Mixed Voxelized-Mesh Phantoms for Accelerated Patient-Specific Monte Carlo-Based HDR Brachytherapy Dosimetry
Abstract
Purpose
Monte Carlo (MC) simulations are the gold standard in high dose rate brachytherapy (HDR-BT) dosimetry, offering accurate dose-to-medium calculations beyond the dose-to-water calculations used clinically. However, clinical adoption remains limited by long simulation times. This work introduces mixed voxelized-mesh (MVM) phantoms to RapidBrachyMCTPS, a Geant4-based MC brachytherapy treatment planning system, designed to significantly accelerate patient-specific dose calculations.
Methods
MVM phantoms use a voxelized region focused on dosimetrically relevant anatomy (target volume and organs at risk plus 5 mm margin), while representing the body’s extent using a triangular surface mesh. RapidBrachyMCTPS generates these meshes from DICOM contours, including a simplification step to reduce mesh complexity. For benchmarking, dose to medium distributions for 10 prostate HDR-BT plans were computed under four scenarios: a fully voxelized phantom, MVM with a RapidBrachyMCTPS-generated mesh, MVM with a baseline mesh generated by 3D Slicer. Simulations used 2x107 192Ir decays and (1 mm)3 voxel resolution, resulting in CTV dose uncertainties <1.7%. Runtimes, dose difference maps, and key DVH metrics were compared.
Results
MVM phantoms using RapidBrachyMCTPS-generated body meshes (mean: 7,200 triangles; range: 5,200-9,300) achieved faster simulation times (mean: 35.6 min; range: 29.6-43.7) versus fully voxelized phantoms (mean: 110.2 min; range: 92.4-150.5), due to a major reduction in voxelized volume size (mean: 1.55x105 voxels; range: 1.92x105-6.70x105) compared to full phantoms (mean: 1.81x107 voxels; range: 7.05x106-3.92x107). The MVM with 3D Slicer meshes (mean: 1.38x105 triangles; range: 6.71x104-2.09x105) had longer runtimes (mean: 156.4 min; range: 52.7-370.0). Both MVM phantom simulation scenarios yielded root-mean-square dose differences <1% relative to the full phantom for all patients. Key DVH metrics (CTV D90%, V100%, V150%; rectum D2cc; urethra D30%, D10%) agreed within 1%.
Conclusion
MVM phantom simulations in RapidBrachyMCTPS were ~3x faster using RapidBrachyMCTPS-generated meshes, while maintaining dosimetric equivalence with fully voxelized phantoms. These accelerated MC dose calculations support research and clinical integration.