"Broadscale" nanodosimetry: Nanodosimetric track structure quantities increase at distal edge of spread-out proton Bragg peaks

This work presents an attempt to addressing the issue of RBE variation in a spread-out Bragg peak (SOBP) of protons based on nanodosimetric track structure analysis. Ionization track structure has been simulated using Geant4-DNA for protons of 100 MeV initial energy propagating in water. The frequency distribution of ionization clusters formed in target volumes corresponding to a 10 base-pairs segment of DNA was obtained as a function of the radial distance between target and proton trajectory for a set of positions along the proton path. Radial dependence of nanodosimetric parameters was analysed using a heuristic model function to obtain an effective track cross section (ETCS) as a function of the proton's residual range. The results were convolved with weighted range distributions suggested in literature for constructing a SOBP. The ETCS shows an increase in the distal end region of the SOBP in qualitative agreement with radiobiological observations of enhanced cell damage in this region. The results demonstrate that nanodosimetric track characteristics may be used for qualitatively predicting the variation of the probability for induction of lethal lesions in cells.