DOSE-RESPONSE STUDY ON THYROTOXIC PATIENTS UNDERGOING POSITRON EMISSION TOMOGRAPHY AND RADIOIODINE THERAPY

With the acknowledged problems associated with assessment of functioni ng thyroid mass and hence radiation dose, our policy had been to give 75 MBq iodine-131 at 6-monthly intervals to patients with Graves' dise ase until they became euthyroid. Since positron emission tomography (P ET) has been available at this hospital, the radiation dose to the thy roid has been calculated with an accuracy of approximately 20%, the th yroid mass being determined from an iodine-124 PET scan. A dose-respon se study has been carried out on 65 patients who have received single or cumulative radiation doses of <80 Gy. The results show that patient s who receive a low radiation dose (<20 Gy) at their first treatment h ave a high probability of remaining toxic at 12 months. In contrast, p atients who receive higher radiation doses (>40 Gy) at their first tre atment have a high probability of control. The probability of becoming euthyroid increases more rapidly with increasing radiation dose than the probability of becoming hypothyroid. Following this dose-response study, a new treatment protocol has been introduced. A I-124 PET trace r study prior to I-131 therapy will be performed to enable a prescribe d thyroid dose of 50 Gy to be delivered to patients with Graves' disea se. Further I-131 therapy will only be considered if patients are stil l toxic at 12 months.