ESTRO 38 Abstract book

S152 ESTRO 38

used in a surveillance program with automated heart exposure monitoring of all breast cancer treatments in a clinic. OC-0300 Experimental validation of an MLC tracking treatment simulator with dose reconstruction A. Hagner 1 , T. Ravkilde 2 , S. Skouboe 3 , C.G. Muurholm 3 , R. Hansen 2 , E.S. Worm 2 , P.R. Poulsen 4 1 Aarhus University, Department of Physics and Astronomy, Aarhus, Denmark; 2 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark; 3 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark ; 4 Aarhus University, Department of Clinical Medicine, Aarhus University, Denmark Purpose or Objective Tumour motion may cause substantial deterioration of planned dose distributions. However, the dosimetric impact of tumour motion can be mitigated by MLC tracking. Current QA for MLC tracking involves pre- treatment delivery of the treatment plan to a moving dosimeter to determine its suitability for MLC tracking. This is a time-consuming process that may be replaced by dose delivery simulations for easier assessment of the suitability of MLC tracking for a given plan. In this study, we simulate delivered doses to a moving dosimeter with and without MLC tracking and validate the simulations by comparison with measurements. Material and Methods MLC tracking experiments were performed on a TrueBeam linear accelerator using iTools Tracking (Varian, CA) guided by an optical marker block. Delivered doses were measured at 72 Hz using a Delta4 dosimeter mounted in a HexaMotion motion stage (Scandidos, Sweden). The motion stage reproduced motion from four liver SBRT patients previously treated using VMAT plans with tumor motion monitoring by kilovoltage intrafraction monitoring. Three VMAT fields per patient were delivered to the dosimeter both with and without MLC tracking. The time- resolved motion-induced 3%/3mm gamma failure rate was determined for each VMAT field delivery faced with motion by comparing measured cumulative dose distributions with a measured static reference. In order to simulate the experiments, two in-house developed programs for 1) treatment delivery simulation and 2) dose reconstruction (DoseTracker) were combined. The treatment simulator took as input the DICOM treatment plans and motion stage trajectories used at the experiments and generated a log file with synchronized target positions and simulated linac parameters (gantry, MLC positions, MU, etc.) with 21Hz resolution. The log file was used by DoseTracker to reconstruct time-resolved delivered doses to all 1069 diodes in the dosimeter and calculate gamma failure rates comparing simulated and planned cumulative doses. Finally, the time-resolved gamma failure rates of simulations and experiments were compared and the root- mean-square deviation (RMSD) calculated. Results The simulated gamma failure rates agreed well with the measurements throughout beam delivery for both MLC tracking and standard non-tracking treatments (Fig 1-2) with a root-mean-square deviation of 2.0 %-points.

Conclusion End-to-end simulations of advanced radiotherapy delivery, from treatment plan to delivered dose distributions, were demonstrated and experimentally validated. The simulator accurately predicted motion- induced dose errors for VMAT liver SBRT to a moving target throughout both MLC tracking and standard non-tracking deliveries. An accurate tracking simulator can eliminate the need for time-consuming experiments and QA measurements for MLC tracking. It allows easy evaluation of the motion robustness of treatment plans and their suitability to MLC tracking for a wider range of motion than practically possible with experiments. OC-0301 Real-time kV image guidance in the treatment of pancreatic SBRT: quantifying the purpose and impact B. Jones 1 , Y. Vinogradskiy 1 , W. Campbell 1 , Y. Ding 1 , T. Schefter 1 , K. Goodman 1 , M. Miften 1 1 University of Colorado School of Medicine, Radiation Oncology, Aurora, USA Purpose or Objective Ablative radiotherapy of the pancreas requires extremely precise delivery in order to achieve local control while minimizing toxicity to the duodenum and