Breast cancer screening study under fire divides researchers


TORONTO – A new article has questioned the scientific process behind the 1980s Canadian National Breast Cancer Screening Study (CNBSS), a controversial study that has divided researchers for years.

Researchers at the Sunnybrook Research Institute in Toronto, the Ottawa Hospital, the University of British Columbia, the University of Alberta and Harvard Medical School published their CNBSS review this week in the Journal of Medical Screening, alleging that the original study had significant flaws, particularly in the randomization aspect of the research.

The CNBSS was a series of trials conducted in the 1980s at 15 testing centers in six provinces: Nova Scotia, Quebec, Ontario, Manitoba, Alberta and British Columbia. Almost 90,000 women aged 40 to 59 participated in the study.

Its objective was to determine whether cancer screening programs were helping to save women’s lives, and divided the participants into two groups: one with women aged 40 to 49 and 50 to 59 who had mammograms and another control group where both age groups underwent a single physical examination.

All participants were followed by the study for several years.

During the five-year screening period, the CNBSS found that 666 invasive breast cancers were diagnosed in the mammogram group and 524 were diagnosed in the control group who underwent physical examination. Of those diagnosed, 180 women in the mammography group and 171 women in the control group died of breast cancer during the 25-year follow-up period.

The overall risk ratio of death from breast cancer diagnosed during the screening period and cumulative mortality from breast cancer were similar between women in the mammogram group and those in the control group, according to the CNBSS.

Researchers then determined from this data that mammograms for women aged 40 to 49 did not reduce breast cancer death rates in women at average risk of contracting the disease.

The results of the CNBSS have influenced health care policies and guidelines on when women should be able to participate in screening programs, the guidelines of the Canadian Task Force on Preventive Health Care (CTFPHC) ) currently recommending breast cancer screening with mammograms for women aged 40 to 49 if they do not have pre-existing factors, such as a direct family member who has had breast cancer or the BRCA gene .

In an emailed statement to on Tuesday, CTFPHC Co-Chair Dr. Brenda Wilson said the organization “is doing rigorous and detailed reviews of the evidence to formulate guidelines. Our guidelines, including those on breast cancer screening, have been recognized as the best in the world… One reason is the careful and detailed process the working group uses to assess the evidence, which is available. in its guidelines documentation.

When asked if the CTFPHC would reconsider the said guidelines in light of the new document’s claims, the statement said that “the working group incorporates the best available evidence into its guidelines. When substantial changes are made to this evidence, the Working Group updates a full review of the body of evidence, including any new evidence. We don’t comment on individual studies or opinion pieces without going through this process. “


One of the co-authors of the new paper, radiologist Dr Jean Seely, who is chief of breast imaging at The Ottawa Hospital, told in a telephone interview on Tuesday that experts around the world had long suspected that there were problems with the science of CNBSS.

“They [the CNBSS] were the only out of eight randomized controlled studies to show no benefit from screening mammography, ”she said. “Many questions have been raised over the years [which] showed that there was likely a problem with the randomization process, which allowed more women with clinically advanced cancers to be assigned to the screening portion of the study. “

Seely says these suspicions are what prompted her and her fellow researchers to take a closer look at the CNBSS and, during their investigation, determined that the trial participants had a clinical breast exam. before being assigned to one of the two test groups.

The new article suggests that breast examination influenced whether women were placed in the control group or in the group that received screening mammograms.

“We have very credible eyewitness testimony showing that this happened at two different sites,” Seely said. “The evidence on the ground is that – with the best of intentions – the nurses who had examined the women attributed them to the mammogram arm with the intention of getting their attention.”

Lead author of the study, Dr Martin Yaffe, of the Sunnybrook Research Institute, echoed Seely’s concerns.

“Our research reveals a number of protocol violations within the CNBSS related to how patients were randomized into trial groups – to receive breast cancer screening or not,” Yaffe said in a statement. . “Given these important issues, trial results are unreliable and should not be used to inform policy on breast cancer screening. “

The authors of the article argue that the CNBSS should not inform screening guidelines because of the “corruption of the randomization process,” according to Seely.

“We now know that many women who had symptomatic breast cancer were actually recruited … so that really shouldn’t inform screening guidelines today,” Seely said, adding that the quality of the photos of mammogram was poor during the testing period.

Seely says she was “astonished” that the alleged corruption of the data could have occurred and that the study is still being published.

“It also bothers me that we have conclusive evidence that this happened… I think as a Canadian myself I was feeling a little embarrassed or ashamed,” she said. “I think the bottom line is that women shouldn’t be hurt by this and that they should be empowered to be healthy and get tested.”

However, Dr. Cornelia Baines, professor emeritus at the University of Toronto’s Dalla Lana School of Public Health, who worked on the CNBSS, flatly denied the evidence of the new articles in an email and phone conversation with .

“Speculation and anonymous remarks do not constitute evidence,” Baines wrote in an email. “The JMS article suggests that women aged 40 to 49 were seeking CNBSS screening mammograms because of symptoms. In fact, women’s access to diagnostic mammograms, regardless of their age, was available in our universal healthcare system. Unlike the United States, Canadian women do not need to seek free care in research trials. “

“The nurse examiners did not need to thwart the randomization protocol due to a positive clinical examination,” she continued. “A positive test required a referral to the study surgeon, who was completely free to order a diagnostic mammogram. “

Baines says screening center coordinators were independent from examiners and there were only four entry criteria for CNBSS: a woman’s age, women were not pregnant, had no history of breast cancer and they had not had a mammogram in the 12 months prior to the study.

“We expected women to be honest and pretty much all of them were,” she wrote. “In 2019, the BMJ ranked our 2014 article on CNBSS’s 25-year results as one of the top five it has published in the decade.”

“I’ll tell you briefly – it’s the radiologists who don’t like our study [and] have been pushing this for three decades, you must be wondering why, after 30 years of such attacks, CNBSS is still highly regarded by people other than radiologists, ”Baines said in a phone conversation with


Currently, there is no universal breast cancer screening program in the country. A woman in her 40s in British Columbia can be screened for breast cancer, just like a woman of the same age in Nova Scotia. In Ontario, however, a woman needs a referral from her family doctor.

The authors of the new article argue that screening should be available to any woman in her 40s to catch possible illness faster.

For law professor Jennifer Quaid, early detection is something she often thinks of after her experience with breast cancer.

“I received excellent care and treatment… but what angers me is that it was preventable. I think it should have been picked up earlier, ”the University of Ottawa professor said in a telephone interview with on Tuesday.

Quaid, who is now in his 50s, found a small lump in November 2019, but delayed seeing his doctor due to the demands of his career and the subsequent COVID-19 pandemic, where in-person visits to the office doctors were often delayed or discouraged. In the summer of 2020, she saw her doctor who asked for a mammogram.

“The technician came back and said ‘the radiologist wants to have a quick chat with you’, at which point I knew something was wrong,” Quaid said. She was eventually diagnosed with stage three breast cancer which required a complete mastectomy and complete lymph node reduction.

“I had just finished a very successful sabbatical where I got all the grants I applied for and wrote some pretty big papers for,” Quaid said. “So I felt like I was ready to take off and all of a sudden it was like someone had just ripped you off the ground. It was probably the worst time.

Quaid underwent surgery and underwent chemotherapy, then radiation therapy until mid-March 2021.

“I don’t want to look like I’m taking the blame,” she said. “You know, people do their best. But my GP knew that… I have breast cancer in my family, but my mother does not, and I have no sisters and my grandmothers did not have breast cancer. So under Ontario rules I was not eligible [for screening]. “

Quaid wants breast cancer screening offered to women sooner, as analysis of tumors removed from her body during surgery revealed they had been there for at least one to two years.

“I will never be able to come back exactly as I was before,” she said. “I also lost a year of productive work at the peak of my career and those things that I will never get back. So if I can stop another person from going through this, it’s worth it. “