SPECIAL CONTRIBUTION 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 radioimmunotherapy and in radionuclide palliation of bone pain. However, skeletal dosimetry is complicated by the intricate nature of its microstructure, which can vary greatly throughout skeletal regions. In this article, a new three-dimensional electron transport model for trabecular bone is introduced, based on Monte Carlo transport and on bone microstructure information for several 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 three-dimensional transport of single electrons. For the marrow spaces, explicit consideration of the site-specific elemental composi tion was made in the transport calculations based on the percentage of active and inactive marrow in each region. The electron transport was performed with the EGS4 electron transport code and th