Office of Biological and Environmental Research

DOE Low Dose Radiation Program Workshop V

2005 Abstract

Title: Comparison of Soft X-Rays and Ions Irradiation in a Model of V79 Mammalian Cell

Authors: B. Ginovska, J.H. Miller, D. J. Lynch and W. E. Wilson

Institutions: School of Electrical Engineering and Computer Science Washington State University Tri-Cities, Richland, Washington

Microbeam irradiation of a model of a V79 mammalian cell with helium-3 ions and focused carbon soft x-rays was simulated using the PITS and XPITS software packages. The model of the cell corresponds to the geometry of cells used in low dose studies of bystander cell-killing with targeted soft x-rays at the Gray Cancer Institute. Two states of cell model geometry were used: the geometry of a cell after 4 hours of seeding and after 8 hours of seeding. The initial beam energy of the ion was 3 MeV, corresponding to stopping power of 100 keV/micron. The energy of the cecer carbon soft x-rays was 278 eV. Computations are carried out based on event-by-event, Monte Carlo-detailed histories of ions and photons and their respective secondaries, i.e. delta-rays and photoelectrons. The energy deposition was scored in a compartmental model of the cell, with a restriction of ensuring that the cell nucleus receives radiation of 1 Gy. The simulation calculated the total energy deposited in each compartment of the cell. It shows that the cell needs about 12 000 photons for the dose to be delivered to the nucleus, whereas only 2 Helium 3 ions are required for the same effect. Figures 1 and 2 show the dose in each compartment when irradiating a model of cell geometry after 4 hours of seeding.

Figure 1 shows the dose deposited by 2 helium-3 tracks, and figure 2 shows the dose deposited by 12 000 carbon soft x-rays.

Although in both cases the dose delivered to the nucleus is 1 Gy, the dose delivered to the other compartment differs. This is due to the different distribution of the energy deposited along the direction of penetration. In both cases the energy deposited is very localized. The photons and the ions do not scatter, so the primary events are always on the beam axis. The secondary events are distributed around the beam entry direction, but they never reach a radial distance greater than 25 nm in the case of photons, and 400 nm in the case of helium-3 ions. Because of this linear characteristic of the energy deposited, describing the effects of the radiation in terms of dose makes no recognition of the fact that all the tracks pass only through a very small fraction of the entire cell volume.

As shown in Figure 3, the amount of the energy deposited by x-rays exponentially decreases with the depth of the penetration. This is a result of the strong attenuation of the x-rays. In case of helium-3 particles, the energy deposited at different depths of penetration dose not vary significantly.

Since the observed results indicate that the different types of irradiation have very different effects on the cell membrane, which is in contact with the cellular medium, our future research will focus on the energy deposition in the section of this membrane that is directly exposed to radiation.

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