Program Information
Validation of Geant4-DNA/TOPAS-NBio Simulations of Water Radiolysis Induced with Monoenergetic Electrons, Protons and Alpha Particles
J Ramos-Mendez1*, J Perl2 , J Schuemann3 , H Paganetti4 , A McNamara5 , K Held6 , B Faddegon7 , (1) University of California San Francisco, San Francisco, CA, (2) Stanford Linear Accelerator Center, Menlo Park, CA, (3) Massachusetts General Hospital, Boston, MA, (4) Massachusetts General Hospital, Boston, MA, (5) Massachusetts General Hospital & Harvard Med. Sch., Boston, MA, (6) Massachusetts General Hospital, Boston, MA, (7) UC San Francisco, San Francisco, CA
Presentations
SU-I-GPD-T-650 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To validate a TOPAS-nBio extension to simulate water radiolysis.
Methods: TOPAS-nBio was extended to configure branching ratios, dissociation schemes, diffusion coefficients, reactions and reaction rates among customized scorers to simulate water radiolysis. The primary yield Gx (number of chemical species per 100 eV deposited energy) was used to compare TOPAS-nBio calculations with experimental data from literature. Two different setups were simulated. For electrons, short-track segments were simulated in a semi-infinite water block with primary energies from 1-1000 keV (LET values of 10.1-0.05 keV/μm). For protons and alphas, a cubic water phantom of length 5 μm was irradiated with monodirectional point beams starting at one side of the phantom. For protons and alphas the energies ranged from 0.5-20 MeV and 1-20 MeV/u, respectively (LET values of 40-2.5 keV/μm and 224.4-32 keV/μm). The simulations consisted of three stages managed by Geant4-DNA (version 10.2.p03). First, the physical stage up to 10-15 s, where ionizations and excitations of the radiation interaction occurred. Second, a pre-chemical stage from 10-15-10-12s, where excited and ionized water molecules dissociated into primary chemical species. Third, a chemical stage from 10-12-10-6s, where all species created in the pre-chemical stage diffused and reacted within each other reducing the primary yields and producing new chemical species. Gx values were scored in the chemical stage, under conditions of neutral pH and ambient temperature conditions.
Results: Gx satisfied the balance material equation in all cases and was in reasonable agreement with published experimental and simulated data. For electrons with an LET of 0.3 keV/μm, the calculated G(-H2O) of 3.97±0.06 molecules/100 eV was 4.3%±1.4% lower than the commonly assumed value.
Conclusion: TOPAS-nBio was extended and validated to simulate the radiolysis of water, allowing indirect effects of radiation on DNA to be modeled.
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