A comparison between track-structure, condensed-history Monte Carlo simulations and MIRD cellular S-values

The S-value is a standard measure in cellular dosimetry. S-values are calculated by applying analytical methods or by Monte Carlo simulation. In Monte Carlo simulation, particles are either tracked individually event-by-event or close events are condensed and processed collectively in different steps. Both of these methods have been employed for estimation of cellular S-values, but there is no consistency between the published results. In the present paper, we used the Geant4-DNA track-structure physics model as the reference to estimate the cellular S-values. We compared the results with the corresponding values obtained from the following three condensed-history physics models of Geant4: Penelope, Livermore and standard. The geometry and source were exactly the same in all the simulations. We utilized mono-energetic electrons with an initial kinetic energy in the range 1-700 keV as the source of radiation. We also compared our results with the MIRD S-values. We first drew an overall comparison between different data series and then compared the dependence of results on the energy of particles and the size of scoring compartments. The overall comparison indicated a very good linear correlation (R-2 > 91%) and small bias (3%) between the results of the track-structure model and the condensed-history physics model. The bias between MIRD and the results of Monte Carlo track-structure simulation was considerable (-8%). However, the point-by-point comparison revealed differences of up to 28% between the condensed-history and the track-structure MC codes for self-absorption S-values in the 10-50 keV energy range. For the cross-absorption S-values, the difference was up to 34%. In this energy range, the difference between the MIRD S-values and the Geant4-DNA results was up to 68%. Our findings suggest that the consistency/inconsistency of the results obtained with different MC simulations depends on the size of the scoring volumes, the energy of the particles, the step-size in electron tracking and the energy cutoff used in the MC codes.