Benchmarking Heterogeneity-Aware 131Cs Intracranial Brachytherapy Using Monte Carlo Simulation and Patient-Specific Voxelized Lattice
Abstract
Purpose
Brachytherapy dose calculations are commonly based on the AAPM TG-43 formalism, which assumes a homogeneous water medium. Although widely used for HDR and LDR brachytherapy, this assumption inadequately represents dose distributions in heterogeneous tissues, particularly for low-energy photon emitters. The purpose of this study was to design and validate a clinically derived, patient-specific voxelized lattice for benchmarking heterogeneity-aware brachytherapy dose calculation algorithms.
Methods
A patient with glioblastoma multiforme treated with GammaTile 131Cs brachytherapy was selected. Patient geometry was converted into a voxelized lattice model in MCNP6, and a 131Cs source model was placed at each implanted seed position. Dose calculations were performed using an energy-deposition mesh tally with 108 histories. For benchmarking, all voxels were initially assigned as water, and resulting dose distributions were compared with those from the treatment planning system and Monte Carlo calculations for a single source. Subsequently, tissues were assigned using HU-based thresholding to classify air, skin, brain, and bone as distinct material universes. Simulations including all implanted seeds were then performed, and dose distributions from the treatment planning system and Monte Carlo were compared within the HR-CTV brain parenchyma. Dose to material was used as the comparison metric.
Results
Material segmentation was successfully achieved for all tissue classes and demonstrated strong spatial agreement with the original CT images. In the water-only benchmark, dose to water agreed within 1% between the treatment planning system and Monte Carlo for both V60 and V100. When tissue-specific densities and materials were applied, the treatment planning system overestimated HR-CTV V60 and V100 by 13.4% and 14.0%, respectively, relative to Monte Carlo results.
Conclusion
A clinically realistic, patient-specific voxelized three-dimensional lattice phantom incorporating tissue heterogeneity was developed and validated, enabling Monte Carlo benchmarking of heterogeneity-aware brachytherapy dose calculation algorithms and supporting implementation of AAPM TG-186 recommendations beyond TG-43.