ESTRO 2024 - Abstract Book

S3404

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

3153

Proffered Paper

Experimental verification of the chemical phase in TOPAS-nBio

David Weishaar 1 , Larissa Derksen 1 , Robin Erdmann 1 , Charlotte Breitenbach 2 , Ulrike Theiss 3 , Klemens Zink 3,1,4 , Boris Keil 1 , Kilian Baumann 1,3,4 1 University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Gießen, Germany. 2 Philipps Universaty Marburg, Faculty of physics, Marburg, Germany. 3 University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany. 4 Marburg Ion-Beam Therapy Center (MIT), Medical Physics, Marburg, Germany

Purpose/Objective:

In particle therapy, the exact dose distribution and the biological effectiveness of the particles are an important part of precise treatment. Monte Carlo simulations can be employed to better understand the physical processes and transport through matter. The TOPAS (Toolkit for Particle Simulation) software offers the possibility to study interactions at the cellular level through the TOPAS-nBio extension. In addition to physics, the evolution of chemical species and their processes are also taken into account. As part of a larger project investigating the relative biological effectiveness of carbon ions in lung tissue, this work aims to experimentally verify the chemical phase as a cornerstone to achieve reasonable and accurate simulation results for later examination of DNA damage, especially indirect damages.

Material/Methods:

To verify the chemical processes, the yield of chemical radicals was determined both experimentally and in silicio. The fluorescent marker "Amplex-Red", which binds to H202 radicals in a 1:1 ratio, was used for the experimental benchmarking. To determine the yield of chemical species, Eppendorf tubes were then filled with deionised water and irradiated in a depth of 3 mm for carbon ions (149 MeV/u) and protons (221 MeV) and 2.4 cm for electrons (6 MeV). All samples were then pipetted into dark well plates, excited at 540 nm and the fluorescence was detected at 590 nm. The G-value, defined as the yield of chemicals per 100 eV, is then derived from a calibration of H202 and the fluorescence. The calibration is highly dependent on the accuracy of the pipettes, while very small amounts of 3% H202 solution are required to produce concentrations in the µM range. Alternatively, G-values can be estimated by plotting the fluorescence against dose. The ratio of the slopes for different ions should be the ratio of the G-values from the simulations for the corresponding ions. For the simulations, the Eppendorf tube is modelled as a cylinder with the same volume. To ensure a similar beam quality as used in the experiment, a phase space file is captured in Topas for each depth. Topas-nBio only provides cross sections for electrons up to an energy of 1 MeV, but it is claimed that the G-value does not increase significantly beyond this energy. The G-value is recorded as the number of H202 species produced divided by the energy deposited in eV multiplied by a factor of 100. The error of the G-value is calculated by the standard deviation from the runs of the independent runs.