Study Reports 15% to 25% of Breast Cancers Are Overdiagnosed

According to a new study out of Norway, about 15% to 25% of cases of breast cancer are overdiagnosed. Published today in the Annals of Internal Medicine, the study reports that 6 to 10 women were overdiagnosed for every 2,500 women invited to screen in the Norwegian Screening Program.

Kalager et al. quantified the rate of overdiagnosis of breast cancer or the percentage of cases of cancer that would not have become clinically apparent in a woman’s lifetime without screening. To estimate the percentage of overdiagnosis attributable to mammography screening, the researchers compared invasive breast cancer incidence in Norwegian counties where the screening program was implemented from 1996 to 2005 to the incidence in countries where the screening program was not yet implemented. A total of 39,888 women with invasive breast cancer were included in the analysis – 7,793 of whom were diagnosed after the screening program started. Through two different analytic approaches, Kalager and colleagues estimated that the rate of overdiagnosis attributable to the screening program ranged from 15-25% and concluded that “Mammography screening entails a substantial amount of overdiagnosis.”


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