Breast cancer screening associated with substantial overdiagnosis in Danish study

1. In this retrospective cohort, breast cancer screening was not associated with a reduction in advanced tumors.

2. Breast cancer screening was associated with higher rates of overdiagnosis of invasive breast cancer tumors and ductal carcinoma in situ (DCIS).

Evidence Rating Level: 2 (Good)

Study Rundown: The purpose of effective breast cancer screening is to reduce the incidence of advanced tumors (tumors larger than 20 mm at detection). Screening mammography detects many small tumors that would not become clinically evident over the course of the lifetime, resulting in increased rates of overdiagnosis. Considering the fact that mammography results in false positive diagnoses, determining whether breast cancer screening actually reduces the rates of advanced tumors has important therapeutic implications. The authors of this Danish study conducted a retrospective cohort evaluation to examine the association of mammography screening with incidence of advanced cancer and to estimate overdiagnosis. In general, they observed that breast cancer screening was associated with substantial overdiagnosis, with no significant reduction in advanced tumors. This study has several limitations. First, the study examined incidence of late stage tumors rather than disease-specific mortality. Additionally, regional differences complicated interpretation of the data. Overall, the study suggests that breast cancer screening does not reduce the incidence of advanced tumors. These results may have clinical implications in the future regarding preventative screening programs in Denmark and globally.

Click to read the study in the Annals of Internal Medicine

Relevant Reading: Implications of Overdiagnosis: Impact on Screening Mammography Practices

In-Depth [retrospective cohort]: The authors conducted a retrospective cohort study utilizing data from all Danish women aged 35 to 85 diagnosed with invasive cancer during 1980 to 2010 from the Danish Breast Cancer Group (DBCD) and the Danish Cancer Registry (DCR). They utilized two different methods to analyze data. From these methods, the estimates of overdiagnosis ranged from 9.9% to 48.3%, depending on whether the estimate included DCIS, which age groups were included in the denominator, and whether trends in women too young to be screened were accounted for. The difference in the incidence rate ratio (IRR) before and after screening for women aged 35 to 49 was 0.71 (95%CI 0.71 to 0.86) for advanced cancer and 1.14 (1.05 to 1.25) for non-advanced cancer. In women aged 70-84, the IRR difference was 1.25 (1.16 to 1.34) for advanced and 1.63 (1.51 to 1.76) for non-advanced cancer.

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