Interaction of ion tracks in spatial and temporal proximity.

In the present work, a systematic analysis of the impact of spatial and temporal proximity of ion tracks on the yield of higher-order radiolytic species as well as of DNA damage patterns is presented. This potential impact may be of concern when laser-driven particle accelerators are used for ion radiation therapy. The biophysical Monte Carlo track structure code PARTRAC was used and, to this end, extended in two aspects: first, the temporal information about track evolution has been included in the track structure module and, second, the simulation code has been modified to enable parallel multiple track processing during simulation of subsequent modelling stages. Depending on the spatial and temporal separation between ion-track pairs, the yield of chemical species has been calculated for incident protons with start energies of 20 MeV, for He(2+) ions with start energies of 1 and 20 MeV, and for 60 MeV C(6+) ions. Provided the overlap of the considered ion tracks is sufficient in all four dimensions (space and time), the yield of hydroxyl radicals was found to be reduced compared to that of single tracks, for all considered ion types. The biological endpoints investigated were base damages, single-strand breaks, double-strand breaks, and clustered lesions for incident pairs of protons and He(2+) ions, each with start energies of 20 MeV. The yield of clustered lesions produced by 20 MeV protons turned out to be influenced by the spatial separation of the proton pair; in contrast, no influence was found for different start times of the protons. The yield of single-strand breaks and base hits was found neither to depend on the spatial separation nor on the temporal separation between the incident protons. For incident 20 MeV He(2+) ions, however, a dependence on the spatial and temporal separation of the ion pair was found for all considered biological endpoints. Nevertheless, spatial proximity conditions where such intertrack effects were obtained are not met in the case of tumour radiation therapy; thus, no impact on radiation effects due to short pulse duration of laser-driven accelerators can be expected from alterations during the chemical stage.