Development of a nonhuman primate computational phantom for radiation dosimetry

Purpose The nonhuman primate (NHP) is an important animal model for evaluating the response of the human body to radiation exposure owing to similarities between its organ structure, genome, life span, and metabolism. However, there is a lack of radiation dosimetry estimations for NHPs. The aim of this work is to construct a computational phantom of NHPs and estimate absorbed fractions and specific absorbed fractions for internal radiation dosimetry. Materials and methods A female rhesus monkey was frozen and sectioned using a cryomacrotome. The transaxial sectioned images were imported into Adobe Photoshop for segmentation of internal organs. A total of 31 organs/tissues were identified and labeled. The segmented voxel phantom was then converted to a boundary representation (BREP) phantom to enable easy alteration of the phantom to mimic monkeys of different stature. The BREP model was then voxelized and imported into the MCNPX Monte Carlo radiation transport code for electron and photon dosimetry calculations. To estimate the appropriateness of using human phantoms as surrogate models for NHPs, absorbed fractions (AFs) and specific absorbed fractions (SAFs) of monoenergetic electrons and photons were calculated and compared with the ICRP reference newborn female phantom and a 1-yr-old female phantom. Results Considerable differences were observed for both self-absorbed and cross-absorbed doses for some organs between the NHP phantom and newborn phantom. For example, the ratios of the self-absorbed SAF((stomach wall) Monkey) to SAF((stomach wall) Newborn) ranged from 0.06 at 10 keV to 0.29 at 3 MeV for photons while the corresponding ratios to cross-absorbed SAF((liver -> kidney) Monkey) to SAF((liver -> kidney) Newborn) ranged from 0.78 at 50 keV to 5.78 at 10 keV for photons. Conversely, values of self-absorbed SAF were much higher (ratios of 2.39-4.19) for the brain and much lower for the uterus (0.51-0.61) in the monkey model compared to the newborn phantom. These dose differences can be attributed to the disparities between organ masses, shapes, and positions between the two phantoms. Conclusion The developed NHP model can be exploited for the assessment of radiation dose to NHPs in preclinical radiation dosimetry studies and radiation therapy research