ESTRO 2021 Abstract Book

S266

ESTRO 2021

Conclusion The proposed ITV-based planning strategy on the avgCT was found to be a clinically feasible approach with adequate tumor coverage and no OAR over-dosage even for large tumor movement. The new proposed 4D robust evaluation, 4DRobAvg, was shown to give an easily interpretable understanding of the effect of respiratory motion dose distribution, and to give an accurate estimate of the dose delivered in the different breathing phases. The proposed planning and evaluation strategy is implemented in clinical practice from December 2020 onwards. OC-0362 What to expect from MR-guided beam gating: Residual motion, efficiency and dose reconstruction S. Ehrbar 1 , S. Braga Magalhaes 1 , M. Chamberlain 1 , J. Krayenbühl 1 , L. Wilke 1 , M. Mayinger 1 , H. Garcia Schüler 1 , M. Guckenberger 1 , N. Andratschke 1 , S. Tanadini-Lang 1 1 University Hospital Zurich and University of Zurich, Radiation Oncology, Zurich, Switzerland Purpose or Objective MR-guided beam gating allows for direct detection of tumor respiratory motion and its compensation during radiotherapy. This study aims to analyze the efficiency, the geometric accuracy and the dosimetric benefit of magnetic-resonance guided beam gating for stereotactic treatments of lung, liver and heart. Materials and Methods Twenty-one patients, treated with the MRIdian system for targets located in the lung (9), liver (10) and heart (2), were analyzed. Breath-hold gating was performed for the lung and liver patients by tracking the lesion directly in sagittal 2D cine-MR images. For heart patients, a surrogate structure at the diaphragm was used for tracking. Cine-MR images were analyzed to determine the gating efficiency, the overall motion and the residual motion during gating. The target offset, or 2D-root-mean-square error (RMSE), from the geometric center of the gate was evaluated. A dose reconstruction workflow based on a convolution of these 2D position- probability maps and the daily 3D dose distribution was used to estimate the daily delivered dose including motion effects. The doses to the target volume and to a 2cm-ring structure around the planning target volume were evaluated. The ring structure was chosen to represent the dosimetric effect on the adjacent parts of the embedding organs at risk. Results The applied gating protocol resulted in a median (range) gating efficiency of 51% (30% to 100%). The median target motion RMSE over all patients and during the whole treatment was 7.9 mm (1.9 to 18.3 mm), which was reduced to 2.7 mm (1.9 to 4.6 mm) during gating. The dose reconstruction showed that this residual motion