Preliminary Monte Carlo Benchmarking of the First U.S. Carbon TPS: Range, Voxel-Wise Differences, and Microdosimetry
Abstract
Purpose
To present preliminary validation results for the first carbon treatment planning system (TPS) in the United States and to benchmark its GPU Monte Carlo dose engine using depth-resolved dose and microdosimetric metrics.
Methods
Pristine carbon-ion beams at 100, 290, and 430 MeV per nucleon and four spread-out Bragg peaks (SOBPs) were simulated in homogeneous 30×30×50 cm³ phantoms composed of water, soft-tissue elemental surrogates (H, C, N, O), calcium, and titanium. Dose was scored on 1 mm isotropic voxels. Independent reference calculations were performed with OpenTOPAS/Geant4 v11.3.0 using the Light-Ion Quantum Molecular Dynamics model. Agreement metrics included distal 80% range (R80), integral dose, and depth-resolved voxel-wise energy-deposition differences normalized to the total reference energy in the slab at Bragg Peak. For SOBPs, microdosimetric spectra were evaluated at six characteristic depths, with derived dose-averaged LET (LETd) and dose-mean lineal energy.
Results
R80 differences were within water-equivalent 0.7 mm while integral dose differences show larger differences from -0.07% to -4.33% across all materials. Depth-normalized voxel-wise analysis showed that most voxels were within 1%, while localized hotspots up to approximately 2% appeared in distal high-gradient regions and higher-Z media. For comparison, a 6 MeV photon beam in a water phantom with 1 × 1, 10 × 10, and 30 × 30 cm² field sizes yielded a maximum voxel-wise difference of 0.34%. For carbon SOBPs, plateau dose agreement was within 1.5%, while microdosimetric spectra showed high agreement with less than 1% error across all depths.
Conclusion
These preliminary results support commissioning of the first U.S. carbon TPS and demonstrate that depth-resolved voxel-wise and microdosimetric analyses provide sensitivity beyond conventional metrics.