ESTRO 2024 - Abstract Book

S3399

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

1 University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany. 2 University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany. 3 Marburg Ion-Beam Therapy Center, Medical physics, Marburg, Germany

Purpose/Objective:

Irradiation with ultra-high dose rates such as FLASH irradiation can lead to increased inter-track interactions within the radiolysis of water, i.e. chemical reactions between the radiolysis products of different primary particles. The effect of inter-track interactions on the dynamics of the chemical stage of the radiation has already been studied using the Monte Carlo toolkit TOPAS-nBio 1.0 [1]. Since the simulation of inter-track interactions was not possible by default with this version, a manual method, called phsp, was developed. Meanwhile, with the update to TOPAS-nBio 2.0, a second method, called TsIRTInterTrack, was implemented so that inter-track interactions can now be simulated by default. In this work, the two independently developed methods for simulating inter-track interactions in TOPAS nBio were compared.

Material/Methods:

In TOPAS-nBio 1.0, chemistry is simulated applying the step-by-step approach (SBS). Simulating inter-track interactions using the TsIRTInterTrack method, inter-track interactions can only be generated in combination of simulating the chemistry applying the independent reaction time (IRT) method. This approach is available in TOPAS nBio 2.0 but not in 1.0. Thus, the phsp method was performed for both chemistry techniques to compare the SBS and IRT approaches and TsIRTInterTrack with the IRT method. In all cases, simulations were performed with TOPAS-nBio 2.0 and inter-track interactions of N=2 to N=60 primary electrons of 4.5 were simulated. To compare the influence of the two methods enabling inter-track interactions on the dynamics of the chemical phase, the G-value (number of molecules/ 100 eV of deposited energy) in a sphere filled with water (r = 5 µm) was investigated.

Results:

In figure 1, the G-values using the phsp method, applying IRT and SBS, and the TsIRTInterTrack method, are shown in dependence of the number N of tracks with inter-track interactions exemplarily for ● OH and H 2 O 2 including the differences between the methods in the bottom panel of each graph. Using the same method, but a different approach to model the chemistry stage, differences are up to 22% for ● OH and at maximum 2.2% for H 2 O 2 . Comparing the phsp and IRTInterTrack method for generating inter-track interactions, differences are up to 1.8% for ● OH and at maximum 4% for H 2 O 2 . In figure 2, time-dependent G-values are presented for N=2 for the same exemplary molecule types. Interestingly, comparing the phsp and TsIRTInterTrack G-values, differences are the highest at the beginning of the chemical stage of the simulation and become less towards the end of the chemical stage. This is particularly interesting, since the same physics lists, the same chemistry models, including reactions, reaction rates and diffusion constants of molecules, and the geometrical set-up were used. The only difference between the simulations of the two methods is the implementation of inter-track interactions in the code. A reason for the high difference of G-values at the beginning of the chemical stage could be a different consideration of the pre-chemical stage influencing the dynamics of the following chemical stage. Considering the generation of chemicals, we could exclude that the number of inelastic interactions in the physical stage generating chemical radicals is treated differently by both methods enabling inter-track interactions, since no significant differences between the number of inelastic processes could be observed.