Voxel-based mouse and rat models for internal dose calculations

The ability to estimate absorbed doses in experimental animals to which radiolabeled material has been administered may be important in explaining and controlling potential radiation toxicity observed during preclinical trials. Most previously reported models for establishing doses to small animals have been styl-ized and mathematically based. This study establishes dose fac-tors for internal sources in realistic models of a typical mouse and a typical rat, based on image data obtained using a dedicated small-animal CT scanner. Methods: A transgenic mouse (body mass, 27 g) and a Sprague–Dawley rat (body mass, 248 g) were imaged using the dedicated small-animal CT scanner. Identified organs were segmented using computer tools that Vanderbilt University applies to process human images for 3-dimensional dosimetry. Monte Carlo N-particle transport code (MCNP) input files were prepared from the 3-dimensional, voxel-based image data. Using methods established for human studies, radiation transport calculations of absorbed fractions (AFs) were performed using MCNP, version 4C, on the seg-mented images, and dose conversion factors for several radio-nuclides were developed. Results: AFs were established at discrete energies for electron and photon sources assumed to be uniformly distributed throughout approximately 10 source and target regions in both models. Electron self-irradiation AFs were significantly less than 1.0 for many organs, at energies above 0.5 MeV, and significant cross irradiation was observed for high-energy electrons, such as those from 90Y or 188Re, in many organs. Calculated dose conversion factors reflected these trends and agreed well with the results of other authors who have undertaken similar investigations. Conclusion: The AFs calculated in this study will be useful in determining the dose to organs for mice and rats similar in size to those studied here. The segmented, voxel-based models developed here can be used for external dose calculations as well. Key Words: radiobiology/dosimetry; radionuclide therapy; radi-ation safety; animal models; radiation effect