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Breast cancer mortality in organised mammography screening in Denmark: comparative study

BMJ 2010; 340 doi: https://doi.org/10.1136/bmj.c1241 (Published 24 March 2010) Cite this as: BMJ 2010;340:c1241
  1. Karsten Juhl Jørgensen, researcher1,
  2. Per-Henrik Zahl, senior researcher2,
  3. Peter C Gøtzsche, professor1
  1. 1The Nordic Cochrane Centre, Rigshospitalet, University of Copenhagen, Denmark
  2. 2Norwegian Institute of Public Health, Oslo, Norway
  1. Correspondence to: K J Jørgensen kj{at}cochrane.dk
  • Accepted 25 November 2009

Abstract

Objective To determine whether the previously observed 25% reduction in breast cancer mortality in Copenhagen following the introduction of mammography screening was indeed due to screening, by using an additional screening region and five years additional follow-up.

Design We used Poisson regression analyses adjusted for changes in age distribution to compare the annual percentage change in breast cancer mortality in areas where screening was used with the change in areas where it was not used during 10 years before screening was introduced and for 10 years after screening was in practice (starting five years after introduction of screening).

Setting Copenhagen, where mammography screening started in 1991, and Funen county, where screening was introduced in 1993. The rest of Denmark (about 80% of the population) served as an unscreened control group.

Participants All Danish women recorded in the Cause of Death Register and Statistics Denmark for 1971-2006.

Main outcome measure Annual percentage change in breast cancer mortality in regions offering mammography screening and those not offering screening.

Results In women who could benefit from screening (ages 55-74 years), we found a mortality decline of 1% per year in the screening areas (relative risk (RR) 0.99, 95% confidence interval (CI) 0.96 to 1.01) during the 10 year period when screening could have had an effect (1997-2006). In women of the same age in the non-screening areas, there was a decline of 2% in mortality per year (RR 0.98, 95% CI 0.97 to 0.99) in the same 10 year period. In women who were too young to benefit from screening (ages 35-55 years), breast cancer mortality during 1997-2006 declined 5% per year (RR 0.95, CI 0.92 to 0.98) in the screened areas and 6% per year (RR 0.94, CI 0.92 to 0.95) in the non-screened areas. For the older age groups (75-84 years), there was little change in breast cancer mortality over time in both screened and non-screened areas. Trends were less clear during the 10 year period before screening was introduced, with a possible increase in mortality in women aged less than 75 years in the non-screened regions.

Conclusions We were unable to find an effect of the Danish screening programme on breast cancer mortality. The reductions in breast cancer mortality we observed in screening regions were similar or less than those in non-screened areas and in age groups too young to benefit from screening, and are more likely explained by changes in risk factors and improved treatment than by screening mammography.

Footnotes

  • Contributors: KJJ and PCG conceived the project. KJJ collected data and wrote the first draft. PHZ performed the statistical analyses. PHZ and PCG revised the manuscript.

  • Funding: No funding was received for this research.

  • Conflicts of interest: All authors have completed the Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare (1) No financial support for the submitted work from anyone other than their employer; (2) No financial relationships with commercial entities that might have an interest in the submitted work; (3) No spouses, partners, or children with relationships with commercial entities that might have an interest in the submitted work; (4) No non-financial interests that may be relevant to the submitted work.

  • Ethical approval: No approval from an ethical committee or informed consent was needed.

  • Data sharing: The mortality data used in our calculations and graphs are available from the corresponding author at kj{at}cochrane.dk.

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