New York Times Article on Mammography Extremely Flawed
EXTREMELY FLAWED CANADIAN STUDY ON MAMMOGRAPHY
An Article published in the British Medical Journal questioning the value of screening mammography is one of the most flawed studies ever published. This misleading analysis was based on the deeply flawed and widely discredited Canadian National Breast Screening Study (CNBSS). The results of this BMJ study, and others resulting from the CNBSS trial, should not be used to create breast cancer screening policy as this would place a great many women at increased risk of dying unnecessarily from breast cancer.
Experts called on to review the CNBSS confirmed that the mammography quality was poor (2). The trial used second hand mammography machines, which were not state of the art at the time of the trial. The images were compromised by “scatter,” which makes the images cloudy and cancers harder to see since they did not employ grids for much of the trial. Grids remove the scatter and make it easier to see cancers. Also, technologists were not taught proper positioning. As such, many women were not properly positioned in the machines, resulting in missed cancers. 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, [I] identified many concerns regarding the quality of mammography carried out in some of the NBSS screening centers. That quality [in the NBSS] was far below state of the art, even for that time (early 1980s).”(3)
To be valid, randomized, controlled trials (RCT) must employ a system to ensure that the assignment of women to the screening group or the unscreened control group is random. Nothing can/should be known about participants until they have been assigned to one of these groups. The CNBSS violated these fundamental rules (6). Every woman first had a clinical breast examination by a trained nurse so that they knew which women had breast lumps, many of which were cancers, and which women had large lymph nodes in their armpits many of which indicated advanced cancer. Before assigning the women to be in the group offered screening or the control women, investigators knew who had large incurable cancers. This was a major violation of RCT protocol. It most likely resulted in the statistically significant excess of women with advanced breast cancers assigned to the screening arm compared to those assigned to the control arm (7). This guaranteed more deaths among the screened women than the control women.
The five year survival from breast cancer among women ages 40–49 in Canada in the 1980s was only 75 percent, yet the control women in the CNBSS, who were supposed to reflect the Canadian population at the time, had a greater than 90 percent five year survival (8). This indicates that cancers may have been shifted from the control arm to the screening arm. 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.