High or low dose radioiodine ablation of thyroid remnants?

Abstract

The need for high dose radioiodine for ablation of remnants in patients with thyroid cancer is still in question. We compared the effectiveness of high and low dose¹³¹I for ablation in patients in a prospective randomized study after surgical thyroidectomy. Twenty patients with differentiatedpT2-3NoMo thyroid cancer were studied. The uptake was 5%–10% at 24 h. Ten patients received 100 mCi, the others 30 mCi¹³¹I. Three months later all patients received a therapeutic dose of 150 mCi¹³¹I. Another twenty patients with known distant metastases (pulmonary and/or bone) of differentiated thyroid cancer were studied. The remnant uptake was between 4%–10%. Ten patients received 300 mCi and ten 30 mCi¹³¹I as ablation dose. Three months later all received 300 mCi¹³¹I. The uptake at day seven was calculated for the same metastases from a whole body scan after both treatments. If effective ablation was defined as 24h uptake in the remnant of less than 1%, then the ablation was effective in eight out of ten of the high dose and in seven out of ten of the low dose group. In pT2-3, NxM1 patients the ablation was effective in seven out of ten cases in both groups. If “effective” ablation was defined as an uptake of less than 0.5%, then the ablation was effective both in NoMo and in NxM1 patients in five out of ten with low dose and in six out of ten with high dose ablation treatment. Seventh day uptake in 20 metastases after the second dose was at mean 3.6 times (range 2.8–4.3) higher than after the first, while there was no significant difference in TSH levels. In two patients,¹³¹I kinetics over a bone metastasis were recorded continuously during 300 mCi ablation treatment and 300 mCi therapeutic treatment without any significant difference. We therefore recommend a 30 mCi ablation dose for all patients with differentiated thyroid cancer after surgical thyroidectomy, followed by a 300 mCi treatment dose in pT2-3N1M x or pT2-3N x M1 patients, while in pT2-3NoMo low dose ablation will be a sufficient treatment.