The radiobiological response of the thyroid. Part II: Response of sheep thyroid cell in vitro to single doses of X rays.

The response of sheep thyroid cells in culture to single doses of X or gamma rays is described. In the absence of cellular proliferation the cells are unusually radioresistant, showing little sign of interphase death at doses up to 9 krad. The follicular morphology characteristic of thyroid cells in vivo is also very radioresistant. Iodide trapping is reduced to 50% of the control value by doses of the order of 2 krad. When proliferation is induced the cells may be assayed for postirradiation survival using a clonogenic endpoint. The survival curves are sigmoid with a Do of 410 rad and a very low extrapolation number.