title: Overdiagnosis in screening mammography in Denmark: population based cohort study.
abstract: OBJECTIVE
  To use data from two longstanding, population based screening programmes to study overdiagnosis in screening mammography.
  
  
  METHODS
  Population based cohort study.
  
  
  METHODS
  Copenhagen municipality (from 1991) and Funen County (from 1993), Denmark.
  
  
  METHODS
  57,763 women targeted by organised screening, aged 56-69 when the screening programmes started, and followed up to 2009.
  
  
  METHODS
  Overdiagnosis of breast cancer in women targeted by screening, assessed by relative risks compared with historical control groups from screening regions, national control groups from non-screening regions, and historical national control groups.
  
  
  RESULTS
  In total, 3279 invasive breast carcinomas and ductal carcinomas in situ occurred. The start of screening led to prevalence peaks in breast cancer incidence: relative risk 2.06 (95% confidence interval 1.64 to 2.59) for Copenhagen and 1.84 (1.46 to 2.32) for Funen. During subsequent screening rounds, relative risks were slightly above unity: 1.04 (0.85 to 1.27) for Copenhagen and 1.14 (0.98 to 1.32) for Funen. A compensatory dip was seen after the end of invitation to screening: relative risk 0.80 (0.65 to 0.98) for Copenhagen and 0.67 (0.55 to 0.81) for Funen during the first four years. The relative risk of breast cancer accumulated over the entire follow-up period was 1.06 (0.90 to 1.25) for Copenhagen and 1.01 (0.93 to 1.10) for Funen. Relative risks for participants corrected for selection bias were estimated to be 1.08 for Copenhagen and 1.02 for Funen; for participants followed for at least eight years after the end of screening, they were 1.05 and 1.01. A pooled estimate gave 1.040 (0.99 to 1.09) for all targeted women and 1.023 (0.97 to 1.08) for targeted women followed for at least eight years after the end of screening.
  
  
  CONCLUSIONS
  On the basis of combined data from the two screening programmes, this study indicated that overdiagnosis most likely amounted to 2.3% (95% confidence interval -3% to 8%) in targeted women. Among participants, it was most likely 1-5%. At least eight years after the end of screening were needed to compensate for the excess incidence during screening.
authors: Department of Public Health, University of Copenhagen, Østre Farimagsgade 5, DK 1014 Copenhagen K, Denmark.
authors: Sisse Helle
authors: SH
authors: Njor
authors: Anne Helene
authors: AH
authors: Olsen
authors: Mogens
authors: M
authors: Blichert-Toft
authors: Walter
authors: W
authors: Schwartz
authors: Ilse
authors: I
authors: Vejborg
authors: Elsebeth
authors: E
authors: Lynge
mesh_terms: D000368:Aged; D001943:Breast Neoplasms / Q000000981:diagnostic imaging* / Q000453:epidemiology; D002277:Carcinoma / Q000000981:diagnostic imaging* / Q000453:epidemiology; D002278:Carcinoma in Situ / Q000000981:diagnostic imaging / Q000453:epidemiology; D018270:Carcinoma, Ductal, Breast / Q000000981:diagnostic imaging / Q000453:epidemiology; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D003951:Diagnostic Errors / Q000706:statistics & numerical data*; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D015994:Incidence; D008327:Mammography / Q000706:statistics & numerical data*; D008403:Mass Screening / Q000706:statistics & numerical data; D008875:Middle Aged