Canadian Breast Screening Study Reignites Controversy Over Mammograms

Beth Fand Incollingo @fandincollingo
Published: Friday, Mar 07, 2014

Patrick I. Borgen, MD, Maimonides Hospital

Patrick I. Borgen, MD

The large Canadian study that has caused a stir by indicating that mammograms are of no use in women aged 40 to 59 years, and in fact can lead to over-diagnosis of breast cancer, is flawed and misleading, according to the program chairman of the Miami Breast Cancer Conference.

The Canadian National Breast Screening Study1—the results of which were published in mid-February in the British Medical Journal and which were reported widely in newspapers and on television—will be the subject of discussion during the conference, the chairman, Patrick I. Borgen, MD, said.

“The Canadian study is probably the single most flawed, confounded trial in the history of trials—certainly in trials of mammography,” Borgen said. “This is a trial from 34 years ago, using ancient technology and a very flawed randomization technique, which put women with breast lumps into the mammography arm and normal-exam women into the control arm—and, of course, after 30 years, it showed no difference. Applying that to today is simply outrageous.” Borgen will address the topic in a talk titled “The Great Mammography Debate” at 8:45 a.m. on Friday, March 7. “We’ll be spending a lot of time at the Miami Breast Cancer Conference dissecting this trial, talking about it, and, frankly, developing a group strategy for how to overcome it,” he said. “Mammograms save lives, and we’ll be getting that message out loud and clear at Miami.”

How Canadian Study Unfolded

In the study, Anthony B. Miller, MD, professor emeritus at the University of Toronto, and colleagues compared 25-year cancer outcomes in 89,835 women aged 40 to 59 years who were randomly assigned to either receive or not receive annual mammography screening. The trial was launched in 1980, and the recruitment period continued through 1985. In women who received mammography, screening took place over 5 years at 15 centers in six Canadian provinces. Two-view film screen mammography was used, with craniocaudal and mediolateral views taken until 1985; after that, in accordance with updated guidelines, craniocaudal and mediolateral oblique views were taken.

Women aged 40 to 49 years in the mammography arm and women aged 50 to 59 years in both arms received annual physical breast examinations, while women aged 40 to 49 in the control arm received a single physical breast examination followed by usual care in the community, the authors wrote. All women in the study were taught breast self-examination by trained nurses prior to randomization. During the 5-year screening period, 666 invasive breast cancers were diagnosed in the mammography arm (nearly three-fourths of them screen-detected), with 180 of those cancers becoming fatal within 25 years. In the control arm, 524 invasive breast cancers were diagnosed, 171 of them fatal within 25 years (Table).

The overall hazard ratio (HR) for death from breast cancers diagnosed during the screening period by mammography was 1.05 (95% confidence interval [CI], 0.85-1.30). Age made virtually no difference in the survival outcomes of these women, according to the authors.

Over the entire study period, 3250 women in the mammography arm and 3133 in the control arm were diagnosed with breast cancer, and 500 and 505, respectively, died of the disease. The cumulative mortality from breast cancer did not significantly differ between women in the mammography arm and in the control arm (HR 0.99; 95% CI 0.88-1.12), Miller et al wrote.

Table. Detected Invasive Breast Cancers and Related Deaths, Canadian National Breast Screening Study

Detection Outcomes (n = 89,835) Mammogram
(n = 44,925)
Physical Exam
(n = 44,910)
Hazard Ratio
Cancers detected at 5-y screening 666 524 n/a
Cancers detected at 5-y screening that became fatal over 25 y 180 171 1.05
Cancers detected over 25 y 3250 3133 n/a
Cancers detected that became fatal over 25 y 500 505 .99

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