Marieke van Son

141 PREDICTION MODELS FOR BF AFTER FOCAL SALVAGE HDR-BT INTRODUCTION Advances in prostate cancer (PCa) treatment have increased cure rates. However, still up to 50% of high-risk PCa patients treated with radiotherapy develop a recurrence within 10 years of treatment[1–3]. These recurrences are often confined to the prostate and frequently located at the site of the primary index lesion[4,5]. Nowadays, recurrenc- es can be assessed at an earlier stage with prostate specific membrane antigen pos- itron emitting tomography CT (PSMA-PET-CT). In this setting, focal therapy, targeting the recurrent lesion while sparing healthy prostate tissue, is an attractive treatment option with the aim of postponing initiation of androgen deprivation therapy (ADT) [5,6]. The main potential advantage of focal over whole-gland salvage treatments is the reduced chance of side-effects and quality of life deterioration, without affecting oncological outcomes[7–13]. One of the treatment options for radiorecurrent PCa is magnetic resonance imaging (MRI)-guided focal salvage high-dose-rate brachytherapy (FS-HDR-BT)[8,9]. In previ- ous studies, we found that around 50% of patients treated with single fraction FS-HDR- BT show biochemical failure (BF) within 2.5 years, caused by either local recurrences, regional recurrences, metastatic disease, or a combination[9]. While several studies have been published on predictive factors for BF after whole-gland salvage radiotherapy treatments[14–16], no studies have been published in patients undergoing focal salvage radiotherapy. Due to differences in patient-, tumor-, and treatment-characteristics, the results from whole-gland salvage studies are not directly applicable to FS-HDR-BT. In the current study we evaluated the predictive value of several pre- and post-salvage variables for BF after FS-HDR-BT for radiorecurrent PCa. Two models were developed, (1) with the aim of enhancing patient selection, based on pre-salvage characteristics, and (2) including additional (post-)salvage characteristics, with the aim of identifying patients at high-risk of BF during follow-up to support patient guidance and counselling. MATERIALS AND METHODS Patient selection For this study we prospectively included 150 patients treated with FS-HDR-BT for lo- calized radiorecurrent PCa between July 2013 and January 2020 at the Radiotherapy of the University Medical Center Utrecht (UMCU). Initially, patients were treated within an institutional review board (IRB)-approved feasibility study (Netherlands Trial Reg- ister number NTR6123), following the criteria: PSA level ≤10 ng/ml, PSA doubling time (PSADT) ≥12 months, tumor stage (MRI) ≤T2c, and acceptable urinary function (Inter- national Prostate Symptom Score <15). Because of favourable toxicity results after 2 years of inclusion, patients beyond the initial inclusion criteria were treated off-pro- tocol. In February 2018, a subsequent phase II study initiated (‘PRostatE Cancer MRI guided focal SalvagE high-dose-rate brachytherapy’, or PRECISE; NTR7014). This study expanded the inclusion criteria from the feasibility study: PSA ≤20 ng/ml, PSADT ≥9 8