The application of voxel phantoms to the internal dosimetry of radionuclides.

Extensive calculations of specific absorbed fractions (SAFs) for monoenergetic photon sources were performed using a Monte Carlo photon transport code together with seven male and female adult voxel models based on computed tomographic data of real persons. These models offer greater realism with respect to organ topology than the mathematical phantoms commonly used in the past. Due to individual anatomical differences, large variations in photon SAFs between the voxel models were found that can amount to orders of magnitude for very low photon energies. However, in many cases, the larger differences were found between MIRD-type and voxel models, since the inter-organ distances tend to be larger in the MIRD-type phantoms than in reality, due to over-simplification of organ shapes. Furthermore, organ absorbed doses per incorporated activity were evaluated for two selected radiopharmaceuticals. Although a method was found to largely eliminate the influence of organ mass on SAFs for organ self-absorption, the absorbed dose coefficients varied by several tens of per cent between the individual voxel models, thus indicating a significant influence of individual photon SAFs for organ cross-fire on organ absorbed dose. Again, 43% of the MIRD organ dose values were outside the range of doses spanned by the voxel models. Effective dose showed a variation of only up to 26% between the single voxel models for the radiopharmaceuticals considered.