S values for a monkey computational model in diagnostic nuclear medicine procedures

Abstract

In the field of nuclear medicine, monkeys have been instrumental in advancing our understanding of how radioactive materials behave within a living organism. However, dosimetric data, including S values, for various source and target regions in monkey's anatomy and radionuclides remain limited. The current study is aimed to develop a library of S values by utilizing a computational monkey model in conjunction with Monte Carlo radiation transport methods. We developed a S value library through the following process. First, we computed specific absorbed fractions (SAFs) for 35 source organs and 35 target organs using the general-purpose Monte Carlo radiation transport code, MCNP6. Monte Carlo simulations were performed for both photons and electrons, covering 27 monoenergetic bins ranging from 0.001 to 10 MeV. Next, we derived the S values from the SAFs for photons and electrons by incorporating energy spectrum data for 299 radionuclides most commonly used in diagnostic nuclear medicine, as provided by ICRP Publication 107. The differences in self-irradiation S values for the large organs likely stem from the type of particles: positron/beta vs. gamma. Cross-irradiation S values mirror the trends seen in SAFs, particularly in relation to the inter-organ distances between source and target regions. The comparison of S values between the monkey models and ICRP reference human models suggests that the monkey model-based S values are unique and may not be surrogated by human model-based S values. This S value library is expected to be a valuable resource for estimating organ doses in monkeys when paired with radionuclide distribution data across their anatomy.