It is widely believed in the realm of medicine that annual mammography is a vital preventative tool against breast cancer. But in a recently reported long-term study of women age 40-59 conducted in Canada, the practice was found not to appreciably reduce cancer death rates after all. This has led to questions as to whether mammography has created a trend of “overdiagnosis” of the disease.
“We found absolutely no benefit in terms of reduction of deaths from the use of mammography,” said study leader Dr. Anthony Miller of the University of Toronto.
The results of the study, as reported Tuesday in the British Medical Journal, may bolster the argument raised by the U.S. Preventive Services Task Force in 2009 that most women under 50 could safely forego the test. This position was based on the finding that the chances of a 40-year-old woman contracting and dying from invasive breast cancer were well below 1%.
The data from the Canadian study suggests that about half of cancers detected by mammography are so small that they would never have become dangerous if left alone; thus the phenomenon of “overdiagnosis.”
The study sampled 89,835 women over a period of 25 years. The entire sample received annual physical breast examinations, while half of them received yearly mammogram screenings for five of those years. The number of breast cancer diagnoses in each group, as well as the number of subsequent deaths, were nearly equal. The mammography arm of the study was somewhat more likely to be diagnosed but no more likely to survive the disease. The research team calculated a 22% rate of overdiagnosis.
According to Dr. Miller, “Modern treatment is so much more effective now that the lead time gained by mammography has little impact on the outcome.”
Dr. H. Gilbert Welch at Dartmouth College’s Geisel School of Medicine co-wrote a book titled “Overdiagnosed: Making People Sick in the Pursuit of Health.” Welch applauded the evidence presented in the Canadian study: “People in the cancer community and the cancer surgery community are aware of the problem of overdiagnosis. They’re aware that mammography was oversold-that its benefits were exaggerated and its harms were kind of downplayed.”