ESTRO 2022 - Abstract Book

S273

Abstract book

ESTRO 2022

the experience as somewhat taxing. A slightly lower satisfaction was noted by three of the first 5 patients whose OSS procedure took >6.0h, as they had to wait at least 3.0h between simulation and treatment.

Conclusion A OSS MR-guided lung SBRT service was successfully implemented. Patients preferred pre-treatment imaging and delivery in one single day, and also appeared more satisfied with shorter stays at the department. Measures to further shorten the workflow are being explored, and use of the OSS implementation will extend to other tumor sites.

MO-0312 Systematic multi-disciplinary sequence evaluation for integration into the MR-linac workflow

S. Alexander 1 , J. Chick 2 , T. Herbert 1 , R. Huddart 3 , M. Ingle 3 , A. Mitchell 2 , S. Nill 2 , U. Oelfke 2 , A. Dunlop 2 , S. Hafeez 3

1 The Royal Marsden NHS Foundation Trust, Radiotherapy, Sutton, United Kingdom; 2 The Royal Marsden Hospital and the Institute of Cancer Research, Joint department of physics, Sutton, United Kingdom; 3 The Royal Marsden Hospital and the Institute of Cancer Research, Uro-Oncology, Sutton, United Kingdom Purpose or Objective Vendor-approved Magnetic Resonance (MR) sequences are provided for MR-linac workflows however alternative ‘off-label’ sequences may offer advantages. Prior to clinical use, the safety, accuracy and overall clinical benefit should be determined. The aim of this study was to develop a systematic approach for off-label sequence evaluation and demonstrate the application for bladder cancer MR-guided radiotherapy (MRgRT) on Unity (Elekta AB, Stockholm). Materials and Methods Two T2-weighted Turbo Spin Echo off-label sequences were proposed, denoted 1.1min3mm and 3.4min2mm, indicating both acquisition time and slice thickness. Each sequence was assessed against the vendor provided 2min1mm sequence. The off-label sequences were acquired during opportunity scanning time on 3 patients receiving daily bladder cancer MRgRT, for up to 3 fractions per patient. Images were exported to Monaco treatment planning system (TPS) for offline review (v5.40.1, Elekta). Microsoft forms employing four-point Likert scales were created to guide qualitative review of image quality and workflow suitability, carried out independently by two doctors and two treatment radiographers. Geometric accuracy, image uniformity and ghosting were assessed using the ACR phantom (acr.org). The impact of the larger slice thickness within the TPS included assessment of margin expansions, optimisation behaviour and dosimetric impact (carried out by physicists). Existing clinical bladder treatment image data (2min1mm) were used in VolumeView (v5.3.31, Philips Medical Systems, Best) to reconstruct 2mm and 3mm slice thickness data sets for plan generation using the same clinical template. Treatment plans were then recalculated on the corresponding 2min1mm sequence and were compared using clinical goals, and dose conformity. To quantify the potential reduction in overall treatment time for the shorter acquisition of the 1.1min3mm sequence, a phantom treatment workflow was carried out. Results Table 1 summarises the results. Both sequences were shown to be accurate, safe and of non-inferior utility to the 2min1mm sequence and facilitated clinically acceptable treatment plans in Monaco. For margins that are not integer multiples of the slice thickness, the effective margin can differ (see Fig 1), however no dosimetric impact was seen. The workflow time reduction of 4min for the 1.1min3mm sequence can be attributed to both shorter acquisition and reduced dataset size. This could translate into a 14% time reduction to the average bladder cancer MRgRT at our centre.