Prospective randomized clinical trial to evaluate the optimal dose of 131 I for remnant ablation in patients with differentiated thyroid carcinoma.

BACKGROUND: Radioiodine has been used for more than a half-century to ablate thyroid remnants following thyroid surgery, but a single optimal dose has not been established. We designed a prospective randomized trial to determine the optimal dose of 131 I for remnant ablation.

METHODS: Using a simple randomization technique, 149 patients with remnant thyroid were incorporated into 4 treatment groups. Twenty-seven of these patients were administered 25 to 34 millicurie (mCi) of 131 I (30 +/- 1.5), 54 received 35 to 64 mCi (50.6 +/- 5.4), 38 received 65 to 119 mCi (88.6 +/- 14) and 30 patients received 120 to 200 mCi (155 +/- 28.7). Six months to 1 year after treatment, all subjects were reassessed after withdrawing L-thyroxine for 4 to 6 weeks. A successful ablation was defined as the absence of thyroid bed activity in 5 mCi 131 I neck scan at 48 hours along with 2 adjunctive criteria which were the neck uptake of <0.2% of the administered activity and the thyroglobulin (Tg) value of <10 ng/mL.

RESULTS: Applying the above criteria, we observed complete ablation of 17 of 27 thyroid gland remnants (63%) in the 30 mCi group, 42 of 54 (77.8%) in the 50 mCi group, 28 of 38 (73.7%) in the 90 mCi group and 23 of 30 (76.7%) in the 155 mCi group. When the radiation-absorbed dose was calculated, a 30 mCi dose delivered approximately 20,000 centigray (cGy), a 50 mCi dose about 30,000 cGy, a 90 mCi dose about 50,000 cGy, and a 155 mCi dose about 130,000 cGy.

CONCLUSIONS: Increasing the empirical 131 I initial dose to more than 50 mCi results in plateauing of the dose-response curve and thus, conventional high dose remnant ablation needs critical evaluation. Based on dosimetry results, one should aim to deliver about 30,000 cGy to the thyroid remnant, as higher doses do not appear to yield a higher ablation rate.