Using an interactive scaling program developed by Segars et al. (12 ), any NURBS model may be scaled to different sizes and shapes. One or more selected organs may be translated or rotated in any direction; scaled linearly in any direction, uniformly in 3 dimensions, or from the center by a fixed factor; or otherwise modified by the user. Instead of spending months or years creating, performing, and perfecting tedious, slice-by-slice segmentations of individual organs from diagnostic imaging data of various animals, we found this method to be much quicker, resulting in a model series that was more internally consistent. We used this program to develop a series of models representing small, medium, and large animals typically used in preclinical research in nuclear medicine—mice weighing about 25, 30, and 35 g and rats weighing approximately 200, 300, 400, 500, and 600 g. The resulting organ and body masses were designed to follow data found in reference literature. During radiation transport, traditionally hollow organs (e.g., stomach, intestines, heart, and bladder) were treated as a uniform organ, with mass equal to that of the wall plus contents, as defined in the NURBS models. This treatment was thought reasonable, because of uncertainties in the exact location of these small structures. The skeleton similarly was treated as a uniform mixture of bone, cartilage, and marrow; development of a detailed bone model with microstructure representing the individual components was beyond the scope of this project and was thought to include uncertainties similar to or greater than those for hollow organs.
Separate models were made for each size rodent. The modified models were saved, converted to a voxelized format, and used in the geometry and tracking particle transport toolkit (GEANT, version 4) (15 ) to perform radiation transport calculations in the voxel-based representations of the various individual models. Cubic voxels of 0.625 mm were used; models started at 512 × 512 × 512 voxels but were trimmed to sizes that removed empty space around each model, to speed up the Monte Carlo simulations. For most organs, the difference between the MOBY and the ROBY reported and voxel model volumes was about 3%–5%. For small organs, however, the difference was sometimes greater. In the absence of well-established information about these species, the tissue compositions and densities recommended for humans (16 ) were used for the corresponding tissues of the animals. Minor changes were suggested in the recently released revision by the International Commission on Radiological Protection (ICRP) (17 ). However, these changes were not deemed large enough to affect calculations from our established Monte Carlo routines, given all other uncertainties in the data and methods, which may be as much as a factor of 2 or more (18 (link)) whereas variations in tissue densities are of the order of a few percentage points. Discrete starting photon and electron energies of 0.01, 0.015, 0.02, 0.03, 0.05, 0.1, 0.2, 0.5, 1, 1.5, 2, and 4 MeV were simulated in available source regions. Typically 600,000 particle histories were followed in the Monte Carlo simulations, which were implemented on the Vanderbilt multinode computing environment (Advanced Computing Center for Research and Education). SAFs were generated for source and target regions in the models, and then organ DFs were generated, using decay data from the RAdiation Dose Assessment Resource (RADAR) (19 (link)). In most cases, uncertainties in the SAFs were under 2%; in a few cases, the variability of the data was high (some small organs or organ pairs that were significantly separated), and reciprocity rules (14 (link)) and smoothing of noisy data were performed in some cases.
Separate models were made for each size rodent. The modified models were saved, converted to a voxelized format, and used in the geometry and tracking particle transport toolkit (GEANT, version 4) (15 ) to perform radiation transport calculations in the voxel-based representations of the various individual models. Cubic voxels of 0.625 mm were used; models started at 512 × 512 × 512 voxels but were trimmed to sizes that removed empty space around each model, to speed up the Monte Carlo simulations. For most organs, the difference between the MOBY and the ROBY reported and voxel model volumes was about 3%–5%. For small organs, however, the difference was sometimes greater. In the absence of well-established information about these species, the tissue compositions and densities recommended for humans (16 ) were used for the corresponding tissues of the animals. Minor changes were suggested in the recently released revision by the International Commission on Radiological Protection (ICRP) (17 ). However, these changes were not deemed large enough to affect calculations from our established Monte Carlo routines, given all other uncertainties in the data and methods, which may be as much as a factor of 2 or more (18 (link)) whereas variations in tissue densities are of the order of a few percentage points. Discrete starting photon and electron energies of 0.01, 0.015, 0.02, 0.03, 0.05, 0.1, 0.2, 0.5, 1, 1.5, 2, and 4 MeV were simulated in available source regions. Typically 600,000 particle histories were followed in the Monte Carlo simulations, which were implemented on the Vanderbilt multinode computing environment (Advanced Computing Center for Research and Education). SAFs were generated for source and target regions in the models, and then organ DFs were generated, using decay data from the RAdiation Dose Assessment Resource (RADAR) (19 (link)). In most cases, uncertainties in the SAFs were under 2%; in a few cases, the variability of the data was high (some small organs or organ pairs that were significantly separated), and reciprocity rules (14 (link)) and smoothing of noisy data were performed in some cases.
