A simplified recycling model for the dosimetry of radioiodide

Following the administration of radioiodide, a process of recycling takes place in which part of the radioiodide taken up and organified by the thyroid is subsequently released to re-enter the cycle. The convolution method has been applied to a simplified three-compartment model to predict individual cycles and overall cycling effects in the compartments and to illustrate the influence of various parameters on the effects of recycling. For example, a turnover half-time of 80 d in the thyroid is increased, as a result of recycling, to about 110 d if the thyroid uptake fraction is 0.3, and to about 153 d if the uptake increases to 0.55. An equation is derived for calculating a recycling factor based on values of the model parameters. This factor, which is common to the three compartments of the model, permits calculation of cumulated activities in the compartments allowing for the effects of recycled radioiodide. It is shown how differences in values of thyroid uptake fraction and discharge rate and of the physical half-lives of iodine radioisotopes affect the value of the recycling factor. Dose calculations for 125I, administered as radioiodide, are used to illustrate the above techniques and to examine the effects of recycling on dosimetry. Although recycling can lead to large increases in the doses to some body organs, the thyroid dose greatly exceeds the dose to any other organ and alone accounts for nearly all the effective dose equivalent.