Microdosimetry study of a multicellular model with mono-energetic electrons using Geant4-DNA simulation toolkit

Introduction:
The goal of any type of radiation therapy in the treatment of tumors, in addition to destroying cancer cells, is to minimizing radiation to nearby healthy cells and thus reducing side damages. For this purpose, targeted radiation therapy (TRT) is more effective in treating of single cells or small cluster of cells. The main factor in the success of this method is that the range of charged particles used in treatment does not exceed the cellular dimensions; this justifies the use of electron emitters with the energy as low as possible (especially Auger electrons).
Materials and Methods:
In this study, the Monte Carlo simulation toolkit, Geant4-DNA, has been used due to the accurate computational models used to simulate the transport of charged particle at the cellular scale (micrometer and lower). So, with the help of this code, a multicellular model (cell cluster) consisting of five spherical cells (containing the nucleus) of liquid water, has been simulated in the medium of tissue material. One of the cells is considered as the source, and with the uniform random sampling of 100,000 mono-energetic electrons with an initial energy of 1-20 keV within this cell, energy deposition and the S-value (mean absorbed dose in the target per unit cumulated activity in the source) in each cell is calculated using microdosimetry approach and the proposed formulation of MIRD committee.
Results:
A comparison of the values obtained for the source cell with the corresponding MIRD values indicates a good agreement, which implies the high ability of Geant4-DNA in microdosimetry simulations. The evaluation of the results suggests that a large part of the energy emitted within the source cell is deposited in the cell itself, which is lowered by increasing energy so that for the energy of 20 keV, it reaches less than 50%. Also, the cell that is far from the source cell (compared to other cells) has a lower energy deposition and S-value.
Conclusion:
From the assessment of the calculated values and their comparison with each other, it is understood that the less the energy of the primary electron is, the less energy is delivered to the adjacent healthy cells. Due to the very low threshold excitation of liquid water (7.4 eV) which is the main compartment of the cells, this is meant to reduce the risk of damage to adjacent healthy cells. Of course, the electron energy should not be so low that reduces its fatality for cancer cells and thus the efficacy of treatment. Therefore, attention to the mentioned cases, as well as the dimensions of the cell studied and its distance with adjacent cells, is crucial for the selection of an efficient electron-emitter radionuclide.