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Electronic Equilibrium in RBE of DSB Induction in Monte Carlo Simulations of Low Energy Photon and Electron Track Structures

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P Pater

P Pater1*, J Seuntjens2, M Bernal3, I El Naqa4, (1) ,,,(2) McGill University, Montreal, QC, (3) UNICAMP, Campinas, Brazil, (4) McGill University, Montreal, QC

SU-E-T-306 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The study of DNA double-strand breaks (DSB) induction by ionizing radiation can provide understanding of linear quadratic formalism failures for complex cases such as stereotactic body radiation therapy. Using Monte Carlo simulations in cell nuclear volumes and adequate DNA modelling, estimation of DSB's is possible for various radiation fields. This study aims to evaluate the impact of electronic equilibrium (EE) and lack-thereof on dosimetric quantities such as dose, number of ionizations and excitations and subsequently on simulated DSB's and on the relative biological effectiveness (RBE).
Methods: Using Geant4 with the newly added Geant4-DNA low energy processes, we simulated track structures of mono-energetic photons and electrons (0.280 to 20 keV), including Auger electrons and fluorescence photons deexcitation products, and scored energy deposition interactions inside liquid water micro-volumes of various shapes and sizes. Electronic equilibrium conditions were varied by changing the relative size of the particle source and volume of interest.
Results: The total number of energy depositions interactions inside the volume varied around 0.62+/-0.04 interactions/eV and was almost constant for all analyzed EE conditions. However, the spatial distribution is affected by EE conditions and lack of interactions around the edges of the volume of 30% were scored. When coupled to a fixed position geometrical DNA model, simulated strand breaks are underestimated by up to 10% influencing the RBE of DSBs induction as a function of particle energy.
Conclusion: In order to speed up miro-dosimetric calculations, source sizes violating electronic equilibrium are often used. Simulated DSB's in a fixed geometrical model for different incident particle energies in electronic deequilibrium conditions, can thus be skewed by up to 10%. To circumvent this effect, sources satisfying electronic equilibrium in the volume for all energies or probabilistic delocalized DNA models need to be used.

Funding Support, Disclosures, and Conflict of Interest: Fond de Recherche en Sante du Quebec Canadian Institutes of Health Research

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