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Biological Dose Characterization of a Research Proton Beam Line

D Smith

D Smith1*, R Stewart1 , E Lee2 , J Eagle3 , N Cao1 , R Hashemian1 , J Schuemann4 , E Ford1 , G Sandison1 , R Emery1 , S Marsh3 , J Schwartz1 , J Meyer1 , (1) University of Washington, Seattle, WA, (2) Cincinnati Children's/UC Health, Liberty Campus, OH, (3) University of Canterbury, Christchurch, Canterbury, (4) Massachusetts General Hospital, Boston, MA


SU-K-108-4 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 108

Purpose: Recently a novel proton research beam has been reported from the University of Washington (UW) specially designed to support image-guided small animal research. The purpose of this study is to characterize both the linear energy transfer (LET) distribution and the variable relative biological effectiveness (vRBE) weighted dose defined for double strand break (DSB) induction of this beam.

Methods: TOPAS Monte Carlo simulations of the LET and vRBE were performed for this cyclotron generated 50.5 MeV proton beam. Simulation of the beam accounted for all beamline components between the cyclotron exit window and the beam’s full range penetration in a water phantom. The model was benchmarked against measured data. Implementation of the Monte Carlo Damage Simulation (MCDS) model for DSB induction in TOPAS was applied to compute the vRBE on a per voxel basis.

Results: Simulations of the energy distribution incident upon the phantom predicted a peak beam energy of 45 MeV with a short tail of lower energies and a resulting Bragg Peak (BP) at 17.0 mm in water, lying within the experimental uncertainty of the measured value. LET predictions for the beam at depth were 3.0 keV/µm at 10 mm, and 3.7 keV/µm at 15 mm, a depth corresponding to the proximal part of the BP. LET increased near-linearly to 15.1 keV/µm at the Bragg peak and reached a maximum value of 43.2 keV/µm at the distal end of the BP. A similar behavior with depth was exhibited for vRBE with predicted values of 1.07, 1.1, 1.4 and 2.2, corresponding to the above depths, respectively.

Conclusion: TOPAS simulations predict a significant variation in RBE with depth for the proton beam, especially within 1 mm either side of the Bragg Peak. The vRBE of this beam allows the exploration of LET-dependent effects in cell cultures and small animal experiments.

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