Dose calculations for 123I, 124I, 125I and 131I in the thyroid gland of the mouse, rat and man and comparison with thyroid function for mice and rats.

To assess the radiotoxicity of various iodine isotopes to the thyroid gland, refined dosimetric calculations of the average dose rate distribution and total absorbed dose were performed using Monte Carlo Procedures applied on a microscopic scale to the electrons emitted by the isotopes and on a macroscopic scale to the photons. The calculated dose rates depended strongly on the average follicle radius, surrounding cell layer thickness and thyroid mass and the total absorbed doses were calculated from them for the mouse, rat and man as a function of the age at the time of administration. The thyroid function of mice and rats after injection of various isotopes, as measured by several authors, was compared with the calculated average absorbed doses in the thyroid and in different parts of the cell layers. Changes in cell layer thickness due to diet were taken into account. The calculated average absorbed dose in the outermost half of the cell layer was found to be better as a prediction of thyroid damage for all species mentioned. The improvement in prediction was obvious for mice and rats, but less important for adult man.