If a study ends before that balancing decline appears, researchers can mistake the early spike for overdiagnosis. Estimates also shift when women in the control group later receive screening themselves.

"When screening is introduced, the number of breast cancer diagnoses initially rises because cancers are detected earlier than they would have been without screening," said Elsebeth Lynge, professor emerita at the University of Copenhagen's Department of Public Health.

Lynge added: "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."

She concluded: "If researchers do not take these factors into account, the initial increase can be mistaken for overdiagnosis."

What changed in the new analysis?

The researchers corrected for three specific timing factors across the trials:

• Whether control-group women later received screening, such as after routine programs launched.
• How many screening rounds each group received, since more rounds create more opportunities to find cancers early.
• How long researchers followed both groups, giving late-appearing cancers time to surface in the control group.

After those adjustments, the overdiagnosis rate aligned with Denmark's real-world estimate of below 5%.

"Taken together, we believe some previous high estimates of overdiagnosis, which influenced screening guidelines and communication, were based on evidence before trial data had fully matured," said Matejka Rebolj, senior epidemiologist at Queen Mary University of London.

Rebolj added: "When interpreted in their full temporal context, randomized trial data are consistent with overdiagnosis of less than five percent, rather than with estimates nearing 50%."

What does this mean for women?

The findings do not eliminate overdiagnosis, which can still lead to unnecessary treatment for cancers that would never have caused harm. But the new analysis suggests that risk has been misrepresented in guideline discussions and patient communications for years.

"Most women will not develop breast cancer, but with this study we can now be reassured that the benefits of detecting breast cancer early and preventing premature death will outweigh the small risk of unnecessary treatment," Njor said.

She added: "With this in mind, we hope this study will provide a framework for a more realistic interpretation of the evidence and help us better inform women when they are invited for screening."

What are the study's limits?

The work is a meta-analysis of existing trial data, not a new randomized trial. The researchers examined both invasive breast cancer and ductal carcinoma in situ (DCIS). Findings rest on the assumption that Denmark's regional rollout patterns accurately reflect what would happen in a longer, fully matured trial.

Funding came from the Novo Nordisk Foundation (grant NNF22OC0076184) and Cancer Research UK (grant C8162/A29083). The paper appears under DOI 10.1093/jnci/djag302.