Nanometer site analysis of electron tracks and dose localization in bio-cells exposed to X-ray irradiation

Abstract

This paper reports on a Monte Carlo simulation analysis of the ionization and excitation clusters in electron tracks, which may contribute to radiation damage of bio-cells with high probabilities. The study is aimed to investigate the energy transfer to the cell nucleus exposed to X-rays from low dose in the environment to high dose in radiation therapy. As an example, we adopted a water phantom (rectangular solid, 30×30×30 cm) exposed to a linac 6 MV X-ray to calculate the expected energy transfer to electrons in liquid water. Then, a track simulation of the electrons produced by the photon interactions was performed. In the track simulation, the behavior of low energy electrons below 1 keV is of particular importance to form clusters of ionization and excitation events within a nanometer scale (<100 nm). We describe the clustering pattern in terms of aggregation index (AI), the distribution of point to point distance below several nanometers between the events. By comparing the distribution with that of the Poisson configuration, the clustering effects of the events including successive radical processes are evaluated.

Modeling of yield estimation for DNA strand breaks based on Monte Carlo simulations of electron track structure in liquid water

“…DSBs are generally defined as the minimum of two strand breaks 4,43 within 10 base pairs (i.e., 3.4 nm). 44 Assuming that ionization and electronic excitation are potential causes to induce DNA strand breaks, 5,22 we scored the number of the events per track Assuming that the number of events per keV N event /E in and that of linkage per keV N link /E in are proportional to the induction yield of a SSB and a DSB, respectively, we defined k SSB and k DSB as proportion coefficients for SSB and DSB inductions (keV/Gy/Da), respectively. The yields of SSB Y SSB and DSB Y DSB (/Gy/Da) as a function of electron incident energy are given by…”

“…1 A variety of DNA lesions, including single-and double-strand breaks (SSBs and DSBs), are induced along the radiation track. [1][2][3] A Monte Carlo code for the track structure simulation at the nanometer scale in liquid water [4][5][6] is a powerful tool for the mechanistic investigation of the DNA damage induction. 7,8 Most of the energy deposition by the ionizing radiation is composed of secondary electrons, 5 thus a reliable code for predicting the track structure of electrons is required for computing the spatial distribution of DNA hits.…”

“…20,21 For this reason, many researchers have investigated the DSB yield by means of Monte Carlo codes 3,7,22 and biological experiments. 2,[23][24][25][26] To evaluate DNA strand break types, several trials for calculating physical, physicochemical, and chemical processes [3][4][5]7,9,22,[27][28][29] have been performed. However, obtaining both the energy deposition and the radical reaction within a cylinder of DNA is a time-consuming process.…”

A Simplified Cluster Analysis of Electron Track Structure for Estimating Complex DNA Damage Yields

“…Ionizing radiation within the human body causes DNA damage [1] both physically (i.e., energy deposition) [2,3] and chemically (i.e., free radical) reacting to the DNA target [4][5][6][7]. Among various 2 of 13 DNA damage types [8,9], DNA double-strand breaks (DSBs), defined as two strand breaks within 10 base pairs (bp) [2,10,11] are conventionally recognized as fatal DNA damage, which can lead to cell death with a certain probability [12].…”

“…A cluster analysis of inelastic interactions based on a radiation track structure in liquid water has been conducted as a powerful tool for estimating DNA damage yield [2,3,8,16,42]. The aim of this study is to develop a simple model for estimating complex DNA damage (i.e., isolated DSB, DSB + BD, DSB + 2BD, 2BD, 3BD, 4BD) based on previous simulation techniques [17,27,28], and to investigate the simulation accuracy and the nature of X-ray-(and electron-) induced complex damage in comparison with experimental data.…”