  Monte Carlo (MC) simulation is considered as the most accurate method for
radiation dose calculations. Accuracy of a source model for a linear
accelerator is critical for the overall dose calculation accuracy. In this
paper, we presented an analytical source model that we recently developed for
GPU-based MC dose calculations. A key concept called phase-space-ring (PSR) was
proposed. It contained a group of particles that are of the same type and close
in energy and radial distance to the center of the phase-space plane. The model
parameterized probability densities of particle location, direction and energy
for each primary photon PSR, scattered photon PSR and electron PSR. For a
primary photon PSRs, the particle direction is assumed to be from the beam
spot. A finite spot size is modeled with a 2D Gaussian distribution. For a
scattered photon PSR, multiple Gaussian components were used to model the
particle direction. The direction distribution of an electron PSRs was also
modeled as a 2D Gaussian distribution with a large standard deviation. We also
developed a method to analyze a phase-space file and derive corresponding model
parameters. To test the accuracy of our linac source model, dose distributions
of different open fields in a water phantom were calculated using our source
model and compared to those directly calculated using the reference phase-space
file. The average distance-to-agreement (DTA) was within 1 mm for the depth
dose in the build-up region and beam penumbra regions. The root-mean-square
(RMS) dose difference was within 1.1% for dose profiles at inner and outer beam
regions. The maximal relative difference of output factors was within 0.5%.
Good agreements were also found in an IMRT prostate patient case and an IMRT
head-and-neck case. These results demonstrated the efficacy of our source model
in terms of accurately representing a reference phase-space file.
