Women who have hesitated over mammograms because of the risk of overdiagnosis now have new evidence to consider. A reanalysis of the major randomized mammography trials, published in the Journal of the National Cancer Institute on Sept. 14, found results consistent with overdiagnosis below 5%. Some past estimates drawn from the same trials ran as high as 30% to 50%.
Overdiagnosis happens when screening finds a cancer that would never have caused symptoms or threatened a woman's life. Because doctors cannot tell which tumors those are, overdiagnosed women still go through surgery, radiation, or hormone therapy. That possibility has been one of the main reasons some women delay or skip screening.
The new work does not change screening guidelines. It does suggest that one risk many women weigh may be smaller than they were told, and it gives patients and doctors a more current number to discuss.
A Fresh Look at Eight Decades-Old Trials
Researchers from the University of Southern Denmark, the University of Copenhagen, Queen Mary University of London and the American Cancer Society reexamined eight randomized trials, including the New York Health Insurance Plan study, Sweden's Two-County trial and the Canadian National Breast Screening Study. They compared the trials against a Danish screening program where overdiagnosis is estimated at under 5%. Organized screening began in some Danish regions 17 years before others, giving researchers a long view of how diagnoses rise and fall.
The team focused on a timing problem. When screening begins, diagnoses jump because cancers are found earlier, and that gap should later shrink as the same cancers eventually show up in the unscreened group. Many trials also offered screening to the control group after the study ended, which blurs the comparison.
After accounting for follow-up time, the number of screening rounds and later screening of control groups, the observed patterns in the trials closely matched what low overdiagnosis would predict. Randomized trial data are "consistent with overdiagnosis of less than five percent, rather than with estimates nearing 50%," said Matejka Rebolj, a senior epidemiologist at Queen Mary, in a university news release.
Weighing the Finding Against Current Screening Guidance
The U.S. Preventive Services Task Force recommends a mammogram every other year for women ages 40 to 74, a guideline finalized in 2024 that lowered the starting age from 50. The Task Force's own review estimated overdiagnosis in the best-designed trials at about 11% to 19%. Its models projected about 14 overdiagnosed cancers per 1,000 women screened every two years from 40 to 74, with a wide range across models.
The Task Force already concluded that the benefits outweigh the harms for women in that age range. The new analysis suggests one of those harms may be smaller than the Task Force assumed. It does not address the more common downside of screening, false alarms, estimated at 1,376 false-positive results per 1,000 women over a lifetime of screening every other year.
The results may matter most for women in their 40s, who were newly advised to start screening in 2024, and for women who have skipped screening over fears of unnecessary treatment. The Task Force notes that Black women are about 40% more likely to die of breast cancer than white women. For women 75 and older, the evidence on screening remains uncertain, and overdiagnosis may be more likely because of competing health problems.
Limits Women Should Know Before Drawing Conclusions
This is a reanalysis of published trial reports, not a new trial. The method assumes the Danish program is a fair reference point, and early trial reports were sometimes incomplete. The paper is posted as an accepted manuscript, so its final version may change slightly.
The analysis also looked at groups of women, not individuals. It cannot tell any one woman whether a tumor found on her mammogram would have stayed harmless. Ductal carcinoma in situ, a noninvasive condition often found on screening, remains an area where doctors still debate how much treatment is needed.
Funding context matters, too. One co-author works for the American Cancer Society, which supports routine screening, and individual authors received support from the Novo Nordisk Foundation and Cancer Research UK. "Randomized trials have often been cited as evidence that overdiagnosis is a substantial problem. Our study shows that this interpretation is not as straightforward as it may seem," lead author Sisse Helle Njor said in the university's release. Other screening researchers have not yet published responses, and the finding will likely draw debate.
Questions to Bring to a Screening Appointment
Women can use this study to have a more specific conversation with a clinician. Useful questions include how overdiagnosis and false positives apply at your age, whether your breast density or family history changes your screening plan, and what happens if a mammogram finds something suspicious. Women with a BRCA mutation, prior chest radiation, or previous breast cancer follow separate, higher-risk guidance.
Cost should not be a barrier for most women. Under federal preventive care rules, most private insurance plans cover Task Force-recommended screening mammograms without a copay, though follow-up imaging and biopsies may carry charges. Uninsured and low-income women can ask about the CDC's National Breast and Cervical Cancer Early Detection Program, which offers free or low-cost screening to those who qualify.
Anyone who notices a new lump, skin dimpling, nipple changes, or discharge should see a clinician promptly, whether or not a screening mammogram is due. Those symptoms call for diagnostic evaluation, not routine screening.
The bottom line is cautious but encouraging. This study suggests overdiagnosis is a smaller risk than many women have been told, yet guidelines have not changed and debate will continue.
Key Questions Answered
What is breast cancer overdiagnosis?
It is the detection of a cancer through screening that would never have caused symptoms or death. Because doctors cannot identify which tumors those are, the women are often treated anyway.
What did the new study find?
After reanalyzing eight randomized mammography trials, researchers found results consistent with overdiagnosis below 5%, far lower than some earlier estimates of 30% to 50%.
Does this change mammogram guidelines?
No. The U.S. Preventive Services Task Force still recommends a mammogram every two years for women ages 40 to 74.
Does this mean mammograms have no downsides?
No. False-positive results remain common and can lead to extra imaging, biopsies, and anxiety. The study did not address that harm.
Are mammograms covered by insurance?
Most private plans cover recommended screening mammograms without a copay. Follow-up tests may involve costs, and eligible low-income women can seek free or low-cost screening through a CDC program.
Who should talk to a doctor about different screening?
Women with a BRCA mutation, previous chest radiation, prior breast cancer or dense breasts should discuss whether different or additional screening is appropriate.
Published by Medicaldaily.com
