  Purpose: This work introduces a new lattice geometry library, egs_lattice,
into the EGSnrc Monte Carlo code, which can be used for both modeling very
large (previously unfeasible) quantities of geometries and establishing
recursive boundary conditions. The reliability of egs_lattice, as well as
EGSnrc in general, is cross-validated and tested. Methods: New Bravais, cubic,
and hexagonal lattice geometries are defined in egs_lattice and their transport
algorithms are described. Simulations of cells and Gold NanoParticle (GNP)
containing cavities are implemented to compare to published Geant4-DNA and
PENELOPE results. Recursive boundary conditions, implemented through a cubic
lattice, are used to perform electron Fano cavity tests. Results: Lattices are
successfully implemented in EGSnrc. EGSnrc calculated doses to cell cytoplasm
and nucleus when irradiated by an internal electron source with a median
difference of 0.6% compared to Geant4-DNA. EGSnrc calculated the ratio of dose
to a microscopic cavity containing GNPs over dose to a cavity containing a
homogeneous mixture of gold, and results generally agree (within 1%) with
PENELOPE. The Fano test is passed (sub-0.1%) for all energies/ cells
considered. Additionally, the recursive boundary conditions used for the Fano
test provided a factor of over a million increase in efficiency in some cases.
Conclusions: The egs_lattice geometry library, currently available as a pull
request on the EGSnrc GitHub develop branch, is now freely accessible as
open-source code. Lattice geometry implementations cross-validated with
independent simulations in other MC codes and verified with the electron Fano
cavity test demonstrate not only the reliability of egs_lattice, but, by
extension, EGSnrc's ability to simulate transport in nanometer geometries and
score in microscopic cavities.
