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We recently built an analytical source model for GPU-based MC dose engine. Inthis paper, we present a sampling strategy to efficiently utilize this sourcemodel in GPU-based dose calculation. Our source model was based on a concept ofphase-space-ring (PSR). This ring structure makes it effective to account forbeam rotational symmetry, but not suitable for dose calculations due torectangular jaw settings. Hence, we first convert PSR source model to itsphase-space let (PSL) representation. Then in dose calculation, different typesof sub-sources were separately sampled. Source sampling and particle transportwere iterated. So that the particles being sampled and transportedsimultaneously are of same type and close in energy to alleviate GPU threaddivergence. We also present an automatic commissioning approach to adjust themodel for a good representation of a clinical linear accelerator . Weightingfactors were introduced to adjust relative weights of PSRs, determined bysolving a quadratic minimization problem with a non-negativity constraint. Wetested the efficiency gain of our model over a previous source model using PSLfiles. The efficiency was improved by 1.70 ~ 4.41, due to the avoidance of longdata reading and transferring. The commissioning problem can be solved in ~20sec. Its efficacy was tested by comparing the doses computed using thecommissioned model and the uncommissioned one, with measurements in differentopen fields in a water phantom under a clinical Varian Truebeam 6MV beam. Forthe depth dose curves, the average distance-to-agreement was improved from0.04~0.28 cm to 0.04~0.12 cm for build-up region and the root-mean-square (RMS)dose difference after build-up region was reduced from 0.32%~0.67% to0.21%~0.48%. For lateral dose profiles, RMS difference was reduced from0.31%~2.0% to 0.06%~0.78% at inner beam and from 0.20%~1.25% to 0.10%~0.51% atouter beam.

An Analytic Linear Accelerator Source Model for Monte Carlo dose  calculations. II. Model Utilization in a GPU-based Monte Carlo Package and  Automatic Source Commissioning