ESTRO 2024 - Abstract Book

S3371

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

1 University Hospital Giessen-Marburg, Department for Radiotherapy and Radiooncology, Marburg, Germany. 2 University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany. 3 Marburg Ion-Beam Therapy Center, Medical Physics, Marburg, Germany. 4 National Physical Laboratory, Medical Radiation Science Group, Teddington, United Kingdom. 5 University College London, Department of Medical Physics and Biomedical Engineering, London, United Kingdom. 6 West German Proton Therapy Center, Medical Physics, Essen, Germany. 7 Maastricht University Medical Centre, Department of Radiation Oncology (Maastro), GROW School for Oncology and Reproduction, Maastrich, Netherlands. 8 MedAustron Ion Therapy Center, Medical Physics, Wiener Neustadt, Austria For the determination of absorbed dose-to-water with air-filled ionization chambers, the beam quality correction factor k Q has to be considered which accounts for the different response of the chamber at the calibration beam quality Q 0 and the user beam quality Q [1]. In the clinical routine for dose determination, values for k Q factors are typically taken from dosimetry protocols such as the IAEA TRS-398 Code of Practice (CoP). In the updated version of this protocol, k Q factors will be presented which are based on both experimental data and Monte Carlo simulations [2,3]. However, those values can only be applied if the same approach for positioning the ionization chamber as used to derive k Q factors is employed in the clinical beam. For plane-parallel ionization chambers, typically two different positioning approaches are commonly used in the clinic: • The reference point of the chamber, which is located at the center of the inner surface on the entrance window, is positioned at the measurement depth • The effective point of measurement, P eff , which accounts for the water-equivalent thickness of the entrance window is positioned at the measurement depth In this study, we investigate the influence of both positioning approaches on the k Q factor of plane-parallel ionization chambers in clinical proton beams and show that this difference is dominated by the gradient of the depth dose distribution at the measurement depth. Purpose/Objective:

Material/Methods:

The Monte Carlo code Geant4 in combination with the toolkit TOPAS was used to determine the overall response function f Q which is the basis of Monte Carlo calculated k Q factors for six plane parallel ionization chambers. For the calculation of these factors, the two different positioning approaches were employed, whereas the water-equivalent thickness of the entrance windows was determined for each ionization chamber model individually. 10 x 10 cm 2 fields of mono-energetic protons of eight different energies between 60 and 250 MeV have been investigated. Additionally, the dose gradient at the measurement depth was calculated to compare it with the change in f Q due to the different positioning approaches.

Results: