Study of rat organ dose conversion coefficients for external photon irradiation based on voxel model and Monte Carlo simulation

A newly voxel rat model was developed from Micro-CT images of a rat to study rat organ dose for external photon irradiation. The voxel model with voxel size of 0.16 x 0.16 x 2 mm(3) weighed about 323 g and contained most of the key organs or tissues. Monte Carlo N-particle code (MCNP) version 4C was utilized to calculate organ dose conversion coefficients for 21 external monoenergetic parallel photon beams between 10 keV and 10 MeV under four different irradiation geometries conditions (left lateral, right lateral, dorsal-ventral, ventral-dorsal). The calculated organ dose conversion coefficients were presented in tables and compared with published data based on a mouse model to investigate the effect of size and weight difference on organ dose. The calculated and comparison results show that the organ dose conversion for both mouse model and rat model exhibited similar energy dependence under the four ideal irradiation geometries, and the organ dose is sensitive to size and weight difference especially at photon energy below 0.1 MeV and above 2 MeV.