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  4. Investigation of Ionization Chamber Responses to Monoenergetic Proton Beams Using Monte Carlo Simulations

Investigation of Ionization Chamber Responses to Monoenergetic Proton Beams Using Monte Carlo Simulations

J Kretschmer1*, A Dulkys1 , L Brodbek1,2 , T Stelljes1 , H Looe1 , B Poppe1 , (1) University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, Germany, (2) Department of Radiation Oncology, University Medical Center Groningen (UMCG), University of Groningen, Groningen, The Netherlands

Presentations

(Wednesday, 7/17/2019) 10:30 AM - 11:00 AM

Room: Exhibit Hall | Forum 7

Purpose: This work presents Monte Carlo calculated ratios of dose to water and dose to the sensitive air volume of ionization chambers in monoenergetic proton beams, fQ. Additionally, the impact of considering the effective point of measurement (EPOM) of cylindrical ionization chambers is investigated.

Methods: Monte Carlo simulations were performed with GATE V8.0/Geant4 10.04.p01. Using detailed chamber models, fQ was determined for a plane-parallel (IBA NACP-02) and four cylindrical ionization chambers (Farmer NE2571, PTW PinPoint 31014, PTW Semiflex 3D 31021, PTW PinPoint 3D 31022). fQ ratios were determined at 2 cm depth for 10x10 cm² fields of monoenergetic protons with incident energies of 70 to 250 MeV. The cylindrical chamber’s response was determined with and without EPOM consideration.

Results: The fQ ratios for the plane-parallel NACP-02 are found to be nearly constant over the considered energy range. The results are in good agreement with published fQ by Goma et al. (2016 Phys. Med. Biol. 61 2389) and Wulff et al. (2018 Phys. Med. Biol. 63 115013) showing a deviation from IAEA TRS398 recommended values of up to 1.7%. For cylindrical chambers placed with their reference point in 2 cm depth, fQ ratios increase with decreasing proton energy. The results for the NE2571 are in good agreement with those by Goma et al. and Wulff et al.. When the cylindrical chambers were placed with their EPOM at the measurement depth, the energy dependence of fQ is less pronounced and fQ ratios are found to be nearly constant. Nevertheless, deviations from fQ values listed in TRS398 of up to 2.4% are found for both chamber positioning approaches.

Conclusion: The results underline that the fQ in TRS398 should be updated and indicate that the EPOM of cylindrical ionization chambers should be considered in the definition of reference conditions for monoenergetic proton beams.

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