Monte Carlo simulation of trabecular bone remodeling and absorbed dose coefficients for tritium and 14C

A Monte Carlo simulation of multiple trabecular bone cavities in adult bone was developed and the absorbed radiation dose factors evaluated for 3H and 14C. The model was developed to assess the dose from radionuclide uptake in quiescent bone, but also the effects of temporal changes in bone turnover by incorporating bone-modelling units (BMU). Absorbed dose fractions were calculated for target regions that include the endosteal layer where radiation-sensitive stem cells in bone marrow are considered to reside preferentially. There were large differences in the absorbed fractions for two types of bone surface, quiescent and forming. Tritium in quiescent bone results in a dose to the endosteum about 20 times that for the same activity in forming bone surface irradiating osteoblasts. When the quiescent bone surface source was extended from an infinitely thin layer to a more realistic 1 mm thick, the tritium absorbed fractions for endosteum and red marrow targets fell by more than 2-fold. (authors)