In a cancer screening program, over-diagnosis refers to identification of cancers that would not otherwise have emerged as a health problem. Researchers in Denmark analyzed information from two regional breast cancer screening programs and concluded that there was little evidence of over-diagnosis. These results were published in the journal Cancer.
Breast cancer is diagnosed in approximately 250,000 women each year in the U.S. alone. Since the disease is highly curable when it is found and treated early, it is recommended that women 40 years of age or older undergo screening mammography every one to two years.
A potential risk of screening for any type of cancer is that screening will detect a cancer that would never otherwise have come to medical attention. This is referred to as “over-diagnosis.” As a result of over-diagnosis, some individuals receive cancer diagnoses and treatments that they might otherwise have avoided.
As an example of over-diagnosis, consider an individual who has cancer, but who will die of another cause before the cancer causes noticeable health problems. Screen-detection of this cancer would lead to unnecessary cancer treatment. This sort of situation is generally not possible to predict in advance, but understanding how frequently it occurs can help women make more informed decisions.
To evaluate the likelihood of over-diagnosis in two Danish mammography programs, researchers evaluated trends in breast cancer diagnoses before and after the screening programs began. Shortly after initiation of a screening program, one expects to see a quick rise in the number of breast cancers diagnosed. This is because the first round of the screening program will detect many existing cancers that, in the absence of screening, would not have been detected until some later time. After this peak, if breast cancer rates remain elevated compared to the pre-screening period, it may suggest over-diagnosis.
- Over time, breast cancer rates in Denmark increased regardless of screening.
- In the regions of the country covered by the screening programs, initiation of screening brought about a steep rise in the number of breast cancers diagnosed. The rate of breast cancer then dropped to roughly what it would have been in the absence of screening.
The researchers conclude that in these two Danish mammography screening programs, there is little evidence of over-diagnosis of breast cancer.
Reference: Svendsen AL, Olsen AH, von Euler-Chelpin M et al. Breast Cancer Incidence After the Introduction of Screening Mammography: What Should Be Expected? Cancer. 2006;106:1883-90.
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