TUESDAY, Sept 15, 2026 (HealthDay News) — Women’s risk of having a harmless tumor caught and treated as a result of breast cancer screening is much lower than once thought, a new evidence review has concluded.
Overdiagnosis — the detection of breast cancers that never would have harmed a woman’s health during her life — occurs in fewer than 5% of cancer screenings, researchers reported Sept. 14 in the Journal of the National Cancer Institute.
That’s far lower than previous estimates, which held that as many as 30% to 50% of breast cancer detected through screening might represent overdiagnosis, researchers said.
These earlier studies were inaccurate because they tracked breast cancer screenings for a limited period of time, the research team argued.
“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,” lead researcher Sisse Helle Njor, a professor at the University of Southern Denmark, said in a news release.
Overdiagnosis occurs when screening detects a cancer that never would have threatened a person’s life or caused symptoms, researchers said in background notes. Without screening, the patient never would have known they had cancer.
For the new study, researchers combined and analyzed data from eight prominent clinical trials of mammography and breast cancer screening.
“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.
The team compared those clinical trial results against long-term data from Denmark, which instituted organized breast cancer screening in some regions as much as 17 years earlier than others.
Specifically, researchers tracked breast cancer diagnoses that occurred at identical time points during both the clinical trials and in the Danish routine screening.
“When screening is introduced, the number of breast cancer diagnoses initially rises because cancers are detected earlier than they would have been without screening,” researcher Elsebeth Lynge said in a news release.
“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,” Lynge said. “If researchers do not take these factors into account, the initial increase can be mistaken for overdiagnosis.” Lynge is a professor emerita of public health with the University of Copenhagen.
Patterns in the clinical trial data confirmed that what was thought to be overdiagnosis was actually the early accurate detection of breast cancers, researchers concluded.
“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,” senior researcher Matejka Rebolj, a senior epidemiologist with Queen Mary University of London, said in a news release.
“When interpreted in their full temporal context, randomized trial data are consistent with overdiagnosis of less than 5%, rather than with estimates nearing 50%,” Rebolj said.
This new evaluation should reassure women who are worried that their mammogram could cause them to receive unnecessary treatment, researchers said.
“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,” Njor said.
More information
The American Cancer Society has more on breast cancer screening.
SOURCE: University of Southern Denmark, news release, Sept. 13, 2026
