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Simulation of nanodosimetric ionization cluster size distributions with Geant4-DNA

31.01.2011

Until recently, only the Monte Carlo program developed at PTB for the simulation of the track structure of ionizing radiation has offered the possibility of calculating nanodosimetric quantities - such as, e.g., ionization cluster size distributions and characteristics of the track structure derived from them. Within the scope of the Geant4-DNA project, the Monte Carlo program package Geant4 was extended to the interaction cross sections of liquid water for the so-called "very low-energetic electromagnetic processes" so that now, another tool is available for numerical simulations in nanodosimetry. The systematic comparison of the track structure parameters derived from numerical simulations performed with the different programs opens up the possibility of obtaining an estimate for those uncertainty contributions which originate from the model assumptions on which the simulation calculations are based.

As a first step into this direction, comparative numerical simulations were carried out with the two Monte Carlo programs, in which electrons, protons and alpha particles were investigated as primary particles. In the case of Geant4-DNA, the additional possibility of applying - alternatively - two different models for the elastic scattering of electrons was made use of. As secondary particles, electron interactions essentially determine the structure of the ions tracks. The nanodosimetric parameters of the track structure were determined for two different target volumes whose dimensions correspond to those of a short segment of the DNA of 10 base pairs or of a nucleosome. Different values of the initial kinetic energy of the primary particles were investigated. They lay between 50 eV and 10 keV for electrons, between 300 keV and 10 MeV for protons and between 1 MeV and 9 MeV for alpha particles.

The results obtained for the two ion types with respect to the mean ionization cluster size are shown in Figures 1 and 2 as the ratio of the values obtained for the two target volumes using the two variants of Geant4-DNA to those obtained by using the PTB code. It is evident that the differences caused by the different models for the elastic electron scattering cross sections are generally considerably smaller than the differences between the two Monte Carlo programs. In addition, also the last-mentioned differences become comparably small for high primary particle energies. This is to be expected, because in both programs the Bethe Born approximation is used for high particle energies. When the initial primary particle energy decreases, larger differences can be observed. For instance, for alpha particles at the smallest energy considered, a relative deviation of up to 60 % is found.

Figure 1: Ratio of the values obtained with Geant4-DNA or with the PTB Monte Carlo code for the mean ionization cluster size generated by protons in a DNA segment or in a nucleosome as a function of the initial particle energy.

Figure 2: Ratio of the values obtained with Geant4-DNA or with the PTB Monte Carlo code for the mean ionization cluster size generated by alpha particles in a DNA segment or in a nucleosome as a function of the initial particle energy.

Literature:

  1. B. Grosswendt:
    Formation of ionization clusters in nanometric structures of propane-based tissue-equivalent gas or liquid water by electrons and α-particles,
    Radiat. Environ. Biophys. 41, 103-112 (2002).
  2. S. Chauvie, Z. Francis, S. Guatelli, S. Incerti, B. Mascialino, F. Moretto, P. Nieminen and M. G. Pia:
    Geant4 Physical Processes for Microdosimetry Simulation: Design Foundation and Implementation of the first Set of Models,
    IEEE Trans. Nucl. Sci. 54, 2619-2628 (2007).