Marieke van Son

140 CHAPTER 8 ABSTRACT Background and purpose Magnetic resonance (MR)-guided focal salvage high-dose-rate brachytherapy (FS- HDR-BT) for radiorecurrent prostate cancer (PCa) shows low toxicity rates. However, biochemical failure (BF) rates after treatment remain high. We developed and internally validated two prediction models for BF (Phoenix definition) with the aim of enhancing patient selection, guidance and counselling before FS-HDR-BT and to identify high-risk patients during follow-up. Materials and methods A prospective cohort of 150 radiorecurrent PCa patients treated with FS-HDR-BT be- tween 2013 and 2020 at the University Medical Center Utrecht was used for model development and validation. Two multivariable Cox proportional hazards models were developed and internally validated. For model 1, only pre-salvage variables were in- cluded as candidate predictors. For model 2, additional (post-)salvage characteristics were tested. The models were calibrated and for each model a nomogram and webtool were constructed. Finally, three risk groups were identified. Results Sixty-one patients (41%) experienced BF after a median of 32.9 months (interquartile range 23.5-43.6). At baseline (model 1), age, gross tumor volume, pre-salvage PSA, and pre-salvage PSA doubling time (PSADT) were predictive of BF. During follow-up (model 2), age, pre-salvage PSA and PSADT, seminal vesicle involvement, post-salvage time to PSA nadir, and percentage PSA reduction were predictive of BF. The adjusted C-statistics were 0.73 (95% CI: 0.66-0.81) and 0.84 (95% CI: 0.78-0.90) with acceptable calibration. Estimated 2-year biochemical disease-free survival was 84%, 70%, and 31% (model 1), and 100%, 71%, and 5% (model 2) for the low-, intermediate-, and high-risk group, respectively. Conclusion Two models are provided for prediction of BF in patients with radiorecurrent PCa treated with FS-HDR-BT. Based on pre- and post-salvage characteristics, we are able to identify patients with a high risk of BF. These findings can aid patient selection, counselling, and guidance at baseline and during follow-up.