Fig. 2
Yearly mean concordance index through time for BI-RADS density or continuous longitudinal breast density. n = 132,439. yC is the yearly mean concordance index. ΔLR-χ2 represents the difference in likelihood-ratio statistics between a model fitted to age at baseline (continuous) and time-varying BMI (continuous) and a model additionally incorporating the density term(s). BI-RADS density and the new longitudinal breast density measure (continuous, including a quadratic term) are time-varying in the proportional-hazards Cox models. ‘Time-varying’ means that a woman’s values for breast density and BMI are updated through time, i.e. at each screening examination. The concordance index decreases through time because the age range in the cohort decreases over time. (Women aged 40 yr get older, and women are censored after age 75 yr.) Orange line represents the yC through time for BI-RADS density. Blue line represents the yC through time for longitudinal density. Error bars represent the standard errors for the yC values at yearly intervals, starting at 0.5 years. BI-RADS Breast Imaging Reporting and Data System, BMI body mass index