SU-E-T-504: Electron Beam Tuning Methodology for TrueBeam Phase-Space Library Generation

Abstract

Purpose: Field-independent phase-space files are an effective means to reduce the time to simulate particle transport through medical linac treatment head components. The purpose of this work is to: (a) introduce the methodology used to newly create the full TrueBeam phase-space library; (b) generate ∼50GB of data for each of seven photon beams; (c) present the results of the validation against recent gold beam data (GBD) measurements. Methods: An accurate representation of the TrueBeam field-independent treatment head components was implemented in Geant4 (v.9.4.p02) and deployed on a large cloud-based cluster with >500 computing cores. The incident electron beam was modeled by assuming Gaussian distributions for the energy spectrum, beam divergence, and spot size. Spot dimensions were fixed by measurements of the electron spots on the same three machines on which GBD was measured. Three remaining electron beam parameters (mean energy, energy spread, beam divergence) were varied one-by-one until the slopes of the least-squares fits to the differences between Monte Carlo and measurement were <1% per fitted region (50 to 300 mm depth for percent depth doses, 0 to ±180 mm off-axis position for in-field profiles). Results: Seven photon beams were tuned to within 1% of GBD. The resulting sets of optimal electron beam parameters were used to generate ∼50GB per beam of IAEA-format phase-space files on a flat scorer (73.3 cm from isocenter) above the Y-jaws for the 4MV, 6MV, 6FFF, 8MV, 10MV, 10FFF and 15MV beams, where FFF is the flattening filter free mode. Conclusion: A consistent methodology of electron beam tuning for megavoltage radiotherapy linac simulations allowed generation of a complete library of phase-space files for TrueBeam freely available to support the majority of modeling needs for TrueBeam worldwide.