Low versus high radioiodine dose in postoperative ablation of residual thyroid tissue in patients with differentiated thyroid carcinoma: a large randomized clinical trial.

OBJECTIVES: Radioiodine ablation of thyroid tissue remains the cornerstone of treatment for patients with differentiated thyroid carcinoma after thyroidectomy. Selecting an optimal dose of radioiodine for successful ablation is a continuous challenge in these patients.

METHODS: We compared the treatment response of 341 patients with thyroidectomy randomly allocated to the high-dose group, 3700 MBq (170 patients), versus the low-dose group, 1110 MBq (171 patients), for radioiodine ablation therapy in a double-blind randomized clinical trial. The response to treatment was defined as successful or unsuccessful according to post-therapy ultrasonography of the neck, serum thyroglobulin (Tg), anti-Tg, and functioning residual tissue after 6-month and 12-month intervals. The major criteria of successful ablation were Tg<2 ng/ml, anti-Tg<100 IU/ml, and absent remnant in the off-levothyroxine state. Additional radioiodine doses were administered in cases showing no significant response to the first therapy. Finally, the initial outcome, the total hospitalization time, and the cumulative I-131 doses during the 12-month course of the study were compared between the subgroups.

RESULTS: The rate of initial successful ablation was 51.6% in all patients, 39.2% in the low-dose group, and 64.1% in the high-dose group. The corresponding success rates at the end of the 12-month follow-up without additional treatment were 55.1, 41.5, and 68.8%, respectively. The relative risk (RR) of unsuccessful ablation for the low-dose versus the high-dose group was 1.695 [95% confidence interval (CI), 1.34-2.14]. In the low-dose group, more patients needed a second dose of I-131, resulting in a higher cumulative activity (median, 4810 vs. 3700 MBq, P<0.0001) and more inpatient time (median 4 vs. 3 days) in comparison with the high-dose group. The covariate factors predicting the treatment response, in order of significance, were radioiodine dose, baseline Tg, baseline thyroid stimulating hormone (TSH) level, efficiency of TSH suppressive therapy, and sex.

CONCLUSION: The higher dose of I-131 (3700 MBq) resulted in successful ablation more often than the low dose (1110 MBq).