Monte Carlo simulations of therapeutic proton beams for relative biological effectiveness of double-strand break

Purpose: The relative biological effectiveness (RBE) values relative to ⁶⁰Co for the induction of double-strand breaks (DSB) were calculated for therapeutic proton beams. RBE-weighted absorbed doses were determined at different depths in a water phantom for proton beams. Materials and methods: The depth-dose distributions and the fluence spectra for primary protons and secondary particles were calculated using the FLUKA (FLUktuierende KAskade) MC (Monte Carlo) transport code. These spectra were combined with the MCDS (Monte Carlo damage simulation) code to simulate the spectrum-averaged yields of clustered DNA lesions. RBE for the induction of DSB were then determined at different depths in a water phantom for the unmodulated and modulated proton beams. Results: The maximum RBE for the induction of DSB at 1 Gy absorbed dose was found about 1.5 at 0.5 cm distal to the Bragg peak maximum for an unmodulated 160 MeV proton beam. The RBE-weighted absorbed dose extended the biologically effective range of the proton beam by 1.9 mm. The corresponding maximum RBE value was inversely proportional to the proton beam energy, reaching a value of about 1.9 for 70 MeV proton beam. For a modulated 160 MeV proton beam, the RBE weightings were more pronounced near the spread-out Bragg peak (SOBP) distal edge. Conclusions: It was demonstrated that a fast MCDS code could be used to simulate the DNA damage yield for therapeutic proton beams. Simulated RBE for the induction of DSB were comparable to RBE measured in vitro and in vivo. Depth dependent RBE values in the SOBP region might have to be considered in certain treatment situations.