignancy.
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.
"We hear the word 'cancer' and we all assume the definition that's in my medical dictionary -- it's a tumor that, left untreated, will inexorably grow and cause death," Welch said. "But now, as we look for really early forms of the disease, we realize the pathologic definition of cancer includes abnormalities that may come and go."
What's more, cancers can now be treated successfully even after they grow large enough to cause symptoms, thereby reducing the benefit of catching them at the earliest stages. He compared screening for breast cancer to screening for pneumonia: "Since pneumonia can be treated successfully, no one would suggest that we screen for it."
The study was roundly criticized by radiologists.