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interfaces, stopping power ratios are playing a role.
Conclusion
As the range of the interface effects is less than commonly
used voxel sizes for MV photon beams, one can in general
use the ratio of attenuation coefficients to convert dose
to medium to dose to water. As a direct consequence,
uncertainties on stopping powers (due to uncertainties on
the ionization potential of tissues) have negligible impact
on the dose to medium to dose to water conversion. Dose
to water obtained by multiplying dose to medium with
stopping power ratios is another concept not consistent
with how dose to water is calculated by conventional TPSs.
OC-0227 The heterogeneous multiscale model for
efficient computation of microscopic dose metrics
M. Martinov
1
, R. Thomson
1
1
Carleton University, Physics, Ottawa, Canada
Purpose or Objective
In the field of radiation therapy, there is increasing
interest in the effects of ionizing radiation on short
(micrometre, nanometre) length scales within
macroscopic (~centimetre) volumes of interest. A common
technique for studying radiation transport and energy
deposition is via Monte Carlo (MC) simulations, requiring
complex simulation geometries that are both reliable and
efficient, two traits often in contention. This work
introduces a general MC framework, the Heterogeneous
MultiScale (HetMS) model, to address these challenges.
Material and Methods
The HetMS model involves combining distinct models of
varying level of detail on different length scales into a
single simulation. We implement and present the HetMS
model using EGSnrc for the context of gold nanoparticle
(GNP) radiation therapy. Our model is verified via
comparison with recently-published results of other MC
GNP simulations. We then consider two scenarios: (A) 20
keV to 1 MeV photon beams incident on a 1 cm radius and
3 cm long cylindrical phantom; (B) low-dose rate
brachytherapy sources at the centre of a 2.5 cm radius
sphere with GNPs diffusing outwards from the centre (Fig.
1). In each simulation, homogenized tissue/gold mixtures
are employed in larger volumes, with distinct subvolumes
containing GNPs discretely modelled in pure tissue. Dose
scored in pure tissue within the subvolumes is compared
to dose scored in homogeneous tissue/gold mixtures and
dose to pure tissue (to compute Dose Enhancement
Factors (DEFs)).
Results
HetMS simulations are able to efficiently account for
important macroscopic and microscopic effects,
successfully modelling the competing effects of photon
fluence perturbation (due to modelling of gold/tissue
mixtures in macroscopic volumes) coupled with enhanced
local energy deposition (due to discrete modelling of GNPs
within subvolumes). Energy deposition is most sensitive to
these competing effects for lower energy sources, with
considerable variations in DEFs for different source
energies, depths in phantom, gold concentrations, and
GNP sizes. For the cylinder phantom with 20 mg Au/g
tissue, DEFs near 3.1 are observed near the phantom
surface and decrease to less than one by 7 mm depth (i.e.
dose decreases, not enhancements). Within the spherical
phantom, DEFs vary with time for diffusion, radionuclide,
and radius; DEFs differ considerably compared to those
computed using a widely-applied analytic approach (Fig.
2). Compared with discrete modelling of GNPs within
entire macroscopic geometry, HetMS simulations offer
efficiency enhancements of up to a factor of 120.
Conclusion
The HetMS framework enables efficient simulation of both
macroscopic and microscopic effects that must both be
considered for accurate simulation of radiation transport
and energy deposition. The HetMS model allows for MC
simulations, typically prohibited by dense parameter
spaces, to be employed in diverse radiotherapy and
radiation protection scenarios.