ESTRO 2020 Abstract book

S73 ESTRO 2020

Amsterdam UMC, where adaptive MRgRT has been delivered since early 2016, main areas of focus have been localized prostate and pancreatic cancer, high-risk lung SBRT patients, and oligometastatic disease (Figure 1). This presentation aims to illustrate the workflow of (adaptive) MRgRT and its clinical results reported in among others (locally advanced) pancreatic cancer, kidney- and prostate cancer, illustrating that clinical MRgRT currently is well- established for selected indications. Figure 1: Patient distribution for adaptive MRgRT.

Symposium: MR-guided Radiotherapy: state of the art and future perspectives

SP-0132 Physics aspects of MRI guided radiotherapy S. Nill 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom Abstract text In room MR image guidance enables the online adaptation of treatment plans based on the current patient anatomy while the patient is on the treatment couch. The two available commercial MR guided systems each combine a MR scanner and a linear accelerator. The integration of both systems into a hybrid device introduces challenges with regards to dosimetry, machine QA, fast online plan adaption and patient workflow. After a brief introduction to the challenges posed by the static magnetic field on dosimetry a summary of tests developed to quality assure a MR linac with exemplary results will be presented. This will be followed by the introduction of a generalized MR Linac workflow including a discussion of the available methods to calculate dose distributions using MR images, contour propagation, and fast online plan adaptation. The presentation will conclude with focusing on the necessity for a fast and automated way of checking the daily generated adapted treatment plan and potential future solutions for reconstructing the daily delivered dose distribution more accurately. SP-0133 MRI guided radiotherapy: a clinical reality A. Bruynzeel 1 , F. Lagerwaard 1 1 amsterdam Umc- Location Vumc, Radiation Oncology, Amsterdam, The Netherlands Abstract text The clinical introduction of magnetic resonance image- guided radiation therapy (MRgRT) is the latest development in Radiation Oncology towards (adaptive) personalized high-precision radiation therapy. In addition to guidance using real-time MR imaging, it offers the capability to optimize each treatment fraction to the anatomy of the moment, i.e. online adaptive delivery. Introducing MRgRT in clinical practice, certainly if this is combined with daily plan adaptation, requires adjusting existing workflows and protocols. Several MRgRT systems have been developed, e.g. MRIdian (ViewRay) and Unity (Elekta), are already in clinical use and other systems will be implemented in the near future. The clinical application of MRgRT was initiated in the United States in 2014 for patients with a variety of tumor sites. In more recent years, daily plan adaptation, real- time tumor tracking and automated gated delivery have been added to the clinical workflow of MRgRT. MR-based tumor setup has clear advantages over (cone-beam) CT- based setup with or without inserted fiducials, because both tumors and relevant organs-at-risk can be defined with higher resolution. One specific challenge of the optional adaptive part of MRgRT is that it needs to be fast and robust since it is performed with the patient in treatment position. Because of this, the majority of centers has developed an online workflow which can be performed within minutes, requiring limited (re- )contouring by the radiation oncologists or radiotherapists. But even using these dedicated workflows, total delivery duration of a single fraction of adaptive MRgRT, including all necessary steps, currently remains long with on average 45 minutes up to one hour. As a consequence, daily adapted MRgRT is particularly suited for treatment with (extreme) hypofractionation, and specifically used for stereotactic body radiotherapy (SBRT). For instance at

SP-0134 The role of the RTT’s in MRI-guided radiotherapy M. Büttner Roed Andersen 1 1 Copenhagen University Hospital, Radiotherapy, Copenhagen, Denmark

Abstract text MRI-guided radiotherapy is a relatively new treatment modality, and in January 2019 the first patient was treated on our MR-Linac at Copenhagen University Hospital in Denmark. A lot of preparation was needed by both physicians, physicists and RTTs prior to the treatment of our first patient. All professions cooperated in the decision-making regarding protocols to the simulation and treatment on the MR-Linac. The preparation phase also included the selection of new immobilization equipment and decisions on how to position the patient during simulation and treatment. A small group of five RTTs were allocated to the MR-Linac and received on-site training by ViewRay staff, in either Ohio or at the clinic in Copenhagen. Not only was the user interface on the MR-Linac new, also the matching protocols were different than the protocols used on the CT-Linacs. The RTTs were used to perform surrogate matching on primarily bony anatomies. Now the improved soft-tissue contrast in the MRI-scans made it possible to perform soft-tissue match close to the target. Also several new RTT assignments were introduced and therefore new skills were needed. This included contouring of the tracking structures used during treatment to ensure exact target placement despite intrafractional movement like bowel movement and breathing. The development on the MR-Linac is rapidly growing. In the beginning we just treated patients in FB (free breathing), e.g. post-operative prostate cancer patients. Now we are gating liver- and lung cancer patients in IBH (inspiration breath hold) and EBH (expiration breath hold) as well. However, this also means, that the overall treatment time is now longer. During gated treatment, the patient can either watch entertainment, follow the live MRI-tracking of the tumor or get audio and/or visual gating instructions on the visual feedback screen.

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