Influence of Thyroid Mass and Shape on the Internal Dosimetry of 131 I.

Radionuclide 131I is of vital importance in radiation protection and nuclear medicine. Its internal dose is mainly concentrated in the thyroid. The S value is a key parameter in the calculation of internal dose. It represents the radiation-weighted energy deposition per unit mass of the target organ when the radionuclide undergoes a single decay of the source organ. As the thyroid is both the source organ and the target organ, its mass and shape may affect the S value. Therefore, this paper studies the influence of thyroid mass and shape on the S value. In this study, based on the thyroid model of the ICRP adult mesh-type reference computational phantoms (MRCPs), a series of thyroids with different masses were generated by proportionally scaling the thyroid volume while maintaining its original shape. The influence of mass on the S value (thyroid←thyroid) is then systematically analyzed using the Geant4 Monte Carlo simulation platform. To further explore the influence of shape, this paper reviews the S values (thyroid←thyroid) simulated by other researchers using thyroid models of different shapes and compares them with the research results of this paper. The research results show that the S value has a good inversely proportional relationship with thyroid mass. It can be precisely fitted as a linear function of the reciprocal of mass, with a correlation coefficient (R2) of 0.9999. The result of this formula has a deviation of within 5.67% compared with the results in the literature. This study demonstrates that the difference in thyroid shape has a relatively weak influence on the S value, and the empirical formula fitted in this paper may have good universality and reliability.