title: Breast Cancer Screening in Denmark: A Cohort Study of Tumor Size and Overdiagnosis.
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).
  
  
  METHODS
  Cohort study.
  
  
  METHODS
  Denmark from 1980 to 2010.
  
  
  METHODS
  Women aged 35 to 84 years.
  
  
  METHODS
  Screening programs offering biennial mammography for women aged 50 to 69 years beginning in different regions at different times.
  
  
  METHODS
  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 advanced 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]).
  
  
  CONCLUSIONS
  Regional differences complicate interpretation.
  
  
  CONCLUSIONS
  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%).
  
  
  BACKGROUND
  None.
authors.affiliation: From Nordic Cochrane Centre, Copenhagen, Denmark, and University of Oslo and Norwegian Institute of Public Health, Oslo, Norway.
authors.forename: Karsten Juhl
authors.initials: KJ
authors.lastname: Jørgensen
authors.affiliation: From Nordic Cochrane Centre, Copenhagen, Denmark, and University of Oslo and Norwegian Institute of Public Health, Oslo, Norway.
authors.forename: Peter C
authors.initials: PC
authors.lastname: Gøtzsche
authors.affiliation: From Nordic Cochrane Centre, Copenhagen, Denmark, and University of Oslo and Norwegian Institute of Public Health, Oslo, Norway.
authors.forename: Mette
authors.initials: M
authors.lastname: Kalager
authors.affiliation: From Nordic Cochrane Centre, Copenhagen, Denmark, and University of Oslo and Norwegian Institute of Public Health, Oslo, Norway.
authors.forename: Per-Henrik
authors.initials: PH
authors.lastname: Zahl
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001943:Breast Neoplasms / Q000175:diagnosis* / Q000453:epidemiology; D002285:Carcinoma, Intraductal, Noninfiltrating / Q000175:diagnosis* / Q000453:epidemiology; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D055088:Early Detection of Cancer; D005260:Female; D006801:Humans; D015994:Incidence; D008327:Mammography; D008403:Mass Screening; D000069342:Medical Overuse / Q000706:statistics & numerical data*; D008875:Middle Aged