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Overdiagnosis in screening mammography in Denmark: population based cohort study

BMJ 2013; 346 doi: (Published 26 February 2013) Cite this as: BMJ 2013;346:f1064
  1. Sisse Helle Njor, statistician1,
  2. Anne Helene Olsen, statistician2,
  3. Mogens Blichert-Toft, professor emeritus3,
  4. Walter Schwartz, chief physician4,
  5. Ilse Vejborg, chief physician5,
  6. Elsebeth Lynge, professor1
  1. 1Department of Public Health, University of Copenhagen, Østre Farimagsgade 5, DK 1014 Copenhagen K, Denmark
  2. 2Institute of Community Medicine, University of Tromsø, Tromsø, Norway
  3. 3Danish Breast Cancer Cooperative Group, 2100 Copenhagen Ø, Denmark
  4. 4Mammography Screening Clinic, University Hospital Odense, 5000 Odense, Denmark
  5. 5Diagnostic Centre, University Hospital Copenhagen, Blegdamsvej, 2100 Copenhagen Ø, Denmark
  1. Correspondence to: S H Njor sissenj{at}
  • Accepted 5 February 2013


Objective To use data from two longstanding, population based screening programmes to study overdiagnosis in screening mammography.

Design Population based cohort study.

Setting Copenhagen municipality (from 1991) and Funen County (from 1993), Denmark.

Participants 57 763 women targeted by organised screening, aged 56-69 when the screening programmes started, and followed up to 2009.

Main outcome measures 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.


  • Contributors: SHN participated in the design of the study, did the statistical analysis, participated in the analysis and interpretation of data, and drafted the manuscript. AHO participated in the design of the study, retrieved and checked data, and revised the manuscript critically. MB-T, WS, and IV contributed data on screening mammography and revised the manuscript critically. EL conceived the study, participated in the design of the study, and finalised the manuscript in collaboration with SHN. All authors have read and approved the final manuscript. All authors had full access to all of the data in the study. SHN is the guarantor.

  • Funding: This study was financially supported by the Esper and Olga Boel Foundation. The funding source had no role in the writing of the manuscript or the decision to submit it for publication.

  • Competing interests: All authors have completed the ICMJE uniform disclosure form at (available on request from the corresponding author) and declare: SHN and EL had financial support from Esper and Olga Boel Foundation; no financial relationships with any organisations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work.

  • Ethical approval: In accordance with Danish legislation, the study was notified to the Danish Data Inspection Agency (J No 2008-41-2191). No further ethical approval is needed for register based studies in Denmark.

  • Data sharing: No additional data available.

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