Analysis of radioinduced DNA damages using Monte Carlo calculations at nanometric scale for different irradiation configurations

Abstract

The mean absorbed dose provides an adequate quantity with regards to the induced biological effects in many applications of ionizing radiations. However, in certain conditions, it does not give a description of the irradiation characteristics detailed enough that would allow predicting properly biological consequences. In particular, the absorbed dose can be misleading when the degree of heterogeneity in the energy deposited in sites exposed to the same radiation field is important compared to the target size. This is the case when investigating the number of radioinduced double strand breaks in the DNA content of a cell population. The study of the heterogeneity in the energy deposition in these nanometric volumes is now possible thanks to the use of Monte Carlo dedicated codes. Indeed, Monte Carlo codes have evolved to include electron transport to very low energy allowing a nanometric description of the energy deposited structure within living matter. In the present work, the Geant4-DNA Monte Carlo processes have been used to study the distribution of the specific energy deposited by photons, taking into account the size and shape of the DNA target of a cell nuclei population. The results of the simulations for a 137 Cs irradiation were compared, for the same macroscopic absorbed dose, to the γ-H2AX foci distributions obtained by an immunofluorescence technique that are related to the detection of DNA double strand breaks by the cell . The comparison of these simulated and experimental distribution variances appears as a possible method to link the irradiation parameters to their early biological consequences. In addition, the study of the evolution of the relative specific energy variance with the mean absorbed dose, in the case of 137 Cs irradiation obtained by Monte Carlo calculation, is also presented on this work. Finally, preliminary results on the calculated specific energy distributions obtained for 137 Cs and 20 keV X-rays were compared in order to study the effect of the radiation quality.