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News • Meta-analysis of mammography trials
Overdiagnosis in breast cancer screening less common than previously thought
When women take part in breast cancer screening, they are informed that screening may detect cancers that would never have caused symptoms or problems during their lifetime. This is known as overdiagnosis.
The extent of overdiagnosis has been one of the most debated issues in breast cancer screening for decades. Estimates from randomised trials have varied widely, with some suggesting that as many as 30-50% of breast cancers detected through screening may represent overdiagnosis. These estimates have played an important role in the international debate about the benefits and harms of population-based breast cancer screening.
Now, a new study from the University of Southern Denmark, Queen Mary University of London, the University of Copenhagen and the American Cancer Society suggests that overdiagnosis in breast cancer screening is less of a problem than previously thought. Results of the meta-analysis were published in the JNCI Journal of the National Cancer Institute.
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
Sisse Helle Njor
"The aim of our study was to bring together the evidence from all randomised controlled trials to get a clearer picture of the extent of overdiagnosis in breast cancer screening," says Sisse Helle Njor, professor at University of Southern Denmark and Lillebælt Hospital, and continues: "Randomised 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."
The researchers found that the excess number of breast cancer cases detected in the randomised trials closely matched what would be expected in a Danish screening population, where overdiagnosis is estimated to be below 5%.
The researchers combined and reanalysed data from all randomised trials of mammography screening. They used Danish data as a reference. Organised breast cancer screening was introduced in some Danish regions 17 years before others, allowing for an observation of how breast cancer diagnoses changed both when screening was introduced and over the longer term.
"When screening is introduced, the number of breast cancer diagnoses initially rises because cancers are detected earlier than they would have been without screening," says Elsebeth Lynge, professor emerita at Department of Public Health, University of Copenhagen. "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. If researchers do not take these factors into account, the initial increase can be mistaken for overdiagnosis."
A comparison of breast cancer incidence at identical timepoints in trials and in Danish routine screening data enabled the researchers to examine the similarity in the observed patterns - and what this means for estimates of overdiagnosis. "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. When interpreted in their full temporal context, randomized trial data are consistent with overdiagnosis of less than 5%, rather than with estimates nearing 50%," says Matejka Rebolj, Senior Epidemiologist, Queen Mary University of London.
Accurate information about the potential benefits and harms of screening and how often they occur is important to help women make informed decisions about their participation in breast cancer screening.
"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," says Sisse Helle Njor. "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."
Overdiagnosis can occur when screening detects a cancer that would never have become life-threatening or caused symptoms during a woman’s lifetime. Without screening, she would never have known that she had the cancer. It can also include women who die from other causes shortly after their breast cancer is diagnosed. These women may have been unlikely to benefit from screening in the first place because of poor health or limited life expectancy, as there was little chance that detecting and treating the cancer would improve their health or prolong their lives.
About the study
The researchers conducted a new analysis of existing research on mammography screening. They included all eight trials in this field: the New York Health Insurance Plan, Malmö, Two-County, Edinburgh, the Canadian National Breast Screening Study, Stockholm, Gothenburg, and UK Age. They used two Danish regional screening programmes as a reference. The analysis included both invasive breast cancer and ductal carcinoma in situ (DCIS).
When reviewing the previous trials, the researchers took three factors into account that can affect estimates of overdiagnosis:
- Whether women in the control group were also screened, for example after the trial ended or following the introduction of a routine screening programme.
- How many rounds of screening the women were offered in each group. More screening rounds provide more opportunities to detect breast cancer earlier.
- How long women in the screening and control groups were followed. A longer follow-up period allows time for cancers detected earlier through screening to subsequently be diagnosed in the control group.
After accounting for these differences in screening and follow-up between the groups, the results suggest that overdiagnosis may be substantially less common than previously thought.
Source: University of Southern Denmark Faculty of Health Sciences
22.09.2026
