Comparative survivals of rat thyroid cellsin vivoafter131I,125I and X irradiations

Rat thyroids increase two or three-fold in weight when the animals are exposed to a 28-day goitrogenic stimulus, and the increase is largely due to cell multiplication. Radiation impairs goitrogenic cell proliferation. Thyroid cell survival curves have been derived after irradiation with X rays, 131I and 125I. The D0 values were 450 rads, 5,500 rads and 9,400 rads, respectively, where dose refers to the mean dose throughout the gland cells. The extrapolation number for X rays was 1·7, but the curves from 131I and 125I were exponential from the origin. The high D0 for X rays is explained on the basis of the limited proliferative properties of rat thyroid cells so that D0 relates to the first or second divisions only. The large difference between values of D0 for 131I and 125I is explained not by dose-rate nor by RBE but chiefly by the inhomogeneity of the distribution of absorbed dose from the 125I across the follicular cells. For the same cell survival, the nuclei of the follicular cells receive about the same dose from 131I and 125I, but radiation doses from 125I to the cell nuclei are only about half the average doses to the gland and about half the doses to the inner parts of the follicular cells. The dosimetry of 125I is presented in detail. The studies show that cell reproductive integrity is determined largely by the mean dose to cell nuclei irrespective of the dose or dose distribution within the cytoplasm of the thyroid cells.