ESTRO 2020 Abstract Book

S333 ESTRO 2020

OC-0592 Implementation of MR-only one-day radiosurgery workflow for non-spinal bone metastases on MR-Linac M. Chamberlain 1 , S. Tanadini-Lang 1 , L. Wilke 1 , A. Moreira 1 , N. Weitkamp 1 , S. Ehrbar 1 , N. Andratschke 1 , M. Guckenberger 1 , H. Garicia Schüler 1 1 University Hospital Zürich, Department of Radiation Oncology, Zurich, Switzerland Purpose or Objective To implement a one-day on-table workflow on the MR- Linac (Viewray) without CT based planning for high dose (12/16Gy) radiosurgery of painful non-spinal bone metastases. Material and Methods Patients with painful bone metastases who were candidates for this treatment were consented and screened for MRI compatibility. The clinician prescribed one fraction of either 12Gy or 16Gy depending on the lesion size. In preparation, a dummy plan was created in the MR-Linac system using previous imaging to prepare a patient specific plan template, this takes approximately 1 hour. The patient was positioned, and an initial MR image was taken. The clinician contoured the GTV on the MR and expanded 5mm to get the PTV. A pseudo-CT was prepared using bulk density overrides of bones, air, lung, fat, and soft tissue for dose calculation. A step-and-shoot IMRT treatment plan was optimised, and approved by the clinician, while the patient stayed on the table. Online QA (monte-carlo recalculation (gamma agreement score 2mm/2%) and point dose calculation using the Clarkson method) was performed. Finally, the single fraction dose was delivered to the patient. Each plan was later measured on a Delta 4 Phantom (ScandiDos) and evaluated using a criterion of 3%/2mm. Evaluation of the pseudo CT technique was performed prior to the development of this workflow. Mean dose difference to the PTV when calculated with a CT based electron density map was compared with a bulk density override based calculation for 20 patients treated in the pelvis, lung, and abdomen. Results A total 7 treatments were performed with this method, all evaluated treatments were prescribed a single fraction of 12Gy based on the size of the tumour. The first 4 were performed over a two day procedure to trial the workflow, then a following 3 treatments were performed with the final single day workflow. The first patient treated in the single day workflow took a total time of 66 minutes, the following two were treated in less than 1 hour. Evaluation of the Pseudo-CT method showed a median dose difference in the PTV compared to CT based calculation of 0.75% (-0.09% – 4.46%). For 6 of the treatments the online QA gave a median passing rate of 98.9%(94.01%-99.07%), and for all 7 treatments a delta4 measurement passed with median passing rate of 99.7% (95.2%-100%) with the criterion of 3%/2mm. The point dose calculation using Clarkson method had a median of -3.8% (-14.4%-0.6%) to the MC calculated point dose. Conclusion One-day radiosurgery with on-table treatment planning has been successfully implemented. A pseudo-CT technique was used, which accurately estimates electron density compared to CT based calculations, and eliminates the need for a planning CT. All treatments were verified with patient specific QA which passed. Patients can now receive treatment for painful non-spinal bone metastases in a single day and single appointment on an MR-Linac as part of a prospective study protocol regarding patient comfort, clinical feasibility, and compliance.

optimal training, content and delivery methods. The objectives were to engage expert RTTs to disclose their thoughts on IGRT training content and delivery; to identify main themes, ideas and to gain consensus on the importance of each area highlighted; and to report a concise list of recommendations for key elements of IGRT training nationally in the UK. Material and Methods A three round modified Delphi was planned using online surveys to meet the study objectives. A purposive sample was used to meet inclusion criteria to ensure an expert panel of RTTs. Round one used semi-structured questions with open text. The data was analysed using content analysis. Statements and content were then sent to participants in round two, using Likert scales, rating statements on importance from 1-5 (Fig. 1). The consensus was accepted at 75% agreeance. Those statements that did not reach concordance in round two were re-presented in round three for arbitration.

Results 40 RTTs from the UK were recruited. Content analysis in round one highlighted nine themes: Staff acquiring and assessing images, Staff training and assessment, troubleshooting and Escalation, Layout of training, Delivery of Training, National Training, Content of training package, Time to train staff, and Competency. 126 statements gained 75% consensus or more. 30 Statements across the round two and three achieved 100% consensus. Including; “Trainees should be taught to detect trends in set up errors” and “Rotations into planning should start at undergraduate level”. 10 new statements derived from feedback in round two. In total 116 statements regarding IGRT training were deemed to gain consensus. Retention throughout the study was 90%. Conclusion To our knowledge this is the first known Delphi study to support the development of RTT IGRT training documentation and guidelines. The next step is to facilitate a national and international guideline for RTT training and assessment to improve IGRT practice.