In 1980, the Canadian National Breast Screening Study (CNBSS) was applauded as one of the most thorough, meticulous, and recognized mammogram studies ever conducted. Now, the study’s methodology is being vilified by other researchers because of the way it determined which participants would receive annual mammograms, and its suggestion that screenings do not reduce mortality rates.
The CNBSS had almost 90,000 participants from six provinces between the ages of 40 and 49 randomly assigned to either have a mammogram, or not have one, over many years. Those in the group who were told to get mammograms continued to do so for 25 years, while those who did not were just left to the care of their family doctor.
During the study in the 1990s, 38 of the nearly 90,000 participants died of breast cancer, 28 of whom did not receive a mammogram.
The new study, published this week in the Journal of Medical Screening, was conducted by researchers from the University of Toronto, University of Ottawa, University of British Columbia, University of Alberta, and Harvard University; it suggests that there are serious problems in the randomization aspect, including many issues in the screening trial.
“In addition to other serious flaws that have been documented elsewhere, there is now direct eyewitness evidence available of subversion of randomization of the CNBSS and the systematic inclusion of symptomatic individuals,” the new study reads. “CNBSS 1 and 2 were not reliably randomized controlled trials, nor were they truly trials of screening.”
CNBSS 1 and 2 refers to sets of data collected in 1995 and 2014, which excluded deaths that occurred from cancers detected in the prevalence screening.
Dr. Jean Seely, a professor of radiology at the University of Ottawa, head of breast imaging at The Ottawa Hospital, and one of the authors of the new study, called for women age 40 and up to get regular mammograms in an interview with CBC News.
“I was astonished. I did not expect to find such clear, clear evidence of what was going on,” Seely said. “Then I think my next reaction was anger. How could this have been allowed?”
CBC interviewed 28 people who worked on the CNBSS study. Some of them confirmed that there were instances in which women with detectable lumps, who were pre-screened by a nurse, were placed in the group that received mammograms, meaning they were not random.
The CNBSS found that annual screening did not reduce breast cancer mortality rates any more than a standard physical exam would. It has since been used around the world to develop guidelines on breast cancer screening.
Since originally being published, the CNBSS study has been widely criticized as misleading due to those factors. The American College of Radiology and Society of Breast Imaging previously issued a statement criticizing the study, calling it “an incredibly misleading analysis based on the deeply flawed and widely discredited Canadian National Breast Screening Study.”
“Future trials should avoid the weaknesses in design methodology and should contain effective measures to prevent the flaws in execution that were found in CNBSS 1 and 2,” the new study reads.
In 2021, more than 27,000 women in Canada will be diagnosed with breast cancer, the Canadian Cancer Society says. That figure makes up one-quarter of all cancer diagnoses nationally. Of those, 5,400 will die, representing 13 per cent of all cancer deaths in women annually.
The Canadian Cancer Society also says that the death rate for breast cancer peaked in 1986 and has been declining since, attributing that to screening and advancements in treatment.