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Parameter Characterization of Electron Beam Monte Carlo Phase Space of TrueBeam Linacs

A Rodrigues

A Rodrigues1,2*, D Sawkey3 , F Yin1,2 , Q Wu1,2 , (1) Duke University Medical Center, Durham, NC, (2) Medical Physics Graduate Program, Duke University Medical Center, Durham, NC, (3) Varian Medical Systems, Palo Alto, CA


SU-D-19A-4 Sunday 2:05PM - 3:00PM Room: 19A

Purpose: For TrueBeam Monte Carlo simulations, Varian does not distribute linac head geometry and material compositions, instead providing a phase space file (PSF) for the users. The PSF has a finite number of particle histories and can have very large file size, yet still contains inherent statistical noises. The purpose of this study is to characterize the electron beam PSF with parameters.

Methods: The PSF is a snapshot of all particles’ information at a given plane above jaws including type, energy, position, and directions. This study utilized a preliminary TrueBeam PSF, of which validation against measurement is presented in another study. To characterize the PSF, distributions of energy, position, and direction of all particles are analyzed as piece-wise parameterized functions of radius and polar angle. Subsequently, a pseudo PSF was generated based on this characterization. Validation was assessed by directly comparing the true and pseudo PSFs, and by using both PSFs in the down-stream MC simulations (BEAMnrc/DOSXYZnrc) and comparing dose distributions for 3 applicators at 15 MeV. Statistical uncertainty of 4% was limited by the number of histories in the original PSF. Percent depth dose (PDD) and orthogonal (PRF) profiles at various depths were evaluated.

Results: Preliminary results showed that this PSF parameterization was accurate, with no visible differences between original and pseudo PSFs except at the edge (6 cm off axis), which did not impact dose distributions in phantom. PDD differences were within 1 mm for R₇₀, R₅₀, R₃₀, and R₁₀, and PRF field size and penumbras were within 2 mm.

Conclusion: A PSF can be successfully characterized by distributions for energy, position, and direction as parameterized functions of radius and polar angles; this facilitates generating sufficient particles at any statistical precision. Analyses for all other electron energies are under way and results will be included in the presentation.

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