Development and tests of a mouse voxel model dor MCNPX based on Digimouse images

Mice have been widely used in experimental protocols involving ionizing radiation. Biological effects (Be) induced by radiation can compromise studies results. Good estimates of mouse whole body and organs absorbed dose could provide valuable information to researchers. The aim of this study was to create and test a new voxel phantom for mice dosimetry from -Digimouse- project images. Micro CT images from Digimouse project were used in this work. Corel PHOTOPAINT software was utilized in segmentation process. The three-dimensional (3-D) model assembly and its voxel size manipulation were performed by Image J. SISCODES was used to adapt the model to run in MCNPX Monte Carlo code. The resulting model was called DM_BRA. The volume and mass of segmented organs were compared with data available in literature. For the preliminary tests the heart was considered the source organ. Photons of diverse energies were simulated and Saf values obtained through F6:p and + F6 MCNPX tallies. The results were compared with reference data. 3-D picturing of absorbed doses patterns and relative errors distribution were generated by a C++ -in house- made program and visualized through Amide software. The organ masses of DM_BRA correlated well with two models that were based on same set of images. However some organs, like eyes and adrenals, skeleton and brain showed large discrepancies. Segmentation of an identical image set by different persons and/or methods can result significant organ masses variations. We believe that the main causes of these differences were: i) operator dependent subjectivity in the definition of organ limits during the segmentation processes; and i i) distinct voxel dimensions between evaluated models. Lack of reference data for mice models construction and dosimetry was detected. Comparison with other models originated from different mice strains also demonstrated that the anatomical and size variability can be significant. Use of + F6 tally for mouse phantoms appears to be more reliable. The Saf data in most simulated cases were statistically equal to those present in Mohammadi and Kinase [2011]. F6:p utilization proved to be unsuitable for photons with higher energies. 3-D visualization of absorbed doses brought improvements in the dosimetric analysis, complementing the information of the organ average dose. Similarly, the depicting of relative errors per voxel can be useful in evaluation of the variance reduction techniques effectiveness. DM_BRA was considered suitable for preliminary dosimetric evaluation. Improvements in model segmentation and voxel size will allow more complexes and accurate studies. (Author)