The influence of thyroid geometry on the response of LiF and CaSO4 thermoluminescent discs to 131I and 125I irradiation.

Recent work has demonstrated that LiF and CaSO4 thermoluminescent discs may be used to monitor the effective half-life of radioiodine in the thyroid in man. To provide a firm basis for this work, the radiation field presented by model thyroids to thermoluminescent discs on the surface of a neck phantom has been examined in detail. The influence of thyroid mass, depth and lobe separation, factors which in practice are highly variable, has also been determined. The results demonstrate that the method is sufficiently sensitive to measure less than 0.5 muCi of 131I or 125I in the human thyroid, and that effective half-life determinations should not be influenced by minor positioning errors which may be encountered when placing thermoluminescent discs on the neck surface.