Supporters of the most recent paper from the Canadian National Breast Screening Study (CNBSS) falsely contend that only radiologists are criticizing this study. This simply is not true. The Canadian study flaws have been well documented for decades.
Robert E. Tarone at the National Cancer Institute (who isn’t a radiologist) wrote in 1995 that there was a statistically significant excess of advanced cancers that were allocated to the mammography group. The World Health Organization long ago excluded the CNBSS from its analyses of screening mammography’s impact of breast cancer mortality. In a recent interview with CNN, the American Cancer Society echoed similar methodological concerns to those raised by American College of Radiology, Society of Breast imaging and others. Breast cancer groups, such as Breastcancer.org, have criticized this study and warned against following the author’s recommendations. CNN’s Dr. Sanjay Gupta detailed the problems with applying such an old study (conducted in the 1980s) to today and recommended regular mammograms for women ages 40 and older.
The study was not blindly randomized. Women were examined by a doctor or nurse before being assigned to the control group (which did not get mammograms) or the group that was screened. This means that the doctors or nurses who may have felt a lump in a woman’s breast, and/or lumps in her armpit that might be lymph nodes to which the cancer spread, were faced with the choice of putting a woman that they strongly suspected had cancer into the group that wouldn’t be screened.
Since these women were assigned from open lists, how many local coordinators, out of common decency, acted to save these women’s lives? The coordinators should never have been put in that position. Study authors contend that local staff did not affect randomization. However, the Canadian government inquiry confirmed that a local coordinator was, in fact, fired because study organizers felt that this person had done so. Leaders of the inquiry admit that they did not talk to local coordinators because they felt coordinators “would have been unlikely to admit” affecting randomization. Coordinators were also not permitted to be interviewed by anyone else. So, how is this question somehow settled?
The CNBSS survival figures also support healthy skepticism. The 5-year survival rate in the CNBSS unscreened group was over 90 percent. At that time in Canada, the 5-year survival rate was 75 percent. This kind of discordance again raises questions about randomization. As the American College of Obstetricians and Gynecologists noted in their press release on the BMJ study, the CNBSS was the only one of many large randomized trials to show no mortality reduction from screening. The CNBSS is also the only one in which organizers knew which women likely had cancers prior to allocation.
The study’s own reference physicist stated in peer-reviewed published studies that the study’s “[mammography] quality was far below the state of the art, even for that time.” Many of the facilities in the trial, which took place in the 1980s, used older mammography machines. Many of the technologists who performed the mammograms across Canada were not properly trained on how to position the woman to include as much breast tissue as possible, so some cancers that could have been detected on the mammogram were missed.
These deficiencies more than explain why only 32 percent of the cancers found in the Canadian study were detected by mammography when most large trials show that mammography detects over 60 percent of cancers and why the cancers found by mammography were more advanced (larger) than those found by mammograms in other trials. Conversely, the false negative rate in the trial (when cancer is present, but missed on a mammogram), was actually worse in the Canadian study than in those done in the 1960s and 1970s.
Criticisms of the CNBSS are well founded and have been well known for 20 years. These factors are why medical professionals worldwide have rejected the trial’s conclusions. Mudslinging doesn’t change the facts. However, it does add to confusion among women regarding when to be screened for breast cancer and may ultimately cost lives.
Geraldine McGinty is chair, commission on economics, American College of Radiology.
