Benefits of Breast Cancer Screening Questioned

Canadian researchers conducting a long-term trial noted no difference in death rates from breast cancer among women who had regular mammograms versus those who did not. The results of the trial add to the confusion on whether the benefits of the annual mammogram, promoted by health professionals as an essential tool in detecting and preventing breast cancer, outweigh the potential for false positives and overtreatment.

In a 25-year follow-up to the Canadian National Breast Screening Study (CNBSS), researchers concluded that the annual mammography “does not reduce mortality from breast cancer beyond that of physical examination or usual care.” Published in a recent report in BMJ, the screening data were originally collected in Canada between 1980 and 1985 from close to 90,000 women aged 40 to 59. The women were randomly assigned to either an experimental group, which received annual mammograms, or a control group, which received none. The CNBSS was designed to evaluate the benefit of mammography screening for women aged 40 to 59 compared to the usual care and the risk benefit of adding mammography to breast physical examination in women aged 50 to 59. It was not deemed ethical for women over age 50 in the control group not to receive screening.

The follow-up reported almost identical results from the two groups. Invasive breast cancers were diagnosed in 666 patients in the mammography group; 524 were diagnosed in the control group. In the mammography group, 180 patients died of breast cancer during the 25-year follow-up; 171 women died in the control group. The 25-year survival rate was 70.6 percent for women with breast cancer detected in the mammography group, but only 62.8 percent  for women with cancers diagnosed in the control group. Although there is a marked difference in survival after a diagnosis of breast cancer between those cancers diagnosed by mammography and those diagnosed by physical examination screening, the researchers attributed this to “lead time, length time bias, and over-diagnosis.”

Overdiagnosis

The researchers determined that overall, 22 percent of the invasive cancers detected during screening were overdiagnosed, meaning they would likely not have presented a danger to the women over their lifetime if they had been left alone. Statistically, that is one over-diagnosed breast cancer for every 424 women who received mammography screening in the trial. Fifteen years after enrollment, the excess became constant at 106 cancers. The research also indicated that, assuming that nearly all over-diagnosed cancers in the Canadian  National Breast Screening Study were non-palpable, 50 percent of mammogram detected, non-palpable cancers were over-diagnosed.

The follow-up is not without its critics. The American College of Radiology and the Society of Breast Imaging issued a joint statement calling the report an “incredibly misleading analysis” and based on a “discredited” study. As organizations for preventive health care in the U.S. review their guidelines this year, it will be interesting to see how they take into account this new research and whether it will have any bearing on current recommendations for yearly mammograms starting at age 40 for women.