Study provides a clearer picture of the extent of overdiagnosis in breast cancer screening

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 randomized 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.

- 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, says Sisse Helle Njor, professor at University of Southern Denmark and Lillebælt Hospital, and continues:

- 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.

The researchers found that the excess number of breast cancer cases detected in the randomized trials closely matched what would be expected in a Danish screening population, where overdiagnosis is estimated to be below 5%.

Researchers take a fresh look at previous studies

The researchers combined and reanalysed data from all randomized trials of mammography screening. They used Danish data as a reference. Organized 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. 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, says Elsebeth Lynge, professor emerita at Department of Public Health, University of Copenhagen.

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 five percent, rather than with estimates nearing 50%, says Matejka Rebolj, Senior Epidemiologist, Queen Mary University of London.

What does overdiagnosis mean for women?

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.