Part II | Chapter 9 182 Abstract Background and purpose Magnetic resonance imaging (MRI)-guided focal salvage high-dose-rate brachytherapy (FS-HDR-BT) is one of the treatment options for radiorecurrent localised prostate cancer. However, due to the invasive nature of the treatment, not all patients are eligible. Magnetic resonance linear accelerator (MR-Linac) systems open up new treatment possibilities and could potentially replace FS-HDR-BT treatment. We conducted a planning study to investigate the feasibility of delivering a single 19.0 Gy dose to the recurrent lesion using a 1.5 Tesla (T) MR-Linac system. Materials and methods Thirty patients who underwent FS-HDR-BT were included. The Clinical Target Volume (CTV) encompassed the visible lesion plus a 5 mm margin. Treatment plans were created for a 1.5 T MRLinac system using a 1 mm Planning Target Volume (PTV) margin. A dose of 19.0 Gy was prescribed to ³ 95% of the PTV. In case this target could not be reached, i.e. when organ-at-risk (OAR) constraints were violated, a dose of ³ 17.0 Gy to ³ 90% of the PTV was accepted. MR-Linac plans were compared to clinical FS-HDR-BT plans. Results Target dose coverage was achieved in 14/30 (47%) FS-HDR-BT plans and 17/30 (57%) MR-Linac plans, with comparable mean D95% and D90%. In FS-HDR-BT plans, a larger volume reached ³ 150% of the prescribed dose. Urethra D10%, rectumD1cm3, and rectumD2cm3 were lower in the FS-HDR-BT plans, while bladder dose was comparable for both modalities. Conclusion Single fraction treatment of recurrent prostate cancer lesions may be feasible using stereotactic body radiotherapy (SBRT) on an MR-Linac system.
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