New Analysis Finds Low Overdiagnosis Rate in Breast Cancer Screening

For decades, one of the most debated questions in breast cancer screening has been how often it leads to "overdiagnosis" — finding cancers that would never have caused symptoms or problems in a woman's lifetime. Estimates from randomized trials have varied widely, with some suggesting that 30% to 50% of breast cancers found through screening represent overdiagnosis. A new analysis, published in the JNCI Journal of the National Cancer Institute, suggests the true rate is far lower.

Researchers led by Sisse Helle Njor of the University of Southern Denmark and Lillebælt Hospital combined and reanalyzed data from all eight randomized trials of mammography screening, using two Danish regional screening programs, where overdiagnosis is estimated at below 5%, as a reference. "The aim of our study was to bring together the evidence from all randomized controlled trials to get a clearer picture of the extent of overdiagnosis in breast cancer screening," Njor said. "Our study shows that this interpretation is not as straightforward as it may seem."

The team found that the excess number of breast cancer cases detected in the randomized trials closely matched what would be expected in the Danish screening population. Elsebeth Lynge, professor emerita at the University of Copenhagen, explained that when screening begins, diagnoses rise at first because cancers are caught earlier, but should later fall as cancers that would have been found anyway are simply diagnosed sooner — a pattern that, if not accounted for, can be mistaken for overdiagnosis.

"When interpreted in their full temporal context, randomized trial data are consistent with overdiagnosis of less than 5%, rather than with estimates nearing 50%," said Matejka Rebolj of Queen Mary University of London, a co-author. Njor said the findings should reassure women that "the benefits of detecting breast cancer early and preventing premature death will outweigh the small risk of unnecessary treatment."