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Scientific news from division 6
First more realistic simulation of the ionization cluster generation in DNA segments

All Monte Carlo programs used previously for studying biological effects of ionizing radiation at the DNA level have calculated the track structure of the radiation in liquid water. Thus, implicitly the cross sections of water were considered as an approximation to those of the DNA. To evaluate the validity of this approximation, it is necessary to carry out numerical simulations which are based on cross sections of DNA constituents.

As a first step towards this objective, the total, differential elastic and double differential inelastic cross sections for the scattering of low energy electrons on tetrahydrofuran were determined last year in Department 6.6 for a worldwide unique range of parameters. (The structure of tetrahydrofuran is very similar to the DNA constituent deoxyribose.) The dataset was parameterised by fitting theoretical models [1, 2] and integrated into the PTB Monte Carlo code for track structure simulation, which is optimized for nanodosimetry. This program was also extended to allow the simulation of a target volume whose chemical composition differs from its surrounding.

The modified code was used for numerical simulations of the ionization cluster size distributions generated by electrons in a nanometric target volume corresponding in size to a short segment of the DNA double helix and surrounded by liquid water. The target volume was assumed to be filled with either liquid water or tetrahydrofuran, for which two different values of the mass density were used. One value was the density of water and the other (0.85 g/cm3) was based a DNA segment containing the same number of all four nucleic bases.

From the calculated ionization cluster distributions, the probability of generating a double-strand break (DSB) in the considered DNA segment was determined for a range of electron energies using a model proposed by Garty et al. [3]. The preliminary results of this study are shown in Figure 1. In all three cases, the probability for DSB formation exhibits approximately the same relative energy dependence. The absolute values, on the other hand, differ considerably from one another. Should this result also be confirmed for the other DNA constituents, this would imply an overestimation of biological radiation effects when using conventional track structure codes based on cross-section data of liquid water.


Figure 1:

Probability for double-strand break (DSB) formation in a target volume (DNA segment of 10 base pairs) filled with water (H2O) or tetrahydrofuran (THF) with the indicated mass densities. These results were obtained from Monte Carlo simulations using measured electron scattering cross sections of THF.

Literature

  1. M. E. Rudd, Differential and total cross sections for ionization of helium and hydrogen by electrons, Phys. Rev. A 44, 1644-1652 (1991).
  2. Y.-K. Kim, M. E. Rudd, Binary-encounter-dipole model for electron-impact ionization, Phys. Rev. A 50, 3954-3967 (1994).
  3. G. Garty, R. Schulte, S. Shchemelinin, C. Leloup, G. Assaf, A. Breskin, R. Chechik, V. Bashkirov, J. Milligan, B. Grosswendt, A nanodosimetric model of radiation-induced clustered DNA damage yields, Phys. Med. Biol. 55, 761-781 (2010).

Contact person:
M. Bug, Department 6.6, Working Group 6.63,
e-Mail: Marion Bug


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