ESTRO 2024 - Abstract Book

S3386

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

3054

Mini-Oral

Underlying mechanism of the magnetic field dependence of a plane-parallel chamber in proton beams

Isabel Blum 1 , Jing Syuen Wong 1 , Krishna Godino Padre 1 , Jessica Stolzenberg 1 , Kilian-Simon Baumann 2,3,4 , Hermann Fuchs 5 , Björn Poppe 1 , Hui Khee Looe 1 1 University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany. 2 University Hospital Giessen-Marburg, Department for Radiotherapy, Marburg, Germany. 3 University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany. 4 Marburg Ion-Beam Therapy Center, -, Marburg, Germany. 5 Division of Medical Physics, Department of Radiation Oncology, Medical University of Vienna, Vienna, Germany

Purpose/Objective:

The response of plane-parallel detectors in magnetic fields in proton beams was investigated experimentally earlier by Fuchs et al [1]. They reported for these detectors investigated in their study a magnetic field dependence. For example, the ratio of the detector’s readings, of the Roos chamber, defined as the factor M 0 /M B , without and with magnetic field, increases initially in magnetic field up to a maximum between 0.25 T and 0.5 T, and decreases again after the maximum. Additionally, the magnitude of ratio M 0 /M B at the maximum also increases with the initial proton energy. Up to date, this behavior of these chambers in magnetic field is not fully understood. The main aim of this work is to investigate the underlying mechanism of this observed magnetic field dependence of plane-parallel detectors using detailed Monte Carlo simulations. In this work, the air-filled plane-parallel Roos chamber (PTW 34001) was studied. All simulations were performed in Geant4/GATE. A 10 cm x 10 cm pencil beam scanning field with an energy of 152 MeV was used. The magnetic field perpendicular to the beam's axis was varied between 0.25 T and 1 T. The chamber was placed with its reference point in 2 cm depth and oriented with its entrance window perpendicular to the beam's axis. Firstly, the influence of different chamber components was examined. For this purpose, the values M 0 and M B , without and with magnetic field, were simulated for three different models (1) “complete” Roos chamber geometry (2) “without wall” by replacing the chamber wall by water; and (3) “sensitive volume + guard ring without wall', as in (2) but by including the air-guard ring as the sensitive volume. In a second step, the contribution of individual particle species to the detector’s response was examined. The total deposited energy, M total , consists of contributions from protons M p , electrons M e- , and to a small extend, various fragments M frag : Material/Methods: