ESTRO 2023 - Abstract Book

S753

Monday 15 May 2023

ESTRO 2023

radiation therapy (RT). Here we present our work on the use of beam current transformers (BCTs) that could replace the standard transmission ion chambers in UHDR beams for beam monitoring and control. Materials and Methods The Mobetron (IntraOp Medical) was updated with two internal BCTs at different positions within the head of the linac. The BCTs were evaluated against known dose-rate independent dosimeters (Gafchromic EBT3 film, TLDs, and OSLDs) and calibrated to a reference point. The possible dependencies against dose, mean dose rate, instantaneous dose rate, pulse width, and dose per pulse was investigated. Furthermore, the reproducibility, long term stability, and response time of the entire system was evaluated to determine the use of BCTs for active beam control of eFLASH deliveries. Results The BCTs were found to be independent of mean- and instantaneous dose rate and showed a linear response to dose and dose per pulse up to the highest values investigated (8 Gy/pulse, >1000 Gy/s). The BCTs showed excellent reproducibility and long-term stability. The overall response time of the entire system (beam-off latency) was on the order of 1-2 ms but was limited by the read-out equipment. Higher temporal resolution equipment is currently being implemented which will likely bring these values down significantly. Conclusion The BCTs were found to be capable of accurately capture all the essential beam parameters for each individual pulse delivered. The two BCTs were calibrated individually to allow redundancy in beam monitoring and control. The system was found to be fast enough for beam control of pulse deliveries up to 500 Hz. Future work is focused on integrating the signal processing into the control system for full beam control. PD-0906 Preventing potential treatment collisions on a TrueBeam system using a 3D video game engine T. Henry 1,2 1 Sahlgrenska University Hospital, Department of Medical Physics and Biomedical Engineering, Gothenburg, Sweden; 2 University of Gothenburg, Department of Medical Radiation Sciences, Institute of Clinical Sciences, Sahlgrenska Academy, Gothenburg, Sweden Purpose or Objective To discover potential collisions between the machine and the patient/treatment couch on a TrueBeam system (Varian Medical System, Palo Alto, USA) during the dose planning process. Materials and Methods A 3D model of a TrueBeam treatment was obtained from Varian Medical Systems and modified to our needs. The model was then imported to the video game engine Unreal Engine 4 (Epic Games, Cary, USA) where a game level was modelled around the TrueBeam model and a graphical user interface, as well as intuitive controls, were implemented. To obtain all the parameters from the treatment plan that needs to be checked against potential collisions, a script was developed in Eclipse (Varian Medical Systems) using the Eclipse Scripting API. The script exports information such as isocenter position and fields data, as well as 3D volume information about the patient and the PTV structure. For this last part, the meshes making up the body and PTV structures in the treatment plan (point positions + triangles information) are exported in a text file. Results Upon starting the collision program, information about the treatment plan and the patient are automatically loaded. The patient and PTV meshes are reconstructed in the level and positioned right in relation to the isocenter. The treatment couch is also adjusted to be in the right position. The program then carries on checking if one of the following scenarios are detected for each field: no-collision, warning (two or more physical entities are less than 30 mm away from each other) or collision (less than 15 mm, to allow some margins). This is done quickly and independently if the fields are static beams or arcs. Within one second of starting the program, a report summary is displayed to the user. The user can then decide to exit the program or to enter the level to check each field manually (figure).

Conclusion A fast and efficient solution to detect potential collisions during treatment delivery on a TrueBeam system using a video game engine was developed. This solution is currently in use at our department to detect risks of mechanical collisions between the gantry and the treatment couch or patient. PD-0907 First evaluation of Bolus-Electron-Conformal-Therapy and intensity modulation for FLASH radiotherapy E. Konradsson 1 , R. Ericsson Szecsenyi 1 , G. Adrian 2,5 , M. Coskun 2 , B. Børresen 3 , M. Arendt 3 , K. Erhart 4 , K. Petersson 2,6 , C. Ceberg 1