For decades, one statistic has hung over the decision to get a mammogram: that somewhere between 30% and 50% of the breast cancers found by screening might be cancers that would never have hurt anyone. On Sept. 14, a team led by Sisse Helle Njor of the University of Southern Denmark published a reanalysis in the Journal of the National Cancer Institute arguing that the real figure is under 5%.

That is not a small correction. It changes how one of screening's best-known harms should be described to patients.

Where the Big Number Came From

Overdiagnosis means finding a cancer that would never have caused symptoms or shortened a life. Doctors cannot tell which tumors those are, so women still get surgery, radiation or hormone therapy. Some earlier readings of the randomized mammography trials put that share at 30% to 50%, according to the study team, and those numbers shaped patient leaflets and guideline debates for years.

The team's argument is that the trials were being read at the wrong moment. When screening starts, diagnoses jump because cancers are found earlier than they otherwise would be. That surplus is supposed to shrink later, as the unscreened group catches up. Read too early, the temporary jump looks like permanent overdiagnosis.

The Danish Natural Experiment

To test this, the researchers needed a place where the catch-up could be watched. Denmark provided one. Organized screening reached some Danish regions 17 years before others, so incidence could be tracked both right after screening began and long afterward. The Danish program has an estimated overdiagnosis rate below 5%.

The team reanalyzed all eight major randomized trials: the New York Health Insurance Plan study, Malmö, the Swedish Two-County trial, Edinburgh, the Canadian National Breast Screening Study, Stockholm, Gothenburg and the UK Age trial. They adjusted for three distortions: screening offered to control groups after the trials ended, different numbers of screening rounds, and unequal follow-up time.

What the Analysis Found

After those adjustments, the excess cancers in the trials closely matched the Danish pattern. Matejka Rebolj, a senior epidemiologist at Queen Mary University of London and a co-author, said in a university release that the trial data fit overdiagnosis below 5% rather than figures near 50%. Njor said the old interpretation of the trials is less straightforward than it looked.

The paper also includes ductal carcinoma in situ (DCIS) as well as invasive cancers, which matters, because DCIS is where much of the overdiagnosis argument lives.

My read: the strongest part of this study is not the new number but the explanation of the old one. If the big estimates came from timing effects, then a whole strand of the screening debate rested on a measurement artifact.

What It Does Not Change

Guidelines stay as they are. As Medical Daily noted on Sept. 29, the U.S. Preventive Services Task Force still recommends a mammogram every other year for women aged 40 to 74, a position finalized in 2024. The Task Force's own review put overdiagnosis in the best-designed trials at about 11% to 19%, so its estimate was already well below the 50% claims. The new work suggests even that may be high, but it does not overturn the guidance.

It also says nothing about false alarms. The Task Force estimates 1,376 false-positive results per 1,000 women over a lifetime of biennial screening, according to Medical Daily. Those extra scans, biopsies and weeks of anxiety are a far more common harm than overdiagnosis, and this paper does not address them.

Screening benefits are uneven, as a 10-year study of 6,000 adults on heart calcium scans recently showed for cardiac risk. Deciding who gains most from any test is the recurring question, and it is the same one behind newer risk tools such as the Mayo Clinic model that flags pancreatic cancer risk years early.

Limits Worth Keeping in Mind

This is a reanalysis of published reports, not a new trial. It assumes the Danish program is a fair yardstick, and the version circulating is an accepted manuscript that could change before final publication. The results describe groups of women, not individuals, so they cannot tell any one woman whether her tumor would have stayed quiet.

Conflicts deserve a mention too. One co-author works for the American Cancer Society, which supports routine screening, and individual authors report funding from the Novo Nordisk Foundation and Cancer Research UK, according to Medical Daily. Other screening researchers had not published responses at the time of writing, and critics of mammography will likely push back on the Danish comparison.

Overdiagnosis may also be more likely in women 75 and older, where other health problems compete, and the evidence for screening at that age remains uncertain.

What This Means for Women Deciding

Nothing here says to change course without talking to a doctor. It does mean the conversation can be more precise. Women in their 40s, who were newly advised to start screening in 2024, may find one worry easier to weigh. Women who skipped screening because of fear of unnecessary treatment have a reason to revisit the question.

The useful questions are practical: how false positives apply at your age, whether breast density or family history changes your plan, and what happens next if something suspicious appears. Anyone who finds a new lump, skin dimpling or nipple changes should get it checked promptly, screening date or not.

The paper's real contribution is to move an argument from percentages to method. When a number as large as 50% survives for decades, it is worth asking when the count was taken.