Validation of particle and heavy ion transport code system (PHITS) in generating dose-voxel kernels for internal dosimetry calculations

Purpose In voxel-based dosimetry, dose-voxel kernels (DVKs) represent the absorbed dose in a target voxel per decay in a source voxel. In this work, we aimed to validate DVKs generated using Particle and Heavy Ion Transport Code System (PHITS) as a first step in establishing a quantitative SPECT/CT-based dosimetry framework for selective internal radiation therapy (SIRT). Methods A 1-MBq source of 90Y, 153Sm, 177Lu, 188Re, and 166Ho located in central cubic voxels of 1 mm, 3 mm, and 6 mm dimension within soft tissue (1.04 g⋅cm−³), was simulated using 2.5 × 10⁷ histories in PHITS. The DVK results were compared to published data (Lanconelli et al., 2012). In a pilot study, 17 MBq of 153Sm microspheres were injected into a liver tumor-bearing rat and imaged 24 h later using SPECT/CT. SPECT/CT images were then converted to cumulated activity and convolved with a 153Sm DVK using Fast Fourier Transform in MATLAB to create an absorbed dose map. Results As expected, DVKs for the radionuclides showed a steep decrease from the source voxel to the maximum continuous slowing down approximation (CSDA) of beta components. Beyond the CSDA, gamma and bremsstrahlung contributed minimally (10⁻⁶ to 10⁻⁴) to voxel doses. Observed DVK changes as voxel size increased were in good agreement with published data, with differences in source voxel ranging from 1 to 24%. Rat imaging revealed 153Sm microspheres localized at injection sites, with mean and maximum doses of 45 Gy and 194 Gy, respectively. Conclusion The DVKs calculated in PHITS were successfully validated. 153Sm dose maps were produced from SPECT/CT images in a rat model. Further studies on the application of DVKs in heterogeneous media will be conducted.

Text (Abstract) Validation of Particle and Heavy Ion Transport Code System -PHITS.pdf Download (120kB) Official URL or Download Paper: https://link.springer.com/journal/12553

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