Radioiodine Dose for Remnant Ablation in Differentiated Thyroid Carcinoma: A Randomized Clinical Trial in 509 Patients

Remnant ablation can be achieved by either administering an empiric fixed dose or using dosimetry-guided techniques. Because of the technical and logistic difficulties, most centers have adapted the fixed-dose or standard-dose technique for remnant ablation using 131I. In the late 1970s, low-dose 131I remnant ablation was introduced, and subsequently many centers confirmed the effectiveness of such therapy. However, the optimal dose (administered activity) of 131I for remnant ablation is not yet settled. In a randomized clinical trial to find out the smallest possible effective dose for remnant ablation in cases of differentiated thyroid carcinoma, between July 1995 and January 2002, 565 patients were randomized into eight groups according to 131I administered activity, starting at 15 mCi and increasing activity in increments of 5 mCi until 50 mCi. In the postrandomization phase, 56 patients were excluded from the study for various reasons, and final analysis was done with 509 patients. The mean age of the patients was 37.5 ± 12.7 yr with a female to male ratio of 2.6. The surgical procedure was total/near-total thyroidectomy in 72% and subtotal or hemithyroidectomy in the rest. Histology was papillary thyroid carcinoma in 80.6% of patients and follicular thyroid carcinoma in the rest. With one dose of 131I, remnant ablation was achieved in 59.6, 63.6, 81.4, 83.6, 79.4, 78.3, 84.4, and 81.8% of patients in the 15- to 50-mCi groups, respectively (overall ablation rate, 77.6%). The successful ablation rate was statistically different in patients receiving less than 25 mCi of 131I compared with those receiving at least 25 mCi [63 of 102 (61.8%) vs. 332 of 407 (81.6%); P = 0.006]. However, there was no significant intergroup difference in outcome among patients receiving 25–50 mCi of 131I. Patients with small tumor size (≤5 cm), adequate surgery (total/near-total thyroidectomy), and radioiodine neck uptake of less than or equal to 10% had odds ratios of 2.4 [confidence interval (CI), 1.3–3.98], 2.6 (CI, 1.6–4.2), and 2.2 (CI, 1.4–3.5), respectively, for successful remnant ablation. Patients receiving at least 25 mCi of 131I had a three times better chance of getting remnant ablation than patients receiving lesser activity of 131I. Any activity of 131I between 25 and 50 mCi appears to be adequate for remnant ablation.