Calculation and evaluation of energy deposition and S-value caused by low-energy electrons in a multicellular model using Geant4-DNA

Today, targeted radiation therapy (TRT) methods for cancer treatment, besides the goal of completely destroying the target tumor, attempts to prevent nearby healthy cells from exposure to ionizing radiation as far as possible. Hence, short-range charged particles, such as low-energy electrons that are suited to achieving these two goals together, play an important role in TRT and so, adoption of precise methods such as Monte Carlo simulations to perform cellular dosimetry of electrons and to get better knowledge of their energy deposition pattern at the cellular level is necessary. In this study, with the help of Geant4-DNA simulation code, four spherical cells (and nuclei) of liquid water in the soft tissue medium were modeled as a cell cluster. One of the cells in this model was considered as a radiation emission source of 1-10 keV monoenergetic electrons, and the energy deposition along with the S-value were calculated in this cell and its neighboring cells for different cell-to-cell distances. In addition to validating the S-values estimated by Geant4-DNA through comparing them to the values reported by Medical Internal Radiation Dose (MIRD) committee as well as the results obtained by previous version of Geant4-DNA, which shows a very good agreement, the evaluation of the results suggests that increasing the intercellular distance is significantly effective (50% or more) in reducing the energy deposition and S-value in cells adjacent to the source cell. Moreover, for electron energies of 3 keV and higher, the energy deposited in the nearby cells is such that it can cause cell destruction and death. Therefore, this is another important issue which should be considered in the selection of the suitable (electron-emitting) radiopharmaceutical as well as optimal treatment design for TRT of tumors in actual cases.