Marieke van Son

123 TOXICITY AFTER ULTRAFOCAL SALVAGE HDR-BRACHYTHERAPY INTRODUCTION Patients with a local prostate cancer recurrence after radiotherapy are potential can- didates for curative salvage treatment, which offers the opportunity to avoid or post- pone palliative androgen deprivation therapy (ADT), thereby preventing patients from its associated metabolic, cardiovascular, sexual and psychological side-effects(1, 2). Whole-gland salvage treatments are generally associated with (severe) side-effects. A recent prospective study on whole-gland salvage brachytherapy reported 14% grade 3 toxicity(3). The aim of focal treatment is to solely target the recurrent tumor and there- fore further reduce toxicity, potentially with comparable cancer control. Improvements in imaging for selection and treatment, most notably prostate specific membrane antigen (PSMA)-PET/CT, have advanced the field of focal salvage treat- ment(4). Across different modalities, toxicity from focal salvage treatment seems lim- ited compared to whole-gland salvage treatment, with event rates of severe (grade 3) genitourinary (GU) and gastro-intestinal (GI) toxicity as low as 5% and erectile dys- function (ED) often reduced, allowing some patients to preserve their potency(5-7). However, reported series in literature are mostly retrospective and small, using a wide range of patient- and physician-reported toxicity outcome measures. This leads to bias and prevents adequate assessment of risk factors which could be used to reduce or avoid associated side-effects of treatment. We previously reported tumor control and functional outcomes of 50 patients after two years follow-up(8) and we investigated patient-reported quality of life of 100 pa- tients treated with MRI-guided ultrafocal salvage high-dose-rate brachytherapy (HDR- BT)(9). With an emphasis on further safety evaluation, the current study reports pro- spectively collected data of physician-graded GU and GI toxicity and ED in a total of 150 treated patients. Additionally, we analyze potential risk factors for toxicity to improve treatment planning and to guide patient counselling. MATERIALS AND METHODS Patients We used data from a single-center prospective registry of patients treated with ultrafo- cal salvage HDR-BT. The first consecutive 150 patients were included, treated between July 2013 and November 2019. As described previously(9), patients were either treated within an institutional review board (IRB)-approved study (Netherlands Trial Register 6123 or 7014) or outside the scope of a study protocol if tumor characteristics were incompatible with study inclusion criteria. All patients (on- or off-protocol) were pro- spectively followed in the same manner. Pre-treatment characteristics varied from lower- to higher-risk disease, but acceptable baseline urinary toxicity (International Prostate Symptom Score [IPSS] <15) was required for all patients. Study patients all signed informed consent. The IRB waived the requirement for informed consent for off-protocol patients. 7