Monte Carlo simulation of chemistry following radiolysis with TOPAS-nBio

Simulation of water radiolysis and the subsequent chemistry provides important information on the effect of ionizing radiation on biological material. The Geant4 Monte Carlo toolkit has added chemical processes via the Geant4-DNA project. The TOPAS tool simplifies the modeling of complex radiotherapy applications with Geant4 without requiring advanced computational skills, extending the pool of users. Thus, a new extension to TOPAS, TOPAS-nBio, is under development to facilitate the configuration of track-structure simulations as well as water radiolysis simulations with Geant4-DNA for radiobiological studies. In this work, radiolysis simulations were implemented in TOPAS-nBio. Users may now easily add chemical species and their reactions, and set parameters including branching ratios, dissociation schemes, diffusion coefficients, and reaction rates. In addition, parameters for the chemical stage were re-evaluated and updated from those used by default in Geant4-DNA to improve the accuracy of chemical yields. Simulation results of time-dependent and LET-dependent primary yields G(x)(chemical species per 100 eV deposited) produced at neutral pH and 25 degrees C by short track-segments of charged particles were compared to published measurements. The LET range was 0.05-230 keV m(-1). The calculated G(x)values for electrons satisfied the material balance equation within 0.3%, similar for protons albeit with long calculation time. A smaller geometry was used to speed up proton and alpha simulations, with an acceptable difference in the balance equation of 1.3%. Available experimental data of time-dependent G-values for (OH)-O-center dot agreed with simulated results within 7% +/- 8% over the entire time range; for e(aq)(-) over the full time range within 3% +/- 4%; for H(2)O(2 )from 49% +/- 7% at earliest stages and 3% +/- 12% at saturation. For the LET-dependent G(x), the mean ratios to the experimental data were 1.11 +/- 0.98, 1.21 +/- 1.11, 1.05 +/- 0.52, 1.23 +/- 0.59 and 1.49 +/- 0.63 (1 standard deviation) for (OH)-O-center dot, e(aq)(-), H-2, H(2)O(2 )and H-center dot respectively. In conclusion, radiolysis and subsequent chemistry with Geant4-DNA has been successfully incorporated in TOPAS-nBio. Results are in reasonable agreement with published measured and simulated data.