ESTRO 2023 - Abstract Book

S773

Monday 15 May 2023

ESTRO 2023

Proffered Papers: Flash and proton measurement

OC-0927 Proton dosimetry with a plane-parallel chamber: determination of magnetic field correction factors B. Gebauer 1,2 , K. Baumann 3,4,5 , D. Georg 6 , H. Fuchs 6 , B. Oborn 1,7,8 , A. Hoffmann 1,2,9 , A. Lühr 10 1 OncoRay – National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany; 2 Institute of Radiooncology-OncoRay, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany; 3 University Medical Center Giessen Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany; 4 University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany; 5 Marburg Ion-Beam Therapy Center, (MIT), Marburg, Germany; 6 Medical University of Vienna, Department of Radiation Oncology, Wien, Austria; 7 Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, Australia; 8 Illawarra Cancer Care Centre, (ICCC), Wollongong, Australia; 9 Department of Radiotherapy and Radiation Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany; 10 Department of Physics, TU Dortmund University, Dortmund, Germany Purpose or Objective In magnetic resonance imaging-integrated proton therapy (MRiPT), the magnetic field-dependent change in the dosage of ionisation chambers is considered by the correction factor k_(B,M,Q), which can be determined experimentally or computed via Monte Carlo (MC) simulations. In this work, k_(B,M,Q) for a plane-parallel ionisation chamber was determined by measurements and MC simulations were used to reproduce these results with high accuracy. Materials and Methods The dose-response of the advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10x10 cm ² mono-energetic fields, using 103.3, 153.1, and 252.7 MeV proton beams was measured in a water phantom placed in the magnetic field (MF) of an electromagnet with MF strengths of 0.32 and 1 T. The detector was positioned at a 2 cm water-equivalent depth with chamber electrodes parallel to the MF lines and perpendicular to the proton beam incidence direction. The measurements were compared with TOPAS MC simulations utilizing COMSOL-calculated 0.32 and 1 T MF maps of the electromagnet. k_(B,M,Q) was calculated for the measurements for all energies and MF strengths based on the equation: k_(B,M,Q)=M_Q/(M_Q^B), where M_Q and M_Q^B were the temperature and air pressure corrected detector readings without and with MF, respectively. MC-based correction factors were determined as k_(B,M,Q)=D_det/(D_det^B), where D_det and D_det^B were the doses deposited in the air cavity of the ionisation chamber model without and with MF, respectively. Results The detector showed a reduced dose-response for all measured energies, and MF strengths resulting in experimentally determined k_(B,M,Q) values larger than 1 (Figure 1). k_(B,M,Q) increased with proton energy and MF strength, except for 0.32 T and 252.7 MeV. Overall, k_(B,M,Q) ranged between 1.006 ± 0.004 and 1.021 ± 0.010 for all energies and MF strengths examined and the strongest dependence on energy was found at 1 T. The MC simulated k_(B,M,Q) values for 0.32 and 1 T showed a good agreement with the experimentally determined correction factors and trends within their standard deviations. The maximum difference between experimentally determined and MC simulated k_(B ,M,Q) values was 0.63%.