ESTRO 2020 Abstract book

S314 ESTRO 2020

Conclusion The overall measurement error of the end-to-end phantom system was approximately 0.1 mm. Based on this system, the accuracy of ART dose delivery at a high-field MRL was found better than 1 mm. The precision of MRL ART dose delivery varied with direction and likely depended on voxel size. PH-0531 Assessment of geometric distortion of eleven clinical MRI scanners across the UK M. Alzahrani 1,2 , D. Broadbent 3 , R. Chuter 4 , B. Al-Qaisieh 3 , S. Jackson 4 , M. Hutton 4 , R. Johnstone 5 , S. Shah 5 , A. Wetscherek 6 , J. Chick 6 , J. Wyatt 7 , H. McCallum 7 , R. Speight 3 1 University of Leeds, Department of Biomedical Imaging Science, Leeds, United Kingdom ; 2 King Abdulaziz University, Department of Diagnostic Radiology, Jeddah, Saudi Arabia ; 3 Leeds Teaching Hospitals NHS Trust, Department of Medical Physics and Engineering, Leeds, United Kingdom ; 4 The Christie NHS Foundation Trust, Department of Medical Physics and Engineering, Manchester, United Kingdom ; 5 Guy’s and St Thomas’ NHS Foundation Trust, Department of Medical Physics and Engineering, London, United Kingdom ; 6 Joint Department at the Institute of Cancer Research and the Royal Marsden NHS Foundation Trust., Department of Medical Physics and Engineering, London, United Kingdom ; 7 Newcastle upon Tyne Hospitals NHS Foundation Trust and Newcastle University, Department of Medical Physics and Engineering, Newcastle, United Kingdom Purpose or Objective MRI is increasingly being used in radiotherapy (RT) in the UK for delineation, MRI-only planning and image guidance on an MR-linac. Geometric distortions are a known

Results The mean difference between dose delivery errors measured using the two different flatbed scanners was 0.1 mm or below with a standard deviation (SD) of 0.1 mm in both CC, LR, and posterior-anterior (PA) direction. The mean ART dose delivery error of the MRL was 0.3 mm (SD 0.7 mm), 0.1 mm (SD 0.2 mm), and 0.5 mm (SD 0.2 mm) in the CC, LR and PA direction, respectively. The dose delivery error depended on the isocentre shift in the CC direction (R=0.52, p<0.001) and PA direction (R=0.63, p<0.001), but not the LR direction (R=0.03, p=0.87) as seen in figure 2. Dose delivery errors above 1.0 mm were only found in the CC direction. Visual inspection of the planning and session images leading to these errors revealed an image misalignment of the automatic registration of the dedicated TPS.