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

Abstract

Any radionuclide that is transported through the blood 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 paper introduces a new three-dimensional electron transport model for cortical bone based on Monte Carlo transport and on bone microstructural information for several cortical bone regions.

Methods: Previously published haversian cavity and bone matrix chord length distributions for cortical bone were randomly sampled to create alternating regions of bone matrix, endosteum and haversian canal tissues during the three-dimensional transport of single electrons. Electron transport was performed using the EGS4 transport code with the parameter reduced electron step transport algorithm. Electron-absorbed fractions of energy were tabulated for three adult cortical bone sites considering three source and target regions: the cortical haversian space, the cortical bone endosteum (CBE) and the cortical bone volume (CBV).

Results: Absorbed fractions assessed with the new model were shown to be highly energy dependent for most combinations of source-target regions in cortical bone. Although chord length data were available for three different bone sites (femur, humerus and tibia), very little variation with bone site was noted in the absorbed fraction data.

Conclusion: International Commission 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 as 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 approximately 0.25, approximately 4 and approximately 1.5 for initial electron energies of 10, 200 keV and 4 MeV, respectively. Consequently, this new transport model of electrons in cortical bone will improve the relatively energy-independent data recommended by the ICRP. This model will also allow consideration of the haversian canals as a potential radiation source.