An Attempt to Predict the Proton Relative Biological Effectiveness using Radical Recombination

Abstract

Most proton treatment facilities have adopted a relative biological effectiveness (RBE) of 1.1 for proton therapy. However, most of the in vitro and in vivo studies indicate that the RBE of the spread-out Bragg peak (SOBP) protons increases with depth. The increase in RBE of proton beams on the SOBP is a well-known phenomenon that is difficult to quantify accurately in vivo studies. The reason for this explains that the RBE increases as linear energy transfer (LET) increases within the SOBP. The fact that intra-track radical recombination can indicate to produce fully competent lesions in room. The purpose of this study was to analyze an impact on radical recombination for the RBE in the SOBP proton beams. First, a depth-dose curve for the 210 MeV proton beam measured using a gel dosimeter and an ionization chamber. Second, the spatial distribution of the physical dose was calculated by Monte Carlo code system PHITS; the role of nuclear interaction was taken into account and the geometry of the apparatus was faithfully reproduced. The simulation results were compared with measured the depth-dose distribution and very good agreement was found, and the spatial distribution of an LET- weighted dose with threshold LET value (4.9 keV/μm) was calculated by the same code. Then, the relative distribution of the radical-recombination was calculated from the physical dose and LET-weighted dose. The relative distribution of the radical-recombination was calculated at each depth as the quotient of relative dose obtained using physical and LET- weighted dose. The agreement between the relative distributions of radical-recombination and RBE was good at the SOBP.KeywordsProton therapyGel dosimeterMonte Carlo methodRadical recombinationRelative biological effectiveness