Part III | Chapter 10 208 cone-beamCT linacs. To be confident the target will receive the prescribed dose, large error margins are needed (³ 3 mm) considering inter- and intrafraction motion and deformations. Especially with high fractional doses, intrafraction motion becomes a problem as the delivery times are well over 10 min. In combination with the less steep dose fall-off with SBRT compared to brachytherapy, dose to the OARs can increase significantly when no intrafraction adaptation is possible that counteracts the effect of intrafraction motion and deformations. The only option to reach such high target doses while respecting the OAR constraints, is to deliver the dose more accurately with small error margins (£ 2 mm). The 1.5 T MR-Linac could open up possibilities to do so. In chapter 9, the planning feasibility of focal salvage SBRT treatment using a 1.5 T MR-Linac was investigated. Thirty patients who were priorly treated with FS-HDR-BT within the PRECISE study, were included. For each patient, a new MR-Linac treatment plan was created using the delineations and imaging (CT and MRI) from the FS-HDR-BT treatment. To account for the additional uncertainties of SBRT, a 1 mm CTV to PTV margin was applied. This 1 mm PTV margin should be achievable when online treatment adaptation is available on the Unity MR-Linac. For treatment planning, the dose constraints used for FS-HDR-BT were applied. A dose of 19.0 Gy was prescribed to ³ 95% of the PTV. Similar to the FS-HDR-BT treatment, a dose of ³ 17.0 Gy to ³ 90% of the PTV was accepted in case this target could not be reached, for example due to violation of OAR constraints. The MR-Linac plans were created using the Monaco treatment planning software for the Unity MR-Linac, which takes into account the magnetic field and beam angles that are possible on this machine. Pairwise comparison of the FSHDR-BT andMR-Linac treatment plans was conducted. This study showed that target dose coverage was achieved in 57% of the MR-Linac plans compared to 47% of the FS-HDR-BT plans, with a comparable median D95% and D90%. However, for the FS-HDR-BT plans, a larger volume reached 150% (median volume 55% versus 1%) and 200% of the pre-scribed dose (median volume 25% versus 0%). No significant differences were found in the median dose to the bladder. On the other hand, urethra D10%, rectum D1cm3, and rectum D2cm3 were significantly lower in the FS-HDR-BT plans, although the differences are probably clinically irrelevant. In conclusion, single fraction treatment of recurrent prostate cancer lesions may be feasible using SBRT on a 1.5 T MR-Linac system. The clinical impact of the (small) differences in dose to the target and OARs should be explored and balanced with the lower costs and potentially improved patient experience. Technological advancements, such as realtime plan adaptation or tracking and exceptional gating, are warranted before a 1 mm PTV margin can be safely adopted. Conclusion and future perspectives The work presented in this thesis provides insight into the first period of the clinical implementation and technological development of MRI-guided radiotherapy for prostate cancer. The developments within MRI-guided radiotherapy are rapidly succeeding and initial outcomes in prostate cancer patients are promising. The presented work is part of the first steps towards MRI-guided, highprecision radiotherapy in one or two fractions for prostate cancer. The results can be used to guide the research that is needed to make single or two-fraction MR-Linac treatment possible, without unacceptable toxicity rates or suboptimal oncological outcomes. To achieve this, further technological developments are needed that allow real-time intervention during radiotherapy
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