A randomized equivalence trial to determine the optimum dose of iodine-131 for remnant ablation in differentiated thyroid cancer.

OBJECTIVE: We conducted a stratified randomized equivalence/noninferiority trial from January 2001 to December 2006 to determine whether lower administered activities are as effective as 3.7 GBq (100 mCi) of iodine-131 (I) for remnant ablation.

PATIENTS AND METHODS: The sample size was found to be 450 on the basis of 80% power (α=5%) and noninferiority margin (δ=0.15). We used an allocation ratio of 2 : 2 : 1 for the 0.93, 1.85, and 3.7 GBq (25, 50, and 100 mCi) groups, respectively. Randomization with concealment was followed for patient group allocation. All patients underwent preablation I whole-body scan, 48-h radioiodine neck uptake measurements and post-therapeutic scans. The patients were advised suppressive L-thyroxine therapy (2 µg/kg/day). Repeat evaluation was performed after 6 months, along with thyroglobulin and antibody assays. The criteria for ablation were as follows: major criterion - negative I whole-body scan; minor criteria - 48-h radioiodine neck uptake less than or equal to 0.2% and stimulated thyroglobulin less than or equal to 10 ng/ml.

RESULTS: A total of 422 patients who fulfilled the inclusion criteria (360 papillary and 62 follicular) could be recruited. As per AJCC, 6th ed., we had 70, 11, and 19% of patients in stage I, II, and III, respectively. First-dose ablation was 81.5, 84.9, 88.5, and 84.2% in the 0.93, 1.85, and 3.7 GBq groups and overall, respectively. Histology had no effect on ablation rate. The equivalence testing of the hypothesis was conducted between the 0.93 and 3.7 GBq groups, the 1.85 and 3.7 GBq groups, and the 0.93 and 1.85 GBq groups. Results showed that, at a significance level of 5%, the null hypothesis was rejected for each pair.

CONCLUSION: First-dose I ablation rates at 6 months with 0.93, 1.85, and 3.7 GBq of I are equivalent with the prespecified clinically acceptable noninferiority margin. We conclude that we are probably administering too much I for remnant ablation (trial registration number: CTRI/002291).