What overdiagnosis means
Overdiagnosis happens when screening finds a cancer that would never have caused symptoms or threatened a person's life. Without screening, the patient would never have known about it. Estimates of how often this happens in mammography have ranged from 1% to more than 50%, and critics of screening have often cited figures of 30% to 50%.
A new look at eight trials
Researchers at the University of Southern Denmark reexamined all eight major randomised trials of mammography screening. They accounted for differences between trial arms in follow-up time, how often women were screened, and screening that continued after a trial ended, factors they say had inflated earlier estimates. After those adjustments, the results point to an overdiagnosis rate below 5%.
The work was published in JNCI: Journal of the National Cancer Institute. Author Sisse Helle Njor said the team hoped the analysis would give a framework for a more realistic reading of the evidence and help doctors inform women when they are invited for screening. Researchers from Denmark, the United States and the Netherlands contributed.
What it does not settle
The analysis is a re-reading of old trial data, not a new trial, so it cannot remove every source of uncertainty. Overdiagnosis is still real, and some women will still be treated for tumours that would never have harmed them. The size of that group matters because it feeds directly into how screening programmes are explained and how benefits are weighed against harms.
Decisions about when to start screening and how often to screen remain individual. Health agencies continue to recommend regular mammography from around age 40 to 50 depending on the country. The authors said they want the findings to make those conversations more accurate rather than to settle the debate.