Dose-response study on thyrotoxic patients undergoing positron emission tomography and radioiodine therapy

Abstract

With the acknowledged problems associated with assessment of functioning thyroid mass and hence radiation dose, our policy had been to give 75 MBq iodine-131 at 6-monthly intervals to patients with Graves' disease until they became euthyroid. Since positron emission tomography (PET) has been available at this hospital, the radiation dose to the thyroid has been calculated with an accuracy of ∼20%, the thyroid mass being determined from an iodine-124 PET scan. A dose-response study has been carried out on 65 patients who have received single or cumulative radiation doses of <80 Gy. The results show that patients who receive a low radiation dose (<20 Gy) at their first treatment have a high probability of remaining toxic at 12 months. In contrast, patients who receive higher radiation doses (>40 Gy) at their first treatment 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 ¹²⁴I PET tracer study prior to ¹³¹I therapy will be performed to enable a prescribed thyroid dose of 50 Gy to be delivered to patients with Graves' disease. Further ¹³¹I therapy will only be considered if patients are still toxic at 12 months.