This article presents a revised voxel S values (VSVs) approach for dosimetry in targeted radiotherapy, allowing dose calculation for any voxel size and shape of a given SPECT or PET dataset. This approach represents an update to the methodology presented in MIRD pamphlet no. 17. Methods: VSVs were generated in soft tissue with a fine spatial sampling using the Monte Carlo (MC) code MCNPX for particle emissions of 9 radionuclides: 18F, 90Y, 99mTc, 111In, 123I, 131I, 177Lu, 186Re, and 201Tl. A specific resampling algorithm was developed to compute VSVs for desired voxel dimensions. The dose calculation was performed by convolution via a fast Hartley transform. The fine VSVs were calculated for cubic voxels of 0.5 mm for electrons and 1.0 mm for photons. Validation studies were done for 90Y and 131I VSV sets by comparing the revised VSV approach to direct MC simulations. The first comparison included 20 spheres with different voxel sizes (3.8-7.7 mm) and radii (4-64 voxels) and the second comparison a hepatic tumor with cubic voxels of 3.8 mm. MC simulations were done with MCNPX for both. The third comparison was performed on 2 clinical patients with the 3D-RD (3-Dimensional Radiobiologic Dosimetry) software using the EGSnrc (Electron Gamma Shower National Research Council Canada)-based MC implementation, assuming a homogeneous tissue-density distribution. Results: For the sphere model study, the mean relative difference in the average absorbed dose was 0.20% ± 0.41% for 90Y and -0.36% ± 0.51% for 131I (n = 20). For the hepatic tumor, the difference in the average absorbed dose to tumor was 0.33% for 90Y and -0.61% for 131I and the difference in average absorbed dose to the liver was 0.25% for 90Y and -1.35% for 131I. The comparison with the 3D-RD software showed an average voxel-tovoxel dose ratio between 0.991 and 0.996. The calculation time was below 10 s with the VSV approach and 50 and 15 h with 3D-RD for the 2 clinical patients. Conclusion: This new VSV approach enables the calculation of absorbed dose based on a SPECT or PET cumulated activity map, with good agreement with direct MC methods, in a faster and more clinically compatible manner.
- 3D dosimetry
- Monte Carlo simulation
- Voxel S values
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging