Radiation Dosimetry Research For Rat Based On The Monte Carlo Method

Rat have been widely used in radiopharmaceutics researches, medical imaging and radiation dosimetry researches, making it important to develop fine structured computational models for rat dosimetry investigations.In this work, based on the same data set of rat, we developed three existing types of computational models:the stylized phantom, the voxel-based phantom and the NURBS-based phantom. Using Monte Carlo method, we simulated photon, electron and proton particles transport in the rat phantom under internal and external irradiation and calculated the absorbed dose for organs. The differences of three kinds of computational phantoms in internal dosimetry, the effect of chemical compositions in organ absorbed dose, the proton dosimetric data for rat, the rat skeletal dosimetry, and the dosimetry for radionuclides in rat liver lobes were investigated in this study.The detailed topics and results are as follows:(1) By comparing the differences of three kinds of computational rat phantoms in internal dosimetry, we found that the NURBS-based phantom presents a precise dose and inborn deformable model in self-absorbed dose calculations, while the stylized phantom might cause an underestimation of the self-absorbed S values for most organs.(2) To evaluate the effect of element composition on rat organ dosimetry, simulations are performed for three different arrangements, changing the chemical composition in the voxel filling liver material, as rat tissue, human tissue and soft tissue analog, respectively. A maximum of 3.5% difference is given for the liver self-absorbed fraction between the rat tissue and human tissue when the photon energy is below 100 keV. Meanwhile, the tissue composition differences were found to be having negligible effects on the electron absorbed dose. The results suggest that, considering the individual differences and inconveniences for accurate evaluation of element composition in specimens, it is acceptable to adopt the tissue compositions recommended for humans in the rat liver dosimetry.(3) We compared the organ dose for rat phantom against external proton exposure under five idealized irradiation conditions. The results seem to suggest that, when the proton energy is low, the anatomic parameters of organs (eg. mass, volume and anatomic position etc.) and the distance between the internal organ and the contour surface present considerably influence on the organ dose of rat. When the incident proton energy is higher than 100 MeV, the irradiation conditions seem to play a relatively minor role in the absorbed dose per fluence for all organs. The calculated results for skeletal system show that the mineral bone and RBM contributes the majority of the skeleton absorbed dose and the RBM dose presents uneven distribution in the whole-body skeletal system.(4) In the rat skeletal dosimetry researches, we established the absorbed fractions (AFs) and specific absorbed fractions (SAFs) for mineral bone, red bone marrow (RBM), and yellow bone marrow (YBM) at 40 different bone sites for monoenergetic photon and electron sources placed in 18 organs and seven bone sites, investigated the effect of tissue density, anatomic position and geometry in skeletal absorbed dose. Thus it is suggested that, in low-energy radiation therapy, since the mineral bone contributes mostly to the cross-organ absorbed dose of the whole body skeletal system and this organ also carries the risk of bone cancer induction, it should serve as an important dose-limiting organ. The photon SAF curves of RBM show that, for photon energies greater than 0.6 MeV, there is an increase in secondary photons emitted from the mineral bone as photon energy increases. The results also suggest that the absorbed dose is non-uniformly distributed throughout the whole-body skeleton system in the intravascular radiation therapy and the RBM in the bone sites which are close to the source and have high cellularity will receive more absorbed dose.(5) In the dosimetry for rat liver lobes, we calculated the photon and electron absorbed fractions and S values for 90Y, 131I,166Ho and 188Re for seven liver lobes. The absorbed dose for different liver lobes present obviously diverged with respect to the lobe masses and the distance between target region and source organ. The results obtained in this work also suggest that, in the rat liver dosimetry experiments, using the combined liver S values to determine the absorbed dose of different liver regions might result in an underestimation of the dose for the source lobe and overestimates of the radiation dose to the other tissue regions.(6) We evaluated the voxel S values in liver and tumor from radionuclides which are used in the therapy researches for the treatment of hepatocellular carcinoma (HCC). The obtained radioactivity distributions of radionuclides suggest that the Ho and Re produce a uniformly distributed high dose in the tumor region and relatively low absorbed dose for surrounding tissues. We consider that the radiopharmaceutical and microspheres loaded with 166Ho and 188Re might be more effective radiopharmaceuticals and will have enormous clinical potential in the radionuclide therapy of HCC.The stylized rat phantom, voxel-based rat phantom and NURBS-based rat phantom developed in this work are all based on the same dataset of rat specimen. The computational rat phantom contains the most identified organs and tissue regions in the published small animal models. The calculated dosimetry data for rat skeleton system, rat liver and external proton exposures may contribute to the systematic multi-particle dosimetric dataset for animals, provide evaluated absorbed organ dose of radiopharmaceuticals in the rat preclinical targeted radiotherapy experiments, help researchers to evaluate the relationship between absorbed dose and biological response, benefit the selection of effective and safety radionuclide, improve the development of new radiopharmaceuticals and advantage the design and optimization of radiation protection for radiation therapy. This study quantified the effect of element compositions in rat dosimetry and the relationship between organ absorbed dose and the types of phantoms, and will contribute to both the advancement and application of computational phantoms in different dosimetry studies.