The first of the two investigations, from Dartmouth University, concluded that mammograms were much more likely to catch small tumors that never would have proved harmful than to detect those destined to become large and life threatening. The second report made news in January 2017, when Danish researchers published strikingly similar findings. They found that as many as one in every three women diagnosed with breast cancer had a tumor that might never become life threatening and had likely received unnecessary treatment as a result.
Both analysis showed that mammography doesn’t reduce the number of large tumors detected, results that contradict the long-held belief that finding tumors when they are small helps save lives.
The Dartmouth investigators analyzed data collected over the past 35 years and reported that since 1975, two-thirds of the invasive tumors detected via mammograms were smaller than two centimeters or were in-situ carcinomas, growths that are not harmful and are unlikely to progress to invasive cancer. They also concluded that while breast cancer deaths declined during the study period, improvements in treatment were responsible for most of this – not early detection of tumors by mammography.
The Dartmouth team looked at breast cancer and the size of the tumors detected in women age 40 and older between 1975 and 1979, when mammograms were not widely available, and compared those data to cases diagnosed between 2000 and 2002 after mammography had become much more common. They then tracked the women affected, their treatment and whether they were alive 10 years later.
They found that more breast cancer was diagnosed among women having routine mammograms and that the tumors were more likely to be smaller than those found in the earlier time period. They also saw a relatively small decline in the number of large tumors detected by mammograms between 1975 and 2012. All told, the investigators determined that 80 percent of the small tumors found never would have proved deadly.
The Danish researchers reviewed the medical records of all women in Denmark diagnosed with invasive breast cancer between 1980 and 2010. Because mammograms became available in different areas of the country at different times, the researchers were able to compare women who were screened with those who weren’t. Overall, they concluded that some 15 to 39 percent of the breast tumors detected would not have become life threatening.
The Danish team leader, Karsten Juhl Jorgensen, deputy director of research for the Nordic Cochrane Centre at the Rigshospitalet in Copenhagen, questioned the value of regular mammograms. He said that breast screening “has not lived up to its promises”.
These conclusions are sure to be controversial, even though they are hardly the first to suggest that mammograms can lead to over-diagnosis and unnecessary treatment. In 2012, researchers at the Harvard School of Public Health, using data from Norway, found that between 15 and 25 percent of all cases of breast cancer revealed by mammograms are over-diagnosed and that those tumors would never be life threatening.
These studies won’t be the last word on the subject. They don’t tell us which women should have screening mammograms and how often. But they do suggest that mammography is overused.
Andrew Weil, M.D.
Gilbert Welch et al, “Breast-Cancer Tumor Size, Overdiagnosis, and Mammography Screening Effectiveness.” New England Journal of Medicine, October 13, 2016, DOI: 10.1056/NEJMoa1600.249
Karsten Juhl Jørgensen et al, “Breast Cancer Screening in Denmark: A Cohort Study of Tumor Size and Overdiagnosis.” Annals of Internal Medicine, January 10, 2017, DOI: 10.7326/M16-0270