Monte Carlo-based evaluation of the relevant RBE values to produced low-energy X-rays by INTRABEAM and Axxent dedicated IORT facilities

Intraoperative radiotherapy (IORT) by low-energy X-rays is a single-fraction modality for tumor bed irradiation following surgery. Two specific machines, including INTRABEAM and Axxent, are available for this purpose. Current study aims to compare the RBE values relevant to produced X-rays by these machines using a hybrid Monte Carlo (MC) simulation approach. Validated MC models of INTRABEAM and Axxent machines with nominal energies of 40 and 50 kV, simulated by GEANT4 toolkit, were employed for RBE assessments. Scored X-ray spectra for each studied machine were used to calculate the corresponding secondary electron spectra at various depths and nominal energies inside the water. Then, Monte Carlo damage simulation (MCDS) code was employed to calculate the initial DNA damages (single and double-strand breaks (SSB and DSB)) and relevant RBE values. Results showed that the scored X-ray spectra for Axxent and INTRABEAM machines are different, a fact which can affect the RBE values for these machines. RBESSB values for Axxent machine were higher than those of INTRABEAM up to 1.4