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.

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.

Fig 1 Study groups in Copenhagen and Funen. Red=programme prevalence screening round; yellow=programme incidence screening rounds; green=0-3 years after end of invitation to screening; pink=4-7 years after end of invitation to screening; blue=≥8 years after end of invitation to screening. Copenhagen birth cohorts: 1 April 1921-31 March 1923; 1 April 1923-31 March 1925; 1 April 1925-31 March 1927; 1 April 1927-31 March 1929; 1 April 1929-31 March 1931; 1 April 1931-31 March 1933; 1 April 1933-31 March 1935. Funen birth cohorts: 1 November 1923-31 October 1925; 1 November 1925-31 October 1927; 1 November 1927-31 October 1929; 1 November 1929-31 October 1931; 1 November 1931-31 October 1933; 1 November 1933-31 October 1934

Fig 2 Relative risks and 95% confidence intervals for incidence of invasive breast carcinoma and ductal carcinoma in situ by time during and after end of invitation to screening. “Prevalence to ≥8 years after” means starting with prevalence round and including follow-up eight and more years after end of invitation to screening