New analysis points to lower rate of overdiagnosis during mammography — ScienceDaily
In Denmark, data from two regional screening programs became a benchmark for a new analysis of eight randomized mammography trials. The researchers concluded that the rate of breast cancer overdiagnosis may be substantially lower than some previous estimates of 30–50% had suggested. The results are consistent with the Danish estimate that screening-related overdiagnosis is less than 5%, ScienceDaily reports.
What overdiagnosis is
Overdiagnosis refers to the detection through screening of cancer that would not have caused symptoms or threatened a woman’s health during her lifetime. Without the examination, the patient might not have learned that such a tumor was present. This concept may also include cases in which a woman dies from another cause soon after diagnosis, and the detection and treatment of cancer would be unlikely to affect her health or life expectancy.
The authors reanalyzed all eight randomized trials of mammography screening: New York Health Insurance Plan, Malmö, Two-County, Edinburgh, Canadian National Breast Screening Study, Stockholm, Gothenburg, and UK Age. For comparison, they used data from two regional screening programs in Denmark. The analysis covered invasive breast cancer and ductal carcinoma in situ (DCIS).
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Why follow-up time matters
After mammography screening is introduced, the number of diagnosed cancer cases initially rises because tumors are detected earlier than they would have been without examination. Some of these cases would have been diagnosed later without screening, so the incidence rate should decline over time. If the follow-up period ends before such a decline occurs, the initial increase in diagnoses may be mistakenly interpreted as overdiagnosis.
The researchers also considered whether participants in the control groups underwent screening after the trials ended or after regular programs were launched, how many rounds of examinations were offered to each group, and the duration of further follow-up. According to the authors, these factors may have influenced the early high estimates of overdiagnosis.
Professor Sisse Helle Njor of the University of Southern Denmark and Lillebælt Hospital stated that, in the researchers’ view, the benefits of early breast cancer detection and preventing premature death outweigh the small risk of unnecessary treatment.
