In a research finding that will add to doubts about the value of breast cancer screening, Canadian researchers determined that women ages 40 to 59 who had yearly mammograms enjoyed no added survival benefit up to 25 years later compared with those who skipped the screening X-rays.
The trial, involving 90,000 women who were randomly assigned to have screening or be in a control group, reached a conclusion at odds with other large studies, which found fewer breast cancer deaths among women who began screening at age 50 or, in a handful of studies, at age 40. The latest finding, published Tuesday in the British Medical Journal, confirms earlier results from the Canadian trial, which first came out two decades ago and is unlikely to discourage many doctors from recommending screening.
But the researchers also determined that 22 percent of breast cancers initially detected on mammograms in the early 1980s were “over-diagnosed”—meaning they never would have been found otherwise and would not have become life-threatening. Most of these women had surgery to remove these tumors which, in hindsight, was unnecessary.
“Overdiagnosis of breast cancer is an important phenomenon that’s been demonstrated by us and other researchers,” said study co-author Dr. Cornelia Baines, a professor emerita at the University of Toronto.
Such findings haven’t, though, swayed the opinions of most women and doctors, who remain more concerned about a life-threatening cancer going undetected than a harmless one being over-treated
“Overdiagnosis occurs with every disease,” said Dr. Richard Wender, chief cancer control officer of the American Cancer Society. “We treat everyone with hypertension but not everyone will end up dying of stroke or a heart attack.”
Combined evidence from eight large clinical trials including the Canadian one indicate that mammography offers a small survival advantage: Women in their 40s who get screened every year or two have a 15 percent reduced risk of dying from breast cancer compared with those who skip the screenings, according to Wender. Women in their 50s have a 20 percent reduced mortality risk.
For this reason, he added, the American Cancer Society recommends that women have yearly mammograms beginning at age 40, though that advice may soften a bit when new recommendations are issued later this year and take into account the latest findings on risks of yearly screening.
The US Preventive Services Task Force, a group of primary care physicians convened by the federal government, stopped universally recommending mammograms for women in their 40s in 2009 after determining that its drawbacks—such as false findings that lead to anxiety, extra X-rays, and surgical biopsies—were about equal to its benefits for those in this age group. Their decision was met with fierce criticism and a Congressional mandate to provide coverage for mammography under the federal health law for all women age 40 and over.
“Mammography is an imperfect test at best, but at this point, it’s the best test we have,” said Dr. Ann Partridge, a breast oncologist at Dana-Farber Cancer Institute. She and others highlighted some potential methodological flaws of the Canadian study.
For example, Partridge said, technology has improved significantly over the past 30 years with X-ray machines and digitized film that yield clearer images.
Others have questioned whether the women in the Canadian study were properly randomized since a significantly higher number of women in the mammography group were diagnosed with advanced cancers during the first year or two of the study than those in the control group.
“This might have been due to chance,” Wender said, “but if the randomization wasn’t done perfectly, some women at higher breast cancer risk might have been put into the mammography group and this might have skewed the results.”
Some radiologists have sharply attacked the study investigators, accusing them of having a bias against mammography by designing a study in which the control group of women in their 50s received breast exams performed by skilled nurses every year instead of mammograms.
“The principal investigator set out to prove that all you needed to do was a physical examination,” said Dr. Daniel Kopans, director of breast imaging at Massachusetts General Hospital, in an e-mail responding to the new study finding. “The nurse examiners were highly trained while the radiologists and technologists [who performed the mammograms] had no training.”
Baines, who has frequently sparred with Kopans through the years, said radiology leaders have remained “really committed” to advocating for mammograms partly for financial reasons, including insurance reimbursements and consultant fees they’ve received from mammography machine manufacturers.
Where many screening experts have found common ground is in their belief that women should be better informed about the downsides of mammography as well as its small potential lifesaving benefit.
“This isn’t a public health imperative, it’s a choice,” said cancer screening researcher Dr. H. Gilbert Welch, a professor of medicine at the Dartmouth Institute, “and it’s a close call.”