Part III | Chapter 10 202 MRI-guided radiotherapy The clinical introduction magnetic resonance imaging (MRI)-guided linear accelerators (MR-Linac) offers new opportunities to further increase treatment accuracy. By using MR imaging before and during treatment, soft tissues can be visualised and for example, the motion of the prostate during treatment can be tracked. This information can be used to adapt the treatment in case of anatomical changes between fractions (interfraction) as well as during a treatment fraction (intrafraction). Currently, MR-Linac systems can be used to adapt the treatment according to the anatomy of the day, thereby already improving treatment accuracy compared to conventional linacs. In the near future, adaptation during the actual treatment delivery will become available. Although many radiotherapy departments have already adopted MR-Linac radiotherapy as a standard of care for some indications, clinical evidence for the use of an MR-Linac for the treatment of prostate cancer is still lacking. Furthermore, technological advancements, such as organ motion tracking and realtime treatment plan adaptation, are needed to optimally treat the target and spare the OARs. Only then, it will be possible to proceed to extremely hypofractionated radiotherapy (i.e. in £ 3 fractions) with small error margins (i.e. < 2 mm). This thesis presents research that was conducted with the aim to contribute to the clinical introduction, implementation, and evaluation of MRI-guided radiotherapy for prostate cancer. Therefore, technological developments are discussed and (early) clinical outcomes are evaluated. In part I of this thesis, we focused on MRI-guided SBRT using an MR-Linac for the treatment of primary localised prostate cancer. In part II, we focused on MRI-guided treatment for locally recurrent prostate cancer after primary radiotherapy. Part I. MRI-guided stereotactic radiotherapy for primary localised prostate cancer A new workflow for prostate cancer radiotherapy treatment At the Department of Radiation Oncology of the UMC Utrecht, the first prostate cancer patient was treated on an MR-Linac in February 2019. During the first year, the online clinical workflow was finetuned. The term online implies that this is the workflow during actual treatment with the patient positioned on the treatment couch. It turned out that the Adapt-to-Shape (ATS) workflow, during which the contours are propagated between the pre-treatment and daily MRI scan and then are manually edited, significantly increased the workload of the treating radiation oncologist (physician). To reduce the workload for physicians, most of the work was carried onto radiation therapists (RTTs). In chapter 2, we assessed the feasibility of assigning the task of contour check and manual editing to the RTTs. Before transitioning the task from physicians to RTTs, RTTs underwent a hands-on training, covering all aspects of MR-Linac treatment. This consisted of offline (without actual patient treatment) and online (with a patient on the treatment couch) workflow training, image registration training, contour adaptation training, and treatment planning and dose check training. Concerning the contour adaptation, RTTs performed 5 offline delineations as well as 15 online contour
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