An inter-comparison between radiobiological characteristics of a commercial low-energy IORT system by Geant4-DNA and MCDS Monte Carlo codes.

Introduction: The need for accurate relative biological effectiveness (RBE) estimation for low energy therapeutic X-rays (corresponding to 50 kV nominal energy of a commercial low-energy IORT system (INTRABEAM)) is a crucial issue due to increased radiobiological effects, respect to high energy photons. Modeling of radiation-induced DNA damage through Monte Carlo (MC) simulation approaches can give useful information. Hence, this study aimed to evaluate and compare RBE of low energy therapeutic X-rays using Geant4-DNA toolkit and Monte Carlo damage simulation (MCDS) code.

Materials and methods: RBE calculations were performed considering the emitted secondary electron spectra through interactions of low energy X-rays inside the medium. In Geant4-DNA, the DNA strand breaks were obtained by employing a B-DNA model in physical stage with 10.79 eV energy-threshold and the probability of hydroxyl radical's chemical reactions of about 0.13%. Furthermore, RBE estimations by MCDS code were performed under fully aerobic conditions.

Results: Acquired results by two considered MC codes showed that the same trend is found for RBE_DSB and RBE_SSB variations. Totally, a reasonable agreement between the calculated RBE values (both RBE_SSB and RBE_DSB) existed between the two considered MC codes. The mean differences of 9.2% and 1.8% were obtained between the estimated RBE_SSB and RBE_DSB values by two codes, respectively.

Conclusion: Based on the obtained results, it can be concluded that a tolerable accordance is found between the calculated RBE_DSB values through MCDS and Geant4-DNA, a fact which appropriates both codes for RBE evaluations of low energy therapeutic X-rays, especially in the case of RBE_DSB where lethal damages are regarded.