Integration of a dose-voxel kernel (DVK) convolution method for voxel-based dosimetry in Particle and Heavy Ion Transport Code System (PHITS)

Objective. Dose-voxel kernel (DVK) convolution provides a physics-based approach for absorbed-dose calculation from nuclear medicine imaging. This study implemented DVK convolution within the particle and heavy ion transport code system (PHITS) and evaluated its performance against Monte Carlo (MC) simulations and clinical dosimetry methods. Approach. Five post-radioembolization 90Y bremsstrahlung single photon emission computed tomography/computed tomography (SPECT/CT) and positron emission tomography/CT (PET/CT) datasets from the University of Michigan Deep Blue Data repository were converted into PHITS-readable formats using the radiotherapy package based on PHITS. Activity distributions were mapped onto CT voxel grids, and DVKs were generated for 24 human tissue materials. A new DVK mode was implemented in PHITS to calculate voxel-wise absorbed-dose rates by convolving tissue-specific DVKs with activity distributions. Performance was evaluated against MC simulations using gamma index analysis, mean absorbed-dose rates within volumes of interests (VOIs), and voxel-wise absorbed-dose differences in liver, lung, and liver–lung interface regions. VOI-based mean absorbed doses were further benchmarked against the local deposition model (LDM) and voxel S-value (VSV) convolution in MIM Software (v7.4.3). Main results. Following one-time DVK generation, DVK mode calculations required ∼40 s compared with >9 h for high-statistics MC simulations. Gamma pass fractions ranged from 92.1%–99.9%, while mean DVK–MC differences in VOI-based dose rates were −1.2 ± 2.9% (SPECT/CT) and −1.0 ± 4.3% (PET/CT). Voxel-wise dose differences showed good agreement between DVK and MC in homogeneous liver tissue, while lung regions exhibited lower absorbed-dose estimates and the liver–lung interface showed increased variability, (P95 up to 39%). Compared with MIM, VOI-based dose differences were 9.7 ± 7.2% for SPECT/CT (LDM-based) and −0.1 ± 8.4% for PET/CT (VSV-based). Significance. The PHITS-integrated DVK implementation enables efficient voxel-based dosimetry while maintaining close agreement with high-statistics MC simulations. This framework provides a practical physics-based approach for patient-specific radiopharmaceutical therapy dosimetry.

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