The Effect of Cell Oxygen Concentration on the Relative Biological Effectiveness of Low Energy X-rays

Background and Aim: Cancer treatment using ionizing radiation is an effective modality for the management and control of various cancers. The presence of oxygen inside the cell causes various damages to the DNA molecule. The purpose of this study is to calculate the relative biological effectiveness (RBE) of low-energy X-rays during cancer treatment using intraoperative radiotherapy at different oxygen concentrations within the cell through a Monte Carlo (MC) simulation approach. Materials and Methods: To estimate the RBE values in this study, the secondary electron spectrum, which was emitted from 1 cm after the bare probe surface, was calculated with the Geant4-validated MC model of the INTRABEAM system. After the calculation of the secondary electron energy spectrum, the RBE values of the emitted low-energy X-rays from the INTRABEAM system at different cell oxygen concentrations were calculated by the MCDS MC code. Results: The results showed that the RBE values increase with the oxygen concentration increment in the cell. When the oxygen concentration increments from 0 to 100%, the RBESSB and RBEDSB values increase by about 1.6 and 2.3 times, respectively. Furthermore, in most of the studied oxygen concentrations, the RBEDSB values were greater than unity, which indicates the high relative biological effectiveness of the considered low-energy X-rays compared to high-energy photons. Conclusion: Accordingly, it can be concluded that cell oxygen level is one of the influential factors for RBE assessments relevant to the emitted X-rays from the INTRABEAM system, where the cell sensitivity to the ionizing radiation decreases at low-oxygen levels.