The biological effects of ionizing radiation at the cellular level are frequently studied using the well-known formalism of microdosimetry, which provides a quantitative description of the stochastic aspects of energy deposition in irradiated media. Energy deposition can be simulated using Monte Carlo codes, some adopting a computationally efficient condensed-history approach, while others follow a more detailed track-structure approach. In this work, we present the simulation of microdosimetry spectra and related quantities (frequency-mean and dose-mean lineal energies) for incident monoenergetic electrons (50 eV-10 keV) in spheres of liquid water with dimensions comparable to the size of biological targets: base pairs (2 nm diameter), nucleosomes (10 nm), chromatin fibres (30 nm) and chromosomes (300 nm). Simulations are performed using the condensed-history low-energy physics models ("Livermore" and "Penelope") and the track-structure Geant4-DNA physics models, available in the Geant4 Monte Carlo simulation toolkit. The spectra are compared and the influence of simulation parameters and different physics models, with emphasis on recent developments, is discussed, underlining the suitability of Geant4-DNA models for microdosimetry simulations. It is further shown that with an appropriate choice of simulation parameters, condensed-history transport may yield reasonable results for sphere sizes as small as a few tens of a nanometer.
Investigation of track structure and condensed history physics models for Investigation of track structure and condensed history physics models for applications in radiation dosimetry on a micro and nano scale in Geant4 applications in radiation dosimetry on a micro and nano scale in Geant4
The concept of nanodosimetry is based on the assumption that initial damage to cells is related to the number of ionisations (the ionisation cluster size) directly produced by single particles within, or in the close vicinity of, short segments of DNA. The ionisation cluster size distribution and other nanodosimetric quantities, however, are not directly measurable in biological targets and our present knowledge is mostly based on numerical simulations of particle tracks in water, calculating track structure parameters for nanometric target volumes. The assessment of nanodosimetric quantities derived from particle-track calculations using different Monte Carlo codes plays therefore an important role for a more accurate evaluation of the initial damage to cells and, as a consequence, of the biological effectiveness of ionising radiation. The aim of this work is to assess the differences in the calculated nanodosimetric quantities obtained with Geant4-DNA as compared to those of an ad-hoc particle-track Monte Carlo code developed at PTB. The comparison of the two codes was done for incident electrons of energy in the range between 50 eV and 10 keV, for protons of energy between 300 keV and 10 MeV, and for alpha particles of energy between 1 MeV and 10 MeV. Good agreement was found for nanodosimetric characteristics of track structure calculated in the high energy range of each particle type. For lower energies, significant differences were observed, most notably in the estimates of the biological effectiveness The largest relative differences obtained were over 50 %, however generally the order of magnitude was between 10 % and 20 %.
P. Lazarakis et al.:Comparison of nanodosimetric parameters of track structure calculated by Geant4-DNA and PTB … 3 / 34
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