Canada’s breast cancer screening policy is based on a flawed study: researchers

Researchers say a decades-old Canadian study that influenced breast cancer screening policy found significant flaws in its approach, leading to a “significant impact” on disease outcomes and potentially contributing to hundreds of “avoidable deaths” per year.

Two studies – collectively known as the Canadian National Breast Screening Study (CNBSS) – found that mammography of 40-year-old women did not reduce breast cancer mortality.

The study was conducted in the 1980s and published in 1992.

But in a comment published this week in the Journal of Medical Screening, researchers at four Canadian institutions and Harvard Medical School say the way participants were selected for the study’s control or screening groups may have affected the results.

They say more recent findings suggest that mammography screening for women under the age of 50 is beneficial, including an observational study in 2014 that showed that mammography scans for 40-year-olds are associated with a 44 percent reduction in breast cancer deaths.

The Canadian Preventive Health Task Force does not currently recommend mammography for 40-49 year olds unless an existing factor puts them at higher than average risk – for example, if a family member has breast cancer or has the BRCA gene.

The task force said on Wednesday that their instructions “are not to be updated immediately”.

But Martin Yaffe, lead author of the comment and senior researcher at the Sunnybrook Research Institute in Toronto, believes change is needed. She estimates that the impact of the CNBSS on politics may have contributed to the avoidable deaths of more than 400 Canadian women each year.

“The idea of ​​doing this experiment was great, but the way it was designed and how it was conducted really makes its results unreliable,” he said. “And basing a policy on it is simply inappropriate.”

Dr. Brenda Wilson, chairman of the Task Force on Preventive Health Care, said the organization’s recommendations, last updated in 2018, “have been recognized as the best in the world.” He added that the group is conducting a “rigorous, detailed examination of the evidence to provide guidance.”

“When substantial changes are made to the evidence, the team will update a full statement of the evidence, including any new evidence,” he said in a statement.

The Canadian Public Health Agency issued a statement on Thursday that it is funding the task force, citing the body as “government arms.”

“It would be inappropriate for PHAC to guide the task force on which studies to include or not to include in its guidelines,” the agency said.

“The Task Force assesses the strength of the evidence used to make all the recommendations. The study described in this report is one of eight studies included in the Task Force guidelines.

Yaffe said the biggest problem with the 1992 study was that participants were given clinical breast examinations at 14 of the 15 study sites before they were assigned to either the screening or control groups. He said this approach could have accidentally affected the results.

She said the nurses who performed the physical breast examinations divided the participants into their groups by writing their names in an open book. If the nurse felt nodules in the woman’s breasts – which could have suggested advanced cancers – she might have been more likely to place her in the screening group “in the best interests of the patient, with all good will, but did not understand how the clinical trial worked.”

“On the mammography side of the study, there was a huge imbalance in the number of advanced cancers found compared to the control … so (the study) found no benefit from screening,” Yaffe said.

“In fact, they found that more women died on the mammography side than in the control group, which was strange because all other studies in older women had shown the benefits of mammography.”

Yaffe said he doubted the investigation methods had been unreliable for years, but the “smoky weapon” came only in March, when an eyewitness testimony from an employee at one of the test sites confirmed that the randomization could be flawed.

The size of the study – nearly 90,000 participants – gave weight to its findings, Yaffe said, which is one reason why it may have influenced politics around the world.

“But if the experiment was done poorly, if it was done a long time ago using approaches that are no longer used, it may not matter,” he said.

The PHAC said mammography screening policy is a “provincial and regional competence.” Jurisdictions can use the guidelines of the task force, but they also develop their own screening programs.

For example, Ontario does not screen patients under the age of 50, but those between the ages of 40 and 49 in Nova Scotia can apply for an annual mammography themselves. British Columbia says it “encourages” people between the ages of 40 and 49 to discuss the benefits and limitations of mammography with their doctor. If screening is chosen, it will be available every two years.

High-risk patients under the age of 50 are screened in jurisdictions, and people who suspect something is wrong can get a mammography referral from a doctor.

Dr. Jean Seely, co-author of the latest comment and director of breast imaging at Ottawa Hospital, said the policy should be updated to allow all women 40 years of age and older to undergo screening mammograms.

“Screening saves lives,” Seely said in a statement. “The five-year survival rate for local breast cancer is 98 percent when detected early.”

This report by The Canadian Press was first published on November 25, 2021.

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