Marieke van Son

144 CHAPTER 8 variables was checked with Martingale residuals. Influential outliers were assessed by calculating dfbeta residuals. Model performance and internal validation The discriminative ability of the model was assessed using Harrell’s C-statistic. Internal validation was performed through bootstrapping with 2000 resamples for each impu- tation set, in which all modeling steps were repeated. The optimism of each model and shrinkage factors were calculated, and the β-coefficients and C-statistic were adjusted accordingly. The predictive accuracy of the optimism-corrected models was visualized with calibration plots at 12, 24, and 36 months. Nomogram and risk group construction For both models a nomogram and webtool were constructed using the optimism-cor- rected coefficients. Finally, for eachmodel separately, three risk groups were identified on the basis of the 25th and 75th percentile of the linear predictor. The Kaplan-Meier method was used to display the biochemical disease-free survival curves for each risk group. All statistical analyses were performed using R studio (version 3.6.1, R Foundation for Statistical Computing, Vienna, Austria, https://rstudio.com ) and the survival, survminer, rms, pmsampsize, ggplot2, mice, psfmi, DynNom, and regplot packages[26]. Reporting was according to the TRIPOD statement[25]. RESULTS Baseline characteristics and Kaplan-Meier survival analysis Baseline characteristics are displayed in Table 1. Median (IQR) follow-up time was 25.1 months (13.5-38.1) for all patients and 18.1 months (9.2-29.6) for patients who did not experience BF (censored). Sixty-one patients (40.7%) experienced BF after a median (IQR) of 32.9 months (23.5-43.6). Median bDFS was 29.7 months (95% CI: 25.0-38.6) (Figure S1 in Supplementary File B). Table 1 – Baseline patient-, tumour-, and treatment-related characteristics Primary treatment n (%) or median (IQR) Missing (%) Primary treatment EBRT LDR brachytherapy HDR brachytherapy 80 (53.3) 67 (44.7) 3 (2) 0 EBRT dose (Gy) 76.0 (71.5-77.0) 12.5 LDR dose (Gy) 145.0 (145.0-145.0) 0 HDR dose (Gy) 19.0 (19.0-38.0) 0 PLND at primary treatment 30 (20.0) 0