An Accurate Method of 131 I Dosimetry in the Rat Thyroid

An accurate method of thyroid /sup 131/I dosimetry was developed by imploying the dose formulation recommended by the Medical Internal Radiation Dose (MIRD) Committee. Six-week-old female Long-Evans rats were injected intraperitonealy with 0.5, 1.9, and 5.4 ..mu..Ci of Na/sup 131/I. The accumulated /sup 131/I activities in the thyroid were precisely determined by integrating the /sup 131/I activities per gram of the thyroid as functions of postinjection time. When the mean thyroid doses derived from this method are compared to those derived from the conventional method, the conventional method over-estimated the doses by 60 to 70%. Similarly, the conventional method yielded effective half-lives of 2.5 to 2.8 days; these estimates were found to be high by factors of 1.4 to 2.0. This finding implies that the biological elimination of iodide from the rat thyroid is much more rapid (up to 2.5 times) that once believed. Results from this study showed that the basic assumption in the conventional method of thyroid /sup 131/I dosimetry in the rat, i.e., that the thyroid iodide retention function is a single exponential, is invalid. Results from this study also demonstrated that variations in animal body weight of 6 to 7-week-old animals and diurnal variation have nomore » significant influence on the mean thyroid doses for a given injected activity of /sup 131/I. However, as expected, variation in iodide content of the animal diets significantly altered the thyroid doses for a given /sup 131/I injected activity.« less