Alpha dosimetry for the skeletal model of the reference adult male

Abstract

Introduction: Dosimetry of bone marrow is challenging due to the complex histology of the skeleton and to the fact that the segmentation is not possible on the macroscopic level. In this context, calculation of absorbed fractions (AFs) of energy from alpha particles is of particular interest. Indeed, the high ionizing power and the short range of alpha particles makes them toxic in the case of internal contaminations or beneficial for radioimmunotherapy applications. Materials and methods: A reference voxelized adult model of the human skeleton, developed at the University of Florida using 3-D microCT images of a 40 year-old male, is used for the calculation of AFs for alpha particles with initial energy ranging from 4 to 10 MeV. The MCNPX Monte Carlo code associated to OEDIPE software will be used to simulate the transport of alphas in 32 skeletal microstructures for the following source regions: trabecular bone volume, active marrow, inactive marrow and endosteum. Radiosensitive skeletal target regions are the active marrow (risk of leukemia) and endosteum (risk for bone cancer). Siteand agespecific percentage of marrow cellularity will be considered. The Monte Carlo calculations will be benchmarked with the GEANT4 Monte Carlo code, which generates secondary electrons in addition of alpha particles. Results: Significant variations in absorbed fractions between different skeletal sites were observed. The parietal bone demonstrates the greatest difference as this particular bone site is characterized by relatively small marrow cavities and thick bone trabeculae. Differences were also observed between this model and the absorbed fractions given in ICRP Publication 30. In fact, the energy deposition in the active marrow is overestimated in the ICRP 30. On the contrary, the AFs are underestimated for the endosteum. Calculation of the AFs as a function of the marrow cellularity shows that the energy deposition decrease with the cellularity and increase with the energy of alpha particles. Conclusion: Results presented here provide a firmer basis for a more realistic dosimetry in alpha emitter radionuclide therapies through the explicit consideration of absorbed fraction variations with particle energy, skeletal site, and marrow cellularity.