Breast Cancer Overdiagnosis Far Lower Than Believed — SkimNews
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- Researchers reanalyzed all eight major randomized mammography trials — including New York Health Insurance Plan, Malmö, Two-County, Edinburgh, Canadian National Breast Screening Study, Stockholm, Gothenburg, and UK Age — and found overdiagnosis consistent with below 5%, rather than the 30-50% estimates cited for decades.
- The team — including Sisse Helle Njor, Elsebeth Lynge, Matejka Rebolj, and Casper Urth Pedersen — adjusted for three timing-related confounders: whether control-group women later received screening, the number of screening rounds, and the length of follow-up in each trial.
- Matejka Rebolj of Queen Mary University of London said earlier high estimates were based on evidence "before trial data had fully matured," arguing that when interpreted in full temporal context, trial data align with overdiagnosis below 5% rather than near 50%.
- Denmark served as a real-world benchmark because organized breast cancer screening began in some regions 17 years earlier than others, letting researchers observe the expected pattern — an initial rise in diagnoses after screening rollout followed by a compensatory drop.
- Sisse Helle Njor of the University of Southern Denmark said the study offers reassurance that for most women, the mortality benefit of early detection will outweigh the small risk of unnecessary treatment.
- The study was published in JNCI: Journal of the National Cancer Institute in 2026, with funding from the Novo Nordisk Foundation (Casper Urth Pedersen) and Cancer Research UK (Matejka Rebolj).
Why it matters: Overdiagnosis estimates of 30-50% have been cited internationally to frame breast cancer screening's harms and have shaped how clinicians counsel patients. This reanalysis — accounting for short follow-up, screening rounds, and post-trial contamination — puts the true rate below 5%, meaning screening guideline committees and patient communications built on the higher figure may need recalibration.
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