A three-dimensional transport model for determining absorbed fractions of energy for electrons within trabecular bone

Bone marrow is generally the dose-limiting organ of concern in radioimmunot herapy and in radionuclide palliation of bone pain. However, skeletal dosim etry is complicated by the intricate nature of its microstructure, which ca n vary greatly throughout skeletal regions, In this article, a new three-di mensional electron transport model for trabecular bone is introduced, based on Monte Carte transport and on bone microstructure information for severa l trabecular bone sites. Methods: Marrow cavity and trabecular chord length distributions originally published by Spiers et al, were randomly sampled to create alternating regions of bone, endosteum and marrow during the thre e-dimensional transport of single electrons. For the marrow spaces, explici t consideration of the site-specific elemental composition was made in the transport calculations based on the percentage of active and inactive marro w in each region. The electron transport was performed with the EGS4 electr on transport code and the parameter reduced electron-step transport algorit hm, Electron-absorbed fractions of energy were tabulated for seven adult tr abecular bone sites, considering three source and target regions: the trabe cular marrow space (TMS), the trabecular bone endosteum (TBE) and the trabe cular bone volume (TBV). Results: For all source-target combinations, the a bsorbed fraction was seen to vary widely within the skeleton. These variati ons can be directly attributed to the differences in the trabecular microst ructure of the different skeletal regions. For many source-target combinati ons, substantial energy dependence was seen in the calculated absorbed frac tion, a factor not considered in values recommended by the international Co mmission on Radiological Protection (ICRP). A one-dimensional model of elec tron transport in trabecular bone, based on range-energy relationships, was also developed to verify the three-dimensional transport model and to eval uate differences between the two modeling approaches, Differences of simila r to 10%-15% were seen, particularly at low electron energies. In the case of a TBV source and a TMS target (or vice versa), differences >50% were see n in the absorbed fraction. Conclusion: The three-dimensional model of elec tron transport in trabecular bone allows improved estimates of skeletal abs orbed fractions. The model highlights both the regional and the energy depe ndency of the absorbed fraction not previously considered in the ICRP model .