Skeletal dosimetry in a microCT image-based rat model for external photon irradiation

Human skeletal dosimetry has experienced great developments in radiation protection in recent years by using the heterogeneous skeletal model. While for the rats experimentally used in radiation medicine, the investigation on skeletal dosimetry were mainly based on the homogeneous skeletal model, leading to inaccurate assessments of dose to radiosensitive tissues of red bone marrow (RBM) and bone surface. The purpose of this study is to develop a rat model with heterogeneous skeletal system and to investigate the dose difference in bone tissues for external photon irradiation. The high resolution of microCT images of a rat weighing 335 g were segmented into bone cortical, bone trabecular, bone marrow as well as other organs to construct the rat model. The absorbed dose to bone cortical, bone trabecular and bone marrow were calculated respectively by using Monte Carlo simulation for 22 external monoenergetic photon beams between 10 keV and 10 MeV under four different irradiation geometries conditions (left lateral [LL], right lateral [RL], dorsal-ventral [DV], ventral-dorsal [VD]). The calculated absorbed dose data were expressed as dose conversion coefficients and presented in this article, and the effect of irradiation conditions, photon energies and bone tissues density on the skeletal dose was discussed. The results showed that the dose conversion coefficients varying the photon energy for bone cortical, bone trabecular and bone marrow exhibit different trends and have the same sensitivity to irradiation conditions. The dose difference in bone tissues indicated that bone cortical and bone trabecular have significant attenuation effect on the energy deposition in bone marrow and bone surface for photon energies below 0.2 MeV. The set of dose conversion coefficients in this work can be used to determine the absorbed dose to skeletal system for external photon irradiation and to supplement the rat skeletal dosimetry.