Marieke van Son

45 MRI-GUIDED ULTRAFOCAL HDR-BRACHYTHERAPY INTRODUCTION Due to PSA-screening and diagnostic advancements prostate cancer has undergone a stage migration to more localised disease(1). Traditionally, whole-gland radical pros- tatectomy and radiotherapy are standard of care. However, neither showed an over- all survival advantage when compared to active monitoring in the ProtecT trial, while side-effects were frequent(2,3). Even though most patients in this trial had low-risk disease potentially eligible for active surveillance, approximately 20% of patients rep- resented an intermediate-risk subgroup who could benefit from curative treatment(2). Focal therapy (FT) could offer a suitable alternative for men with clinically significant prostate cancer in terms of cancer control, while decreasing genitourinary (GU) and gastro-intestinal (GI) toxicity, and erectile dysfunction (ED)(4). The largest cohorts of FT after a median follow-up of 5 years reported biochemical disease-free survival (BDFS) rates of 88-92%(5,6), with favourable toxicity profiles(4,6). Recent advancements in magnetic resonance imaging (MRI) and MRI-guidance during treatment have made an ultrafocal approach possible, as opposed to segmental ablation. We have previously described preliminary results of our prospective feasibility study of MRI-guided ultrafocal high-dose-rate (HDR)-brachytherapy at median 2 years follow-up(7). In the current paper, we report updated results regarding toxicity, quality of life (QoL) and tumour control at 4 years median follow-up. MATERIALS AND METHODS Between May 2013 and April 2016, 30 consecutive patients underwent MRI-guided ultra- focal HDR-brachytherapy within a feasibility study. The study was approved by the In- stitutional Review Board (IRB) of the UMC Utrecht and written informed consent was ob- tained from all patients. Patient selection criteria, treatment procedures and follow-up assessment have been described previously(7,8). They are briefly discussed below. Patient selection criteria Patients were eligible if they had the following clinical characteristics: 1. Age >65 years, 2. Karnofsky score ≥70, 3. T-stage ≤T2c, 4. Gleason sum score ≤7, 5. PSA <10 ng/mL, and 6. IPSS <15. Diagnostic procedures and treatment Imaging of the intraprostatic lesion was performed using 3T multiparametric (mp)- MRI, consisting of T2-weighted, diffusion-weighted imaging (DWI) and dynamic con- trast enhanced (DCE) sequences. PET-scans were not part of the diagnostic work-up. Tumour lesions were pathologically verified using systematic biopsies. Radiological concordance with positive biopsy location was required. All MRI-sequences were used for delineations of the gross tumour volume (GTV), prostate and organs at risk (ure- thra, bladder, rectum). An intraprostatic margin of 5mm was applied around the GTV to 3