Breast cancer screening guidelines based on flawed Canadian study, new paper says

A new paper calls into question a decades-old Canadian study that has informed breast cancer screening guidelines for women in their 40s around the world, which generally do not recommend a yearly mammogram.

The commentary — co-written by researchers at The Ottawa Hospital, Sunnybrook Research Institute in Toronto, the University of British Columbia, the University of Alberta and Harvard Medical School — will be published in the Journal of Medical Screening this week.

It specifically points to randomization issues with the Canadian National Breast Screening Study (CNBSS), originally conducted in the 1980s, which involved tens of thousands of women and eventually took place in 15 different urban centres across the country.

The randomized trials set out to determine whether or not screening helped save women's lives. In the first study, women aged 40 to 49 were randomly assigned to receive mammograms or placed in a control group where they had a single physical exam, with all participants followed up with for several years.

Over the course of the study, there were 38 deaths from breast cancer in the mammography group and 28 among those who didn't receive mammogram screenings.

As a result, Canadian researchers concluded that annual screening in women aged 40 to 49 at average risk does not reduce breast cancer mortality any more than a physical exam.

The study has gone on to inform guidelines both here and around the world. The most recent guidelines issued by the Canadian Task Force on Preventive Health Care still do not recommend screenings for women in that age group.

A key issue of earlier screening is the risk of false positives and over-diagnosis. There are lumps that aren't cancerous or harmful to women, but if discovered, would require a biopsy.

Experts have warned any kind of breast cancer treatment — whether it be radiation, chemotherapy or surgery — may carry harm.

But the CNBSS had several problems, according to Dr. Jean Seely, co-author of the new paper and a professor of radiology at the University of Ottawa and head of breast imaging at The Ottawa Hospital.

She and her co-authors are calling for mammograms for women aged 40 or older, should they want them.

Seely said there is now "conclusive confirmation" the CNBSS was conducted — particularly in some trial locations — with women who weren't randomly assigned.

When interviewing 28 staff members who worked on the original study, some confirmed there were multiple instances in which women with detectable lumps, who were pre-screened by a nurse, were in fact purposefully placed in the mammography group, she said.

"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?"