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Article by David Brousseau, M.D., PhD
I have been asked many questions about the recent BMJ publication regarding screening mammography effectiveness in Canada. This is my summary of its limitations. The BMJ article on breast cancer screening effectiveness is grossly flawed and misleading. Why?
1) The trial used mammography equipment, which was not state of the art at the time of the trial. The images were grossly compromised by “scatter,” and did not employ grids for much of the trial. Also, technologists were not taught proper positioning. And the CNBSS radiologists had no specific training in mammographic interpretation. The CNBSS own reference physicist stated that "...in my work as reference physicist to the NBSS, That quality [in the NBSS] was far below state of the art, even for that time (early 1980s)."
2) Only 32 percent of cancers were detected by mammography alone. This extremely low number is consistent with poor quality mammography. Based upon well-established mammographic standards, at least two-thirds of the cancers should be detected by mammography alone. Poor image quality is supported by the mean tumor size which was 19 mm in the screening group and 21 mm in the control group… only a 2 mm difference. Screen detected tumors should have been less than 15 mm at the time of the study and near 10 mm today. The poor quality of the mammography screening alone explains these results and should disqualify the CNBSS as a valid scientific study of modern mammography screening.
3) The assignment of women was biased and not randomized between the control and mammography arms. The CNBSS violated the fundamental rules of a randomized trial because every woman first had a clinical breast examination by a trained nurse so that they knew which women had lumps, many of which were cancers.
Before assigning women to the screening or control groups, investigators knew who was more likely to have a cancer and It likely resulted in a statistically significant excess of women with breast cancers assigned to the screening arm compared to those assigned to the control; thus guaranteeing more deaths among the screened women than the control women. This is supported by the fact that survival from breast cancer in the control group had a 90 percent five year survival compared to the expected 75 percent rate of the average age-matched Canadian women at that time. This supports a suspicion of selection bias that cancers were being shifted from the control arm to the screening arm based upon the pre-identified physical findings.
Summary: Coupling the fundamentally corrupted allocation process with the documented poor quality of the mammography should have long ago disqualified the CNBSS as a legitimate trial of modern screening mammography.
Mammography remains the gold standard for breast cancer detection and numerous well designed studies continue to support its effectiveness.
Dr. David Brousseau, M.D., PhD is the Medical Director of Breast Imaging at the Los Angeles Center for Women's Health where he specializes in radiology and breast imaging. For more information, contact the Los Angeles Center for Women's Health at 213.742.6400.