Part I | Chapter 2 38 using the in-house developed software Volumetool®6 by experienced radiation oncologists on the pre-treatment MR scan. The Gross Tumour Volume (GTV) contained the MR visible tumour with a 4 mm isotropic margin for microscopic extension (GTV+4mm), excluding the organs-at-risk (OARs).7 The Clinical Target Volume (CTV) encompassed the prostate body including the GTV+4mm. For intermediate-risk patients, up to 1.0 cm of the seminal vesicles were included based on judgement of the treating physician. The Planning Target Volume (PTV) included the CTV with a 5 mm isotropic margin. Intensity modulated radiation therapy (IMRT) treatment plans were created using the Elekta Monaco® treatment planning system (Version 50.40.01, Elekta Inc., Stockholm, Sweden), prescribing a dose of 36.25 Gy to the PTV. Online workflow The ATS workflow is depicted in Figure 1.4 During each fraction, after positioning the patient on the treatment couch, a daily online T2-weighted MR scan was obtained with an acquisition time of 2 min, a field-of-view of 0.448 x 0.448 x 0.300 m, and a reconstructed voxel spacing of 0.8 x 0.8 x 2.0 mm.3 For the CT-based workflow, during the first fraction, the pre-treatment CT was registered to the online MR scan and the contours were propagated from the CT scan to the MR scan using DIR that is part of the Monaco treatment planning software for the Unity MR-Linac (Version 50.40.01, Elekta Inc., Stockholm, Sweden). For fractions 2 to 5, the online MR scan of the first fraction was registered to the daily MR scan and likewise the contours from fraction 1 were propagated to the daily MR. For the MR only workflow, contours were propagated from the pre-treatment MR to the daily online MR scan for all fractions. After DIR and contour propagation, the contours were checked andmanually adapted by certified RTTs. During each first fraction, a radiation oncologist was present for approval of the adapted contours. For the remaining fractions, only in case of specific questions or concerns a radiation oncologist was present. After approval of the daily contours, the treatment plan was recalculated, and a position verification (PV) MR scan was obtained. In case the CTV was no longer covered by the PTV, a dose shift was applied, also known as ‘Adapt-to-Position’ (ATP).4 During dose delivery, 3D cine-MR images were acquired for analysis of intrafraction prostate motion.8 Training and certification of RTTs To become a certified MR-Linac RTT, RTTs with either experience in treatment planning or clinical image processing, and preferably MRI experience, are trained both offline and online for four to six weeks. Training consists of general workflow training, image registration, contour adaptation, treatment planning, dose check, and treatment plan approval, depending on the profile of the RTT. The training for contour adaptation by RTTs consisted of two phases. During the first phase, RTTs performed five offline prostate CTV delineations on T2-weighted MR images. These were examined by an experienced prostate cancer radiation oncologist. To improve their contouring skills, any disagreement on the contours was discussed between the RTT and the physician. In case the physician was satisfied with the offline contours, 15 online contour adaptations were performed in conjunction with a radiation oncologist. Once the online contour adaptations were performed satisfactorily, RTTs obtained their certificate and were allowed to perform the adaptations without
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