Breast cancer screening overdiagnosis may be far lower than trials suggested

September 26, 2026 at 08:56 PM2 min

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A reanalysis of all randomized mammography trials found that breast cancer overdiagnosis rates closely match Denmark's routine screening, where overdiagnosis is below 5%. The findings challenge earlier estimates of 30-50% that influenced international screening guidelines. Researchers attribute the discrepancy to immature trial data and post-trial screening contamination.

Key Facts

The reanalysis of all randomized mammography trials found overdiagnosis patterns similar to Denmark's routine screening, where overdiagnosis is below 5%.
Earlier randomized trial estimates suggested 30-50% of screen-detected breast cancers were overdiagnosed, influencing international screening guidelines.
Denmark's organized breast cancer screening began in some regions 17 years earlier than in others, providing a real-world reference for diagnosis patterns.
Researchers attribute inflated overdiagnosis estimates to trial data that had not fully matured and to post-trial screening contamination.

Trial Reanalysis

Researchers combined and reanalyzed results from all randomized trials of mammography screening and compared them with data from Denmark. Denmark offered a useful real-world reference because organized breast cancer screening began in some regions 17 years earlier than in others. That difference allowed researchers to track how breast cancer diagnoses changed immediately after screening was introduced and how those patterns evolved over longer periods. The researchers compared breast cancer incidence at matching points in time in the randomized trials and in Denmark's routine screening programs.

Overdiagnosis Estimates

The researchers found that the additional breast cancer cases detected in randomized trials closely resembled patterns seen in Denmark, where overdiagnosis associated with screening is estimated to be below 5%. For decades, estimates from randomized trials have differed sharply, with some studies suggesting that 30-50% of breast cancers detected through screening could fall into the overdiagnosis category. Those figures have influenced international discussions about both the benefits and possible harms of population-based breast cancer screening. Sisse Helle Njor, professor at University of Southern Denmark and Lillebælt Hospital, said the aim of the study was to bring together evidence from all randomized controlled trials to get a clearer picture of the extent of overdiagnosis.

Methodological Factors

When screening is introduced, the number of breast cancer diagnoses initially rises because cancers are detected earlier than they would have been without screening. Over time, this should be followed by a drop, as some of these cancers would otherwise have been diagnosed later. This pattern can also be affected if women in either group continue to undergo screening after the trials had ended, which was common. Elsebeth Lynge, professor emerita at Department of Public Health, University of Copenhagen, said that if researchers do not take these factors into account, the initial increase can be mistaken for overdiagnosis. The researchers believe some previous high estimates of overdiagnosis, which influenced screening guidelines and communication, were based on evidence before trial data had fully matured.