Focal salvage radiotherapy for recurrent prostate cancer 183 Introduction Up to 50% of the high-risk prostate cancer patients treated with primary dose-escalated radiation therapy develop a biochemical recurrence within 10 years of treatment.1,2 In more than half of the patients with a recurrence detected on imaging, the recurrence is confined to the prostate and/or seminal vesicles.3 A treatment option for local radiorecurrent prostate cancer is magnetic resonance imaging (MRI)-guided focal salvage high-dose-rate brachytherapy (FS-HDR-BT).4,5 With FS-HDR-BT, patients are treated in a single treatment session with a dose of 19.0 Gy to the Clinical Target Volume (CTV). The radiation is delivered locally through catheters inserted via the perineum into the prostate using an Ir-192 source.4 However, due to the invasiveness of the treatment and the need for spinal anaesthesia, not all patients are eligible. Furthermore, a non-invasive treatment option will increase patient comfort. The recent clinical introduction of magnetic resonance linear accelerator (MR-Linac) systems opens up a non-invasive treatment possibility that uses stereotactic body radiotherapy (SBRT).6 An MRLinac integrates an MRI scanner with a linear accelerator.7 This technology enables the visualisation of the anatomy before and during treatment, allowing for daily adjustment of treatment plans.8,9 This leads to improved targeting compared to current conventional SBRT. Consequently, radiation oncologists can reduce safety margins and deliver a higher dose per fraction. These aspects could make the MR-Linac a viable non-invasive alternative to FS-HDR-BT for treatment of radiorecurrent prostate cancer in a single fraction. We conducted a planning study to evaluate the feasibility of delivering a single 19.0 Gy dose to a locally radiorecurrent prostate cancer lesion using a 1.5 Tesla (T) MR-Linac system. The MR-Linac plans were compared to the clinically delivered FS-HDR-BT plans. Materials and methods Patient characteristics For this study, we included 30 patients that had previously been treated with FS-HDR-BT at the Department of Radiation Oncology of the UMC Utrecht between November 2014 and April 2019. Patients were included from two pre-existing FS-HDR-BT studies (Netherlands Trial Register numbers NTR6123 and NTR7014) approved by our institutional review board. Written informed consent was obtained for use of the data for this planning study. Tomake the results more generalisable, different tumour locations within the prostate were identified and patients were divided accordingly. Next, the 30 patients were selected randomly (using a random number table) from our patient database, taking into account the general tumour location, but without knowledge of the exact anatomy and FS-HDR-BT dose distribution. Baseline tumour characteristics of these 30 patients are displayed in Table S1 (Supplementary A). The different tumour locations are visualised in Figure 1. 9
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