Local distribution of energy deposition in and around the follicles of a 125I contaminated thyroid.

Iodine-125, which is used for the clinical diagnosis of the thyroid gland, has a high probability of decay by K-capture followed by the emission of low-energy electrons having ranges of up to 23 micrometer. Thus one has high local concentrations of energy deposition comparable in size to that of the smaller follicles. In this paper the local distribution of energy deposition inside and outside the follicles of the human thyroid gland is calculated for follicle sizes of 20--400 micrometer. Also the local dose rate at the position of the nuclei of the follicle cells is determined as a function of the follicle size. Possible dosimetric approaches to the problem of radiobiological effectiveness are discussed, at first in general and then for the example of the inactivation of follicle cells due to the incorporation of 125I.