Focal salvage radiotherapy for recurrent prostate cancer 191 focal recurrent tumour using a volumetric modulated arch therapy (VMAT)-like technique resembles SBRT. Several studies on ultra-hypofractionated and single session radiotherapy for prostate cancer use only high-dose to low-volume constraints, thereby showing low toxicity rates.18,22,23 Accordingly, we argue that D1cm3 is a sensible and important parameter allowing for comparison between the two treatment modalities. Fourthly, due to the small stratified sample sizes, we were unable to draw any definitive conclusions from the dosimetric analysis per tumour location. Lastly, by using the intraoperative delineations from the FS-HDR-BT treatment, we took into account the effect of volume changes (oedema) that might have occurred during brachytherapy due to the insertion of catheters. While no volume changes will occur with MR-Linac treatment, we used the same delineations for MR-Linac treatment planning. Therefore, the volumes might not be completely representative for MR-Linac treatment. However, for a fair comparison of the dose distributions, no adaptations were made to the delineations to include identical target volumes for both treatment plans. Conclusion In conclusion, we showed that for the majority of the patients treated with FS-HDR-BT, we could create an acceptable and comparable MR-Linac plan. This demonstrated the feasibility of SBRT treatment planning for radiorecurrent prostate cancer using a single 19.0 Gy dose on a 1.5 T MRLinac system, while respecting FS-HDR-BT OAR constraints. Hence, MR-Linac systems may provide a non-invasive alternative to FS-HDR-BT. Research should be conducted on the necessity of the larger high-dose volumes achieved in FS-HDR-BT, which may favour oncological outcomes in the longterm. Before clinical application of salvage MR-Linac treatment, real-time intrafraction adaptation and dose calculation should be technically enabled, and clinical feasibility has to be tested in early phase clinical trials. 9
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