A new report from the Canadian National Breast Screening Study (CNBSS) found no reduction in mortality in women who received annual screening mammograms and has set off a firestorm of criticism and an impassioned defense.
The study, published in the February 11th issue of the British Medical Journal (BMJ), is one of the longest prospective, randomized trials of screening mammography ever completed.1 Launched in 1980, the trial recruited almost 90,000 women between ages 40 and 59 years who had no history of breast cancer. The women received a breast examination from a trained nurse and instruction in breast self-exam and were then randomly assigned to receive or not receive mammography. Women assigned to the mammography arm were also offered four more annual mammograms and physical examinations.1
In the control arm, women aged 40 to 49 years received usual care from their primary care physician, while women aged 50 to 59 years received annual physical breast examinations from a nurse (it was considered unethical not to offer screening to women aged 50 to 59 years).1
Women with abnormal findings on mammography or physical examination were referred for follow-up care. In both age groups, compliance in the mammography arm exceeded 85%.1
No Mortality Benefit Found
At the time of study publication, participants had been followed for a mean of 21.9 years (maximum 25 years). During the screening period, 666 breast cancers were detected in the mammography arm and 524 in the control arm. During the follow-up period, 2,584 breast cancers were detected in the mammography arm and 2,609 in the control arm.1
Data analysis showed no reduction in breast cancer deaths among women in either age group who had annual mammograms compared with the women who didn’t. Through the end of follow-up, there were 500 breast cancer deaths in the mammography arm and 505 in the control arm, a nonsignificant difference (hazard ratio [HR], 0.99; 95% CI: 0.88-1.12).1 All-cause mortality was also similar between the mammography and control arms (HR, 1.02; 95% CI: 0.98-1.06).1
Women whose cancers were diagnosed by mammography were significantly more likely to survive 25 years than women whose cancers were diagnosed by physical examination. Similarly, women with nonpalpable cancers survived significantly longer than women with palpable cancers. However, the study authors attributed this survival advantage to early diagnosis and overdiagnosis—that is, the cancer was found earlier but treatment did not prolong life, or the cancer didn’t need to be treated in the first place.1
Not only did screening mammography show no evidence of mortality benefit, it appeared to result in harm—the study authors estimated that overdiagnosis accounted for 22% of cancers detected by screening mammography.1 Put another way, for every 424 women who underwent mammography screening, one underwent treatment for a cancer that never would have caused symptoms.
The Critics Respond
Reaction to the report was fast and furious. The American College of Radiology issued a statement calling the study “incredibly misleading…deeply flawed, and widely discredited.”2
In a statement from the Society of Breast Imaging, Dan Kopans, MD, a professor of radiology at Harvard Medical School in Boston MA, said, “The CNBSS data were long ago shown to be unreliable. Longer follow-up does not improve the results.”3