The current study offers fresh evidence linking routine screening to over-diagnosis of non-aggressive tumors because it compares outcomes over a single time period in two regions of Denmark – one that offered biennial mammography for women aged 50 to 69 and one that didn’t.
January 2017 Study Abstract
Background
Effective breast cancer screening should detect early-stage cancer and prevent advanced disease.
Objective
To assess the association between screening and the size of detected tumors and to estimate overdiagnosis (detection of tumors that would not become clinically relevant).
Design
Cohort study.
Breast Cancer Screening in Denmark: A Cohort Study of Tumor Size and Overdiagnosis, Annals of Internal Medicine, DOI: 10.7326/M16-0270, 10 JANUARY 2017.
Philips Mammography Truck, Brazil via philips_newscenter.
Setting
Denmark from 1980 to 2010.
Participants
Women aged 35 to 84 years.
Intervention
Screening programs offering biennial mammography for women aged 50 to 69 years beginning in different regions at different times.
Measurements
Trends in the incidence of advanced (>20 mm) and nonadvanced (≤20 mm) breast cancer tumors in screened and nonscreened women were measured. Two approaches were used to estimate the amount of overdiagnosis: comparing the incidence of advance and nonadvanced tumors among women aged 50 to 84 years in screening and nonscreening areas; and comparing the incidence for nonadvanced tumors among women aged 35 to 49, 50 to 69, and 70 to 84 years in screening and nonscreening areas.
Results
Screening was not associated with lower incidence of advanced tumors. The incidence of nonadvanced tumors increased in the screening versus prescreening periods (incidence rate ratio, 1.49 [95% CI, 1.43 to 1.54]). The first estimation approach found that 271 invasive breast cancer tumors and 179 ductal carcinoma in situ (DCIS) lesions were overdiagnosed in 2010 (overdiagnosis rate of 24.4% [including DCIS] and 14.7% [excluding DCIS]). The second approach, which accounted for regional differences in women younger than the screening age, found that 711 invasive tumors and 180 cases of DCIS were overdiagnosed in 2010 (overdiagnosis rate of 48.3% [including DCIS] and 38.6% [excluding DCIS]).
Limitation
Regional differences complicate interpretation.
Conclusion
Breast cancer screening was not associated with a reduction in the incidence of advanced cancer. It is likely that 1 in every 3 invasive tumors and cases of DCIS diagnosed in women offered screening represent overdiagnosis (incidence increase of 48.3%).