ESTRO 2024 - Abstract Book

S4707

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

Purpose/Objective:

This study aims to model the particle fluences in carbon ion beams using Monte Carlo simulations. Special attention has been paid to the underlying fragmentation processes and their impact on the water-to-air stopping power ratio crucial for dosimetry with air-filled ionization chambers. For this purpose, different physics lists were compared regarding the fragmentation products. Based on the results, a comprehensive approach for the determination of the stopping power ratio has been derived.

Material/Methods:

Particle fluences were scored for monoenergetic carbon ion beams of 10 cm x 10 cm field size with initial energies from 140 MeV/u to 400 MeV/u. Simulations were performed with the software package GEANT4 v11.0.2 / GATE9.2 using four different physics lists: QBBC; QGSP_BIC; FTFP_BERT and QMD_BIC. The comparison of the resulting particle fluences between the physics lists, including the different isotopes of the fragmentation products, was performed in 0.5 cm depth in a homogenous water phantom. For the calculation of stopping power ratios, the electronic stopping powers were primarily taken from the ICRU90 appendix when available. For other particle species, the stopping powers from the SRIM database were used. The resulting water-to-air stopping power ratios were calculated for energies from 100 MeV/u to 429 MeV/u and depths between 0.5 cm and 30.8 cm in the phantom and compared to results in earlier studies. Monte Carlo simulations showed that inelastic nuclear interactions, produce fragments with masses between 1 amu and 16 amu are produced through inelastic nuclear interaction. The heaviest fragments observed in the simulations were oxygen isotopes, where four different isotopes of oxygen constituted more than 99% of the oxygen fluence in the scoring volume (see fig.1). The stopping powers associated with the different oxygen isotopes deviate by up to 10% from the stopping power of O-16 (see fig. 2). These differences are taken into account in the calculation of the stopping power ratios. For the lower initial beam energies, 1% of the fragments exceed a mass of 4 amu. However, the portion of fragments as well as the heavier fragments increases for higher initial energy and at larger depths. For the initial energy of 100, 140, 278 and 429 MeV/u, the computed water-to-air stopping ratios are 1,1265, 1,1260 ,1,1230, and 1,1220, respectively. Therefore, the values differ by less than 0.5 % over the entire investigated energy range. The results obtained in this study are in good agreement with previously published results (Burigo und Greilich 2019; Bolsa ‐ Ferruz et al. 2021), where they reported values in the range of 1.1262 up to 1.1240 for the same energy range. Additionally, the choice of physics list was shown to play negligible role for the determination of stopping power ratios. Although differences in the particle fluences were observed between the lists. Results:

allow for further optimisation of particle therapy and associated dosimetry.