Prediction models for biochemical failure after FS-HDR-BT 159 Introduction Advances in prostate cancer treatment have increased cure rates. However, still up to 50% of highrisk prostate cancer 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, recurrences can be assessed at an earlier stage with prostate-specific membrane antigen positron emitting tomography CT (PSMA-PET/CT).6,7 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).5,8 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.9–15 One of the treatment options for radiorecurrent prostate cancer is magnetic resonance imaging (MRI)-guided focal salvage high-dose-rate brachytherapy (FS-HDR-BT).10,11 In previous studies, we found that around 50% of patients treated with single fraction FS-HDR-BT show biochemical failure within 2.5 years, caused by either local recurrences, regional recurrences, metastatic disease, or a combination.11 While several studies have been published on predictive factors for biochemical failure after whole-gland salvage radiotherapy treatments16–18, no studies have been published in patients undergoing focal salvage radiotherapy. Due to differences in patient, tumour, and treatment characteristics, the results fromwhole-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 biochemical failure after FS-HDR-BT for radiorecurrent prostate cancer. Twomodels 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 biochemical failure 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 localised radiorecurrent prostate cancer between July 2013 and January 2020 at the Radiotherapy of the University Medical Centre Utrecht (UMCU). Initially, patients were treated within an institutional review board (IRB)-approved feasibility study (Netherlands Trial Register number NTR6123), following the criteria: PSA level £ 10.0 ng/mL, PSA doubling time (PSADT) ³ 12 months, tumour stage (MRI) £ T2c, and acceptable urinary function (International Prostate Symptom Score < 15). Because of favourable toxicity results after 2 years of inclusion, patients beyond the initial inclusion criteria were treated off-protocol. 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 £ 20.0 ng/mL, PSADT ³ 9 months, and tumour 8
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