RADIATION-DOSE ASSESSMENT IN RADIOIODINE THERAPY - DOSE-RESPONSE RELATIONSHIPS IN DIFFERENTIATED THYROID-CARCINOMA USING QUANTITATIVE SCANNING AND PET

Dose-response charts have been constructed to determine the tumouricid al dose for differentiated thyroid carcinoma metastases and thus enabl e precise activities of radioiodine to be prescribed in order to maxim ise tumour kill and minimise morbidity. Tumour and normal residual thy roid absorbed doses from radioiodine-131 have been determined with inc reased precision using a dual-headed whole-body rectilinear scanner wi th special high-resolution low-sensitivity collimators. Improved accur acy in the estimation of functioning tumour mass has been achieved usi ng positron emission tomography (PET) with a low-cost large area PET c amera. Dose-response data have been obtained for 33 patients. Followin g near-total thyroidectomy and 3.0 GBq I-131, a mean absorbed dose of 410 Gy achieved complete ablation of thyroid remnants in 75% of patien ts. Patients who had persistent uptake in the thyroid region on subseq uent radioiodine scanning had received a mean dose of only 83 Gy. Cumu lative absorbed doses in excess of 100 Gy were found to eradicate cerv ical node metastases. Patients with bone metastases, who generally hav e a poor prognosis, were found to have received doses of the order of only 20 Gy to the tumour deposits. The dose-response data explain the spectrum of clinical responses to fixed activities of radioiodine. In future, they will enable precise prescription of radioiodine to achiev e tumouricidal doses whilst avoiding the morbidity and expense of inef fective therapy.