[Dose-response relationship in radioiodine therapy of hyperthyroidism]

For the analysis of the dose-response relationship during radioiodine therapy, the effective dose was calculated based on measurements in 1171 patients in the course of treatment. Early results of the radioiodine therapy of these cases were checked up from six to nine months afterwards. The probability of incidence of a residual hyperthyroidism or of the induction of hypothyroidism respectively, in dependence on dosage, is represented by a dose-response curve. The dose-response curve for residual hyperthyreoses shows an exponential shape, whereas the curve of induced hypothyroidism is sigmoidally shaped. The results of experimental biophysics concerning dose-response curves allow the statement that there is no reactivity of the thyroid gland in cases of residual hyperthyroidism, whereas distinct compensation mechanisms are counteracting the induction of hypothyroidism. Thus, the conception of fractionated radioiodine therapy is supported by the shape of dose-response curves. The fractionation of the dose leads to accumulated inactivation effects on the hyperthyroidism, while the compensational mechanisms against hypothyroidism are strengthening by intermittent recovery and the rate of induced hypothyroidism is decreasing. The favorable effect of multiple treatment on the number of induced hypothyreoses could be verified by our own material. During the radioiodine therapy of hyperthyroidism, a defined interrelation is found between the effective dose, the thyroid mass and the effect of the dose. A formula of the isoeffective dose in dependence of the thyroid mass is derived analogically to percutaneous radiation therapy. The deviation of the effective dose from the dose previously calculated is represented as a distribution function. The mean value of this distribution function can be regarded as an expectation of the effective dose. Hence, a previous calculation of the dose is considered useful also in future. The isoeffective dose is proposed to be chosen in dependence on the thyroid mass according to a fifty-per-cent-probability of residual hyperthyroidism. Relative to the intensity of residual hyperthyroidism, a second treatment considering the same standards should be performed after four to nine months. The number of reiterated treatments needed depends on individual radiation sensitivity resulting from biological factors.