Part II | Chapter 8 158 Abstract Background and purpose Magnetic resonance-guided focal salvage high-dose-rate brachytherapy (FS-HDR-BT) for radiorecurrent prostate cancer shows low toxicity rates. However, biochemical failure after treatment occurs frequently. We developed two prediction models for biochemical failure (Phoenix definition) with the aim of enhancing patient counselling before FS-HDR-BT and during follow-up. Materials and methods A prospective cohort of 150 radiorecurrent prostate cancer patients treated with FS-HDR-BT between 2013 and 2020 was used for model development and internal validation. Multivariable Cox Proportional Hazards regression was applied. For model 1, only pre-salvage variables were included as candidate predictors. For model 2, additional (post-)salvage characteristics were tested. After calibration, nomograms and web tools were constructed. Finally, three risk groups were identified. Results Sixty-one patients (41%) experienced biochemical failure. At baseline (model 1), age, Gross Tumour Volume, pre-salvage PSA, and pre-salvage PSA doubling time (PSADT) were predictive of biochemical failure. 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 biochemical failure. The adjusted C-statistics were 0.73 (95% CI: 0.66-0.81) and 0.84 (95% CI: 0.780.90), respectively, with acceptable calibration. Estimated 2-year biochemical disease-free survival for the low-, intermediate-, and high-risk groups were 84%, 70%, and 31% (model 1), and 100%, 71%, and 5% (model 2). Conclusion Two models are provided for prediction of biochemical failure in patients with radiorecurrent prostate cancer treated with FS-HDR-BT. Based on pre- and post-salvage characteristics, we are able to identify patients with a high risk of biochemical failure. These findings can aid patient counselling for FS-HDR-BT.
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