  Purpose: To enhance an in-house graphic-processing-unit (GPU) accelerated
virtual particle (VP)-based Monte Carlo (MC) proton dose engine (VPMC) to model
aperture blocks in both dose calculation and optimization for pencil beam
scanning proton therapy (PBSPT)-based stereotactic radiosurgery (SRS). Methods
and Materials: A block aperture module was integrated into VPMC. VPMC was
validated by an opensource code, MCsquare, in eight water phantom simulations
with 3cm thick brass apertures: four were with aperture openings of 1, 2, 3,
and 4cm without a range shifter, while the other four were with same aperture
opening configurations with a range shifter of 45mm water equivalent thickness.
VPMC was benchmarked with MCsquare and RayStation MC for 10 patients with small
targets (average volume 8.4 cc). Finally, 3 patients were selected for robust
optimization with aperture blocks using VPMC. Results: In the water phantoms,
3D gamma passing rate (2%/2mm/10%) between VPMC and MCsquare were
99.71$\pm$0.23%. In the patient geometries, 3D gamma passing rates (3%/2mm/10%)
between VPMC/MCsquare and RayStation MC were 97.79$\pm$2.21%/97.78$\pm$1.97%,
respectively. The calculation time was greatly decreased from 112.45$\pm$114.08
seconds (MCsquare) to 8.20$\pm$6.42 seconds (VPMC), both having statistical
uncertainties of about 0.5%. The robustly optimized plans met all the
dose-volume-constraints (DVCs) for the targets and OARs per our institutional
protocols. The mean calculation time for 13 influence matrices in robust
optimization by VPMC was 41.6 seconds. Conclusion: VPMC has been successfully
enhanced to model aperture blocks in dose calculation and optimization for the
PBSPT-based SRS.
