Chapter 1 24 Thesis outline This thesis covers a wide range of topics with a common denominator: MRI-guided radiotherapy for the treatment of prostate cancer. It provides evaluations of technological and clinical aspects of MRIguided radiotherapy in patients with primary localised and locally recurrent prostate cancer. Part I of this thesis focuses on the clinical introduction, technological developments, and clinical evaluation of MRI-guided SBRT using a 1.5 T MR-Linac for the treatment of primary localised prostate cancer. In February 2019, the first prostate cancer patient was treated on an MR-Linac at the Department of Radiation Oncology of the UMC Utrecht. During the first year, the online clinical workflow was fine-tuned. To reduce the workload of physicians, some of the work was carried onto radiation therapists (RTTs). This included the check and manual adaptation of contours. In chapter 2, the feasibility of this RTT-led workflow for prostate cancer treatment is assessed, with a focus on the clinical quality of the contours used for the actual treatment of patients. Since manual contour adaptation for daily adaptive treatment is quite labour-intensive and timeconsuming, our work has focussed on optimising the online clinical workflow. By improving the interfraction propagated target and OAR contours, the workload can be reduced. Furthermore, fast, online adaptive workflows can be enabled with accurate intrafraction contour propagation. In chapter 3, we assess the clinical quality and usability of propagated contours that were created by a deformable image registration algorithm. This work paves the way for exploring intrafraction adaptive workflows. So far, only low- and intermediate-risk patients have been treated with SBRT using an online adaptive ATS workflow on the MR-Linac. Patients with high-risk disease often have more extensive disease with involvement of the seminal vesicles. Currently, these patients are still treated in ³ 20 fractions. Before progressing to SBRT-like treatments in these patients, information on intrafraction seminal vesicle motion during treatment is warranted. At our department, a soft tissue-tracking algorithm was developed that uses 3D cine-MR images to track the motion of the prostate during radiotherapy treatment. In chapter 4, a similar method is described to determine the intrafraction motion of the seminal vesicles from 3D cine-MR images using this soft tissue-tracking algorithm. This work serves as a basis for future MR-Linac treatment of high-risk prostate cancer patients. Real-time, intrafraction adaptive treatments are – at the moment of writing – not yet clinically available on 1.5 T MR-Linac systems. Therefore, intrafractionmotion can still have a significant impact on the accuracy of dose delivery. This is especially true for SBRT treatments with relatively long beam-on times. In chapter 5, a new intrafraction adaptive workflow for 1.5 T MR-Linac systems is presented. The sub-fractionation workflow enables the delivery of multiple treatment plans within a single treatment session. This allows the delivery of a fraction in multiple parts (sub-fractions), e.g. a fraction of 7.25 Gy can be delivered in two sub-fractions of 3.625 Gy each. For each sub-fraction, the plan is updated based on the latest anatomy and this way, systematic (drift) intrafraction motion can potentially be counteracted. For efficiency, imaging and treatment planning are performed
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