ESTRO 2024 - Abstract Book

S3381

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

3020

Poster Discussion

Investigation of the response of cylindrical ionization chambers in magnetic field in proton beams

Isabel Blum 1 , Jing Syuen Wong 1 , Krishna Godino Padre 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, Vienna, Germany

Purpose/Objective:

The recent developments in magnetic resonance guided particle therapy (MRgPT) require extensive investigations on the associated detector’s behavior to ensure safe clinical implementations. Air-filled cylindrical ionization chambers represent an important tool in dosimetry of proton beams. The aim of this work is to investigate the behavior of two cylindrical chambers (a Farmer-type and a PinPoint chamber) in magnetic fields irradiated by proton beams. Special emphasis is placed to understand the role of chamber’s geometry using detailed Monte Carlo simulations and direct comparison between the two investigated chambers.

Material/Methods:

Two cylindrical ionization chambers have been studied in this work: the PTW 30013 Farmer-type chamber and the PTW 31016 PinPoint 3D chamber. The Farmer chamber has a sensitive volume of 0.6 cm3, whereas the PinPoint 3D chamber has a much smaller sensitive volume of 0.016 cm3. Monte Carlo simulations were performed in GATE (version 9.2) and Geant4 (version 11.0.2). The proton beam used has a field size of 10 cm x 10 cm and initial energies of E = 97.4, 152, 223.4 and 252 MeV. The detectors were modelled according to the detailed constructional drawings provided by the manufacturer. The axis of the Farmer chamber and the PinPoint 3D chamber was orientated perpendicular to both, the beam’s axis and the magnetic field. The reference point of each chamber was placed at z = 2 cm depth in water. The production cut was set to 1 μm within a region (cutbox) encompassing a 0.5 cm margin from the chambers’ outer dimensions in each direction. The magnetic field was orientated perpendicular to the beam’s axis with nominal magnetic field strengths B = ±0.25, ±0.32, ±0.5, ±0.75 and ±1 T; depending on the sign, the Lorentz force acts in the opposite direction. The ratios M 0 /M B , defining the response of the chambers, were scored as the ratio of energy deposited in the sensitive volume of each chamber without and with magnetic field. Additionally, the role of the chamber stem and the sensitive air volume on the ratio M 0 /M B has been evaluated. This has been performed by repeating the simulations after replacing the chamber stem with water and by considering the complete air volume as sensitive, respectively.

Results: