  To introduce the heterogeneous multiscale (HetMS) model for Monte Carlo
simulations of gold nanoparticle dose-enhanced radiation therapy (GNPT), a
model characterized by its varying levels of detail on different length scales
within a single phantom; to apply the HetMS model in two different scenarios
relevant for GNPT and to compare computed results with others published. The
HetMS model is implemented in EGSnrc; the code is tested via comparisons with
published data from independent gold nanoparticle (GNP) simulations. Two
distinct scenarios for the HetMS model are considered: (1) monoenergetic photon
beams incident on a large cylinder; (2) isotropic point source at the center of
a large sphere with GNPs diffusing from the center. Dose enhancement factors
(DEFs) are compared for different source energies, depths, gold concentrations,
GNP sizes, and modeling assumptions. Simulation efficiencies are investigated.
The HetMS MC simulations account for the competing effects of photon fluence
perturbation coupled with enhanced local energy deposition. DEFs are most
sensitive to these effects for lower source energies, varying with distance
from the source; DEFs below unity can occur at energies relevant for
brachytherapy. Compared to discrete modeling of GNPs throughout the
gold-containing volume, efficiencies are enhanced by up to a factor of 122 with
the HetMS approach. For the spherical phantom, DEFs vary with time for
diffusion, radionuclide, and radius. By combining geometric models of varying
complexity on different length scales within a single simulation, the HetMS
model can effectively account for both macroscopic and microscopic effects.
Efficiency gains with the HetMS approach enable diverse calculations which
would otherwise be prohibitively long. The HetMS model may be extended to
diverse scenarios relevant for GNPT, providing further avenues for research and
development.
