Part III | Chapter 10 204 Intrafraction motion of the seminal vesicles So far, most patients treated with SBRT on anMR-Linac at the UMC Utrecht had low- or intermediaterisk prostate cancer. Patients with high-risk disease often have more extensive disease with seminal vesicle involvement. They are currently treated with ³ 20 fractions. Before treating these patients with less fractions and with smaller error margins, information on intrafraction seminal vesicle motion is warranted. At our department, a soft tissue-tracking algorithm was developed that uses 3D cine-MR images to track the prostate during radiotherapy treatment. The information on the intrafraction motion that is extracted from these 3D cine-MR images, could be used as an input for online adaptation possibilities. In chapter 4, we applied an improved version of this soft tissuetracking algorithm to a new dataset of 50 prostate cancer patients who were treated on a 1.5 T MRLinac. All patients were treated with 5 x 7.25 Gy. During treatment delivery, 3D cine-MR images were acquired, with a temporal resolution of 9.4 sec; i.e. each 9.4 sec a full 3D set was acquired, referred to as ‘dynamic’. A total of 15938 cine-MR dynamics were included (over 247 fractions in 50 patients). The left and right seminal vesicle were delineated by a physician on each first cine-MR dynamic of each fraction, and the contours were used as input for the tracking algorithm. The results showed that the translational motion of the seminal vesicles is larger in the cranial direction compared to the motion of the prostate. Prostate motion, however, was larger in the posterior and caudal direction. On the other hand, seminal vesicle motion was comparable to prostate motion in the leftright and anterior direction. Concerning rotational motion, a significantly larger spread was observed in all axes (left-right, cranial-caudal, anterior-posterior) for the seminal vesicles compared to the prostate. To estimate the margins needed for treatment, a coverage probability analysis was conducted. This showed that 99% of the volume of both seminal vesicles could be covered in at least 95% of the fractions when using 5 mm isotropic volume expansion. To our knowledge, this is the first study that quantified seminal vesicle motion in six dimensions using 3D cine-MR imaging acquired during actual MR-Linac SBRT treatment. This work serves as a basis for future MR-Linac treatment of high-risk prostate cancer patients. Intrafraction adaptive treatment 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 and thus limits further dose escalation and/or error margin reduction. In chapter 5, we presented a new and efficient intrafraction adaptive workflow for 1.5 T MR-Linac systems. The sub-fractionation workflow enables the delivery of multiple treatment plans within a single treatment session, and therefore allows the delivery of a fraction in multiple parts (sub-fractions). Since for each sub-fraction the treatment plan complies with the dose constraints, this approach does not require online dose accumulation. To enable the delivery of multiple treatment plans within a single treatment session, the so-called ‘JamTool’ was developed. For each sub-fraction, a new treatment plan is created, which is adapted (shifted) to the latest anatomy just prior to delivery. The hypothesis was that splitting the fraction into multiple sub-fractions, all of which are adapted to the latest anatomy, would reduce the systematic intrafraction prostate motion compared to single-plan delivery. The workflow was clinically applied in 15 prostate cancer patients
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