In February, a study was published in the British Medical Journal (BMJ) that spawned a number of headlines around the U.S. including “Vast Study Casts Doubts on Value of Mammograms” (New York Times), “Mammogram screenings don’t reduce cancer death rates, study finds” (Los Angeles Times), and “Study disputes value of routine mammograms” (Washington Post).
The study in question was presented in a BMJ article titled “Twenty–five year follow–up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomized screening trial.” I have been asked by many colleagues and friends to weigh in on this study’s findings given my own expertise and research interests related to mammography and breast cancer.
|Dr. Etta D. Pisano|
There are many opinions being voiced by those in the field that are critical of the findings of the Canadian National Breast Screening Study (CNBSS). The American College of Radiology (ACR) issued a release on Feb. 12, stating that the British Medical Journal article “is an incredibly misleading analysis based on the deeply flawed and widely discredited Canadian National Breast Screening Study.” It goes on to state that the “results of this BMJ study . . . should not be used to create breast cancer screening policy as this would place a great many women at increased risk of dying unnecessarily from breast cancer.” Specific issues identified by the ACR include poor mammography quality (attributed to use of second-hand mammography machines which were not state–of–the art at the time of the trial, poorly trained technologists, and radiologists with no specific training in mammographic interpretation) and significant problems with randomization of study subjects.
Dr. Daniel Kopans, professor of radiology and senior radiologist in the Breast Imaging Division at Harvard/Massachusetts General Hospital, stated, “The analysis of the results from the CNBSS have been suspect from the beginning” and that the “compromise of the CNBSS trial is indisputable.”
Dr. Laszlo Tabar, professor emeritus of radiology at the University of Uppsala School of Medicine and former medical director of the Department of Mammography at Falun Central Hospital, stated that the “Canadian trials stand apart as the only ones among the published screening trials that failed to show any effect on mortality from breast cancer.” He goes on to state that “the Canadian trials are uniquely poorly designed and carried out and therefore have been much criticized for decades.”
There have been seven other large randomized trials of screening mammography that showed a mortality benefit from screening. Why should we believe this one that failed to show benefit when the other seven showed the opposite result?
I am in agreement with the statements issued by ACR, Kopans and Tabar. It is my opinion that the Canadian study randomization schema was flawed. Nurses randomized women by entering their names on alternative lines in a local notebook, which means they could assign women with suspicious physical exam findings to the mammography arm over the control group. That appears to be what happened since there were many (30 percent) more large (probably palpable) cancers in the screened group. That means the population studied was not a pure screening (asymptomatic) population. Given that fact, the all–cause mortality differences between the women who had mammography and those who did not is understandably small.
The fact remains that the Canadian screening trial is the only one of eight large randomized trials of screening mammography that failed to show a benefit for women from mammography screening. It was also the only one with this non–centralized randomization methodology. All the other studies showed a significant reduction in mortality in the group that was screened with mammography and physical examination over the control group. The benefit is larger in women over 50 than for women in their 40s, but still significant for both age cohorts.
Practically speaking, every woman should discuss her own personal health risks and choices with her physician and should make her own decision as to whether to undergo a regular mammogram. As a woman, a radiologist, a breast imager and a researcher, it is my belief that screening mammography saves lives.
Editor's Note: Etta D. Pisano, M.D. is vice president for Medical Affairs, College of Medicine dean and professor of radiology.