Calculation of microdosimetric spectra for protons using Geant4-DNA and a μ-randomness sampling algorithm for the nanometric structures

Abstract

Purpose

Through introducing stochastic quantities that can be connected to the dimensions of the microscopic structures exposed to radiations, microdosimetry is concerned with the substantive specifications of radiation quality that could help gain insight into radiation effects. Utilizing the μ-randomness method and Geant4-DNA code, we calculated microdosimetry quantities for nanometric structures in a spherical body of water irradiated with protons. To gain more insight into the effects of radiation on microscopic structures and validate the code parameters, we made a comparison between our results obtained within Geant4-DNA and results from other simulations.

Materials and methods

We calculated microdosimetric quantities through irradiating a spherical body of water of 6 μm diameter with 0.5–100 MeV protons. Microdosimetric quantities were derived for cylinders with diameter × height values of 23 × 23, 50 × 100, and 300 × 300 Å × Å, which would resemble the typical sizes of sub-cellular organisms such as the DNA, nucleosome, and chromatin fiber. We exploited the concept of μ-randomness to introduce convex bodies of random positions and directions for calculating microdosimetric quantities. We used the Geant4-DNA Monte Carlo simulation toolkit for transporting protons and secondary particles and calculating the frequency- and dose-mean lineal and specific energies in cylindrical volumes. Specifically, for same-sized cylindrical volumes, microdosimetric parameters obtained by Nikjoo et al. using the KURBUC code were used for evaluation.

Results

For the energy range investigated, the frequency-mean lineal energy, dose-mean lineal energy, frequency-mean specific energy, and dose-mean specific energy vary within [2.34,47.06] (keV/μm), [10.40,68.55] (keV/μm), [0.04,39.38] × 10⁶ cGy, and [0.16,90.29] × 10⁶ cGy, respectively. Regardless of the proton energy, our specific-energy results showed higher sensitivity to volume change, for smaller cylinder volumes rather than larger ones. Regardless of both proton energy and volume of the cylinder under study, we observed a generally better agreement between our frequency-mean, than dose-mean, specific energy results and the KURBUC results.

Conclusion

Using Geant4-DNA to account for the stochastic nature of energy depositions due to physical interactions between radiation and matter, we calculated microdosimetry parameters concerning proton irradiation. By employing microdosimetry concepts in conjunction with simulation results of our previous work on radiation effects on the DNA, we pinpointed and quantified correlations between microdosimetry parameters and DNA damage. As such, for a volume with comparable mass and mean chord length to the DNA, we could observe the clear correspondence of the mean lineal and specific energy results with the double-strand-break yields of protons in Gy⁻¹.Gbp⁻¹.

Keywords:

• Microdosimetry
• stochastic
• specific energy
• lineal energy
• Monte Carlo simulations
• protons
• μ-randomness

Disclosure statement

The authors (Mojtaba Mokari, Hossein Moeini, and Marzieh Soleimani) have no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

Additional information

Notes on contributors

Mojtaba Mokari

Mojtaba Mokari, Ph.D. in Physics from Isfahan University of Technology, Isfahan, Iran in 2018. He is an Assistant Professor in the Physics Department, Behbahan Khatam Alanbia University of Technology, Behbahan, Iran. He has been working on the microdosimetry, radiation therapy, and cellular response to ionizing radiation.

Hossein Moeini

Hossein Moeini, Ph.D. in Nuclear Structure Physics from University of Groningen (Kernfysisch Versneller Instituut), Groningen, Netherlands in 2010. He is an Assistant Professor in the Physics Department, Shiraz University, Shiraz, Iran.

Marzieh Soleimani

Marzieh Soleimani, M.Sc. in Nuclear Physics from Isfahan University of Technology, Isfahan, Iran in 2014. She is a researcher in the Physics Department Isfahan University of Technology, Isfahan, Iran. She has been working on the microdosimetry, radiation therapy, and the process of ionizing radiation with the mater.