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

Any radionuclide that is transported through the brood stream will also be carried through the haversian canals within cortical bone. These canals are lined with a layer of endosteum that contains radiosensitive cells. This p aper introduces a new three-dimensional electron transport model for cortic al bone based on Monte Carlo transport and on bone microstructural informat ion for several cortical bone regions. Methods: Previously published havers ian cavity and bone matrix chard length distributions for cortical bone wer e randomly sampled to create alternating regions of bone matrix, endosteum and haversian canal tissues during the three-dimensional transport of singl e electrons. Electron transport was performed using the EGS4 transport code with the parameter reduced electron step transport algorithm. Electron-abs orbed fractions of energy were tabulated for three adult cortical bone site s considering three source and target regions: the cortical haversian space , the cortical bone endosteum (CBE) and the cortical bone volume (CBV). Res ults: Absorbed fractions assessed with the new model were shown to be highl y energy dependent for most combinations of source-target regions in cortic al bone. Although chord length data were available for three different bone sites (femur, hu me rus and tibia), very little Variation with bone site w as noted in the absorbed fraction data. Conclusion: International Commissio n on Radiation Protection (ICRP)recommended absorbed fractions for cortical bone are given only for the CBE as target region and for the CBE and CBV a s source regions. Comparisons of these recommended absorbed fractions with the absorbed fractions calculated in this study show large differences. For example, ratios of self-absorbed fractions to the CBE in this model and in the ICRP 30 model are similar to 0.25, similar to 4 and similar to 1.5 for initial electron energies of 10, 200 keV and 4 MeV, respectively. Conseque ntly, this new transport model of electrons in cortical bone will improve t he relatively energy-independent data recommended by the ICRP. This model w ill also allow consideration of the haversian canals as a potential radiati on source.