title: Comparison of Danish dichotomous and BI-RADS classifications of mammographic density.
abstract: BACKGROUND
  In the Copenhagen mammography screening program from 1991 to 2001, mammographic density was classified either as fatty or mixed/dense. This dichotomous mammographic density classification system is unique internationally, and has not been validated before.
  
  
  OBJECTIVE
  To compare the Danish dichotomous mammographic density classification system from 1991 to 2001 with the density BI-RADS classifications, in an attempt to validate the Danish classification system.
  
  
  METHODS
  The study sample consisted of 120 mammograms taken in Copenhagen in 1991-2001, which tested false positive, and which were in 2012 re-assessed and classified according to the BI-RADS classification system. We calculated inter-rater agreement between the Danish dichotomous mammographic classification as fatty or mixed/dense and the four-level BI-RADS classification by the linear weighted Kappa statistic.
  
  
  RESULTS
  Of the 120 women, 32 (26.7%) were classified as having fatty and 88 (73.3%) as mixed/dense mammographic density, according to Danish dichotomous classification. According to BI-RADS density classification, 12 (10.0%) women were classified as having predominantly fatty (BI-RADS code 1), 46 (38.3%) as having scattered fibroglandular (BI-RADS code 2), 57 (47.5%) as having heterogeneously dense (BI-RADS 3), and five (4.2%) as having extremely dense (BI-RADS code 4) mammographic density. The inter-rater variability assessed by weighted kappa statistic showed a substantial agreement (0.75).
  
  
  CONCLUSIONS
  The dichotomous mammographic density classification system utilized in early years of Copenhagen's mammographic screening program (1991-2001) agreed well with the BI-RADS density classification system.
authors.affiliation: Center for Epidemiology and Screening, Department of Public Health, University of Copenhagen, Copenhagen, Denmark ; Danish Institute for Study Abroad, Copenhagen, Denmark.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Hodge
authors.affiliation: Center for Epidemiology and Screening, Department of Public Health, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Sophie Sell
authors.initials: SS
authors.lastname: Hellmann
authors.affiliation: Center for Epidemiology and Screening, Department of Public Health, University of Copenhagen, Copenhagen, Denmark.
authors.forename: My
authors.initials: M
authors.lastname: von Euler-Chelpin
authors.affiliation: Diagnostic Imaging Centre, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark.
authors.forename: Ilse
authors.initials: I
authors.lastname: Vejborg
authors.affiliation: Center for Epidemiology and Screening, Department of Public Health, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Zorana Jovanovic
authors.initials: ZJ
authors.lastname: Andersen
keywords: Breast cancer; Kappa; breast density; mammographic density; mammographic screening