ESTRO 2022 - Abstract Book

S694

Abstract book

ESTRO 2022

The 15 patients exhibit 32 PTV targets and 216 organs at risk (OARs) in total. Several points of the dose volume histogram (DVH) were compared; the 98%, 50% and 2% (near, median and max coverage) for the targets, and the 2% and mean dose (serial and parallel type indicators) for the OARs. The discrepancies were normalized relative to the prescribed dose Results Figures 1 and 2 show the relative discrepancies between the doses for the MR-only and CT approach. The dash red lines and bands indicate mean and ±1SD of the data. For all target DVH points in figure 1, all discrepancies are <2% with a systematic shift in mean value of -0.16% For the 2 OARs DVH points, the 2 the highest discrepancy is <1.4% and mean value <-0.08%. In general, the deviations for the OARs show less spread as compared to the targets which suggests less dosimetric uncertainty (more robustness for ED assignment) of the OARs than for the targets

Conclusion This study evaluates the possibility of using MR-only for MRgOART in the upper/abdominal-thoracic region. With MR-only using assigned ED of water and lung, the discrepancies with the CT is less than 2% and 1.4% for the targets and OARs, respectively. Therefore, variations of less than 2% are expected when the CT-based ED map does not follow the MR image in adaptive radiotherapy workflows for the upper abdominal/thoracic region. We believe that the way forward is to assign an RED for all the structures based on e.g. the ICRU 46 report or synthetic CT generation directly from the MRI. This will allow for a more thorough comparison between ED methods

OC-0776 Preliminary report on Longitudinal Stability of MRI QA up to 2 years on 7 clinical 1.5 T MR-Linacs

A. Wetscherek 1 , E. v.d. Bijl 2 , A.L.H. v. Lier 3 , S. Jackson 4 , S. Gibson 5 , V.W.J. v. Pelt 6 , S.W. Hasler 7 , R.H. Tijssen 8

1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Dept of Physics, London, United Kingdom; 2 Radboud University Medical Center, Dept of Radiation Oncology, Nijmegen, The Netherlands; 3 University Medical Center Utrecht, Dept of Radiotherapy, Utrecht, The Netherlands; 4 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom; 5 Townsville Hospital and Health Service, Townsville Cancer Centre, Townsville, Australia; 6 The Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands; 7 Odense University Hospital, Dept of Oncology, Odense, Denmark; 8 Catharina Hospital, Dept of Radiation Oncology, Eindhoven, The Netherlands Purpose or Objective MR-guided RT leverages the excellent soft-tissue contrast of MRI for daily treatment adaptation, facilitating hypo- fractionated and dose-escalated treatments. To deliver precision RT on MR-Linacs, geometric fidelity and long-term stability of MR imaging components is essential. These components differ from conventional MRI systems due to the need for radio-translucency and exposure to scattered ionizing radiation. We report preliminary data on temporal stability of periodic MR imaging QA on MR-Linacs, which could inform frequency and action levels. Materials and Methods Seven centers contributed longitudinal MRI QA data acquired over 3-24 months on their 1.5 T MR-Linac systems (Elekta AB, Stockholm). To ensure consistent data across institutes, only phantoms provided with the MR-Linac system were used and a prescribed scan protocol was followed. A large cylindrical phantom (37 cm diameter) was used to measure B 0 and B 1 homogeneity and B 0 dependence on gantry position. In addition to a monthly axial measurement, coronal and sagittal scans were performed every three months. For B 0 mapping, phase images were acquired in separate TRs with TE 1 /TE 2 =5.4/6.9 ms to minimize phase wrapping, while a double flip angle technique with α 1 / α 2 =60°/120° was used for B 1 mapping. All tests were conducted at gantry angle 0° except gantry dependence for which phase images were acquired without re-shimming at TE=15.7 ms for 13 gantry angles from -180° to 180°. For a circular ROI of diameter 35 cm through the isocenter, we report mean, standard deviation and range from 1 st to 99 th percentile for each measurement. The vendor-provided PIQT phantom–a multi-purpose phantom with 20 cm diameter, similar to the ACR phantom–was used for weekly image scaling and SNR tests. Here we report on flood field uniformity and spatial linearity. A phantom consisting of 7 slabs with position markers was used to assess spatial accuracy of the MR images in a large field of view. We report change in mean and 99 th percentile over time for the distance to known position for markers within a spherical DSV of 35 cm.

Results