Annual screening in women ages 40 to 59 does not reduce mortality from breast cancer beyond that of physical examination or usual care, according to a Canadian study spanning 25 years.
Furthermore, the study shows that 22% of screen-detected breast cancers were over-diagnosed, representing one over-diagnosed breast cancer for every 424 women who received screening in the trial. Over-diagnosis refers to the detection of harmless cancers that will not cause symptoms or death during a patients lifetime.
Women with non-palpable breast cancer detected by screening have better long-term survival than women with palpable breast cancer. But it is not clear whether this survival difference is a consequence of organized screening or of lead time bias meaning screening increases perceived survival time even though a patient would have died at the same time regardless of screening and over-diagnosis, according to a news release.
As published Feb. 11 on the website of the British Medical Journal, researchers with the University of Toronto compared breast cancer incidence and mortality over a span of up to 25 years in more than 89,000 women ages 40 to 59.
Women in the mammography arm of the trial had a total of five mammography screens, one a year over a five-year period. Those in the control arm were not screened.
Women ages 40-49 in the mammography arm and all women ages 50-59 in both arms also received annual physical breast examinations. Women ages 40-49 in the control arm received a single examination followed by usual care in the community.
During the 25-year study period, 3,250 women in the mammography arm and 3,133 in the control arm were diagnosed with breast cancer and 500 and 505, respectively, died of breast cancer.
Thus, the cumulative mortality from breast cancer was similar between women in the mammography arm and in the control arm, the authors wrote.
At the end of the five-year screening period, an excess of 142 breast cancers attributable to over-diagnosis occurred in the mammography arm compared with the control arm, and at 15 years the excess remained at 106 cancers.
The author said this statistic implies that 22% of the screen detected invasive cancers in the mammography arm were over-diagnosed that is, one over-diagnosed breast cancer for every 424 women who received mammography screening in the trial.
They stress these results may not be generalizable to all countries, but in technically advanced countries, our results support the views of some commentators that the rationale for screening by mammography should be urgently reassessed by policymakers.
While they believe that education, early diagnosis and excellent clinical care should continue, they conclude that annual mammography does not result in reduction in breast cancer-specific mortality for women aged 40-59 beyond that of physical examination alone or usual care in the community.
In an accompanying editorial, Mette Kalager, MD, of the University of Oslo in Norway, and colleagues wrote that long term follow-up does not support screening women under 60.
They agree with the study authors that the rationale for screening by mammography be urgently reassessed by policymakers, but pointed out this task would not be easy because governments, research funders, scientists and medical practitioners may have vested interests in continuing activities that are well established.