Relative Biological Effectiveness (RBE) of Protons in Pristine Bragg Peaks
S Streitmatter1*, R Stewart2, G Sandison2, (1) University of Utah, Salt Lake City, UT, (2) University of Washington, Seattle, WAWE-E-108-3 Wednesday 2:00PM - 3:50PM Room: 108
Quantify spatial variations in proton RBE in the plateau region and near the distal edge of pristine Bragg peaks.
The linear energy transfer (LET) and dosimetric characteristics of pristine Bragg peaks were simulated for 30 and 200 MeV proton beams using MCNPX Version 2.7.0. To assess proton RBE, the Monte Carlo Damage Simulation (MCDS) was used to determine the double strand break (DSB) yield as a function of proton energy. To model reproductive cell death, mechanistic formulas derived from the Repair-Misrepair-Fixation (RMF) model were used to explicitly link DSB induction to estimates of linear-quadratic (LQ) model parameters α and β. These formulas were input into MCNPX (DE/DF tally modifier) and used to compute dose-averaged estimates of α and β at selected locations in pristine Bragg Peaks. The modified MCNPX dose tallies provide all of the information needed to compute the RBE-weighted dose (RWD) for the endpoints of reproductive cell death and DSB induction.
The RBE for reproductive cell death is greater than or equal to the RBE for DSB induction, regardless of location within the Bragg Peak. RBE and RWD are predicted to decrease as α/β increases. The ratio of the maximum RWD (distal edge of peak) to minimum RWD (plateau region) is 12.1 for a 30 MeV beam and 3.7 for a 200 MeV beam (α/β = 1.5 Gy). The lateral spread in RWD is much more significant for the degraded clinical beam than for the 30 MeV beam.
A low-energy proton beam has attractive physical and biological characteristics for the precise delivery of a wide range of RWD in vitro and in vivo. However, models are needed to translate the results of preclinical studies with low-energy protons to the more clinically relevant proton beam energies used on human patients.