ESTRO 2024 - Abstract Book

S3378

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Kilian Baumann 1,2,3 , Larissa Derksen 2 , Jessica Stolzenberg 4 , Pascal Saße 4 , Klemens Zink 1,2,3 , Sebastian Adeberg 1,3 , Hui Khee Looe 4 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 Carl von Ossietzky University, University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Oldenburg, Germany

Purpose/Objective:

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 ionization chamber at the calibration beam quality Q 0 and the user beam quality Q [1]. While k Q factors in clinical photon beams have been investigated extensively, data for proton and especially carbon ion beams are scarce. In recent years, Monte Carlo simulations have been employed to determine k Q factors in proton beams [2,3] reducing the overall uncertainty of tabulated k Q factors to 1.4%. However, for carbon ion beams, the overall uncertainty is still quite large with 2.8% for plane parallel and 3.2% for cylindrical ionization chambers due to missing data. Hence, the goal of this study is to use the Monte Carlo code FLUKA to calculate k Q factors in both modulated and monoenergetic clinical carbon ion beams for four cylindrical and six plane-parallel ionization chambers.

Material/Methods:

The Monte Carlo code FLUKA version 2021-2-9 was used to calculate k Q factors in mono-energetic carbon ion beams with energies of 279 MeV/u and 429 MeV/u. Cylindrical ionization chambers were positioned with their effective points of measurement and plane-parallel ionization chambers with their reference points at a depth of 5 cm in water. Ionization chambers were modelled following detailed descriptions and blue prints from the manufacturers. Additionally, k Q factors were calculated in a modulated carbon ion beam resulting in a spread-out Bragg peak (SOBP) with an extension between 9 cm and 15 cm in water. For this simulation, the ionization chambers were positioned at a depth of 10 cm. In order to investigate the beam quality at the measurement depth, the dose fraction of carbon ions and all possible target and projectile fragments as well as the spectral fluences of these particles were scored for 429 MeV/u carbon ions.

Results:

In table 1, k Q factors are shown for two exemplary cylindrical ionization chambers for mono-energetic carbon ion beams and the simulation in the SOBP together with values from the literature where k Q factors were determined using calorimetry [4]. Monte Carlo calculated k Q factors are larger than the values presented in the literature by up to 2.4%.

Table 1: k Q factors for two exemplary ionization chambers in clinical carbon ion beams and comparison with values from the literature. The values within parenthesis correspond to one standard uncertainty in the last digit(s).

PTW 30013

IBA FC65-G