Comparison of Photon and Electron Absorbed Fractions in Voxel-Based and Simplified Phantoms for Small Animals

*Internal dosimetry on non-human biota is getting more important from the viewpoint of radiation protection of the environment. The International Commission on Radiological Protection (ICRP) proposed the Reference Animals and Plants using simplified phantoms, such as ellipsoids and spheres, and assessed absorbed fractions (AFs) for the whole bodies. In this study, photon and electron AFs in whole body of voxel-based frog and mouse phantoms were evaluated and compared with AFs in simplified phantoms. The evaluations were done by Monte Carlo methods for voxel-based and simplified phantoms. The monoenergetic photon or electron source was considered to be distributed uniformly in whole body. There were very small differences (less than 2%) between whole-body AFs in voxel-based and simplified mouse phantoms, however, the differences were up to 24% for the voxel-based and the Reference Frog phantoms. Whole-body AFs in voxel-based and simplified phantoms demonstrated that not only mass but also shape of whole body affected AFs significantly. The results of this study may suggest the replacement of the Reference Animal phantoms by voxel-based animal phantoms to improve the accuracy of the whole-body AFs.