  Purpose: Very fast Monte Carlo (MC) simulations of proton transport have been
implemented recently on GPUs. However, these usually use simplified models for
non-elastic (NE) proton-nucleus interactions. Our primary goal is to build a
GPU-based proton transport MC with detailed modeling of elastic and NE
collisions. Methods: Using CUDA, we implemented GPU kernels for these tasks:
(1) Simulation of spots from our scanning nozzle configurations, (2) Proton
propagation through CT geometry, considering nuclear elastic scattering,
multiple scattering, and energy loss straggling, (3) Modeling of the
intranuclear cascade stage of NE interactions, (4) Nuclear evaporation
simulation, and (5) Statistical error estimates on the dose. To validate our
MC, we performed: (1) Secondary particle yield calculations in NE collisions,
(2) Dose calculations in homogeneous phantoms, (3) Re-calculations of head and
neck plans from a commercial treatment planning system (TPS), and compared with
Geant4.9.6p2/TOPAS. Results: Yields, energy and angular distributions of
secondaries from NE collisions on various nuclei agree well with the Geant4
Bertini and Binary cascade models. The 3D-gamma pass rate at 2\%-2 mm for
treatment plan simulations is typically 98\%. The net calculation time on a
NVIDIA GTX680 card, including all data transfers, is $\sim$20 s for
$1\times10^7$ proton histories. Conclusions: Our GPU-based MC is the first of
its kind to include a detailed nuclear model to handle NE interactions of
protons with any nucleus. Dosimetric calculations are in very good agreement
with Geant4/TOPAS. Our MC is being used to perform fast routine clinical QA of
pencil-beam based treatment plans, and has also been adopted as the dose engine
in a clinically-applicable MC-based IMPT TPS. The detailed nuclear modeling
will allow us to perform very fast linear energy transfer and neutron dose
estimates on the GPU.
