Low activity versus high activity: noninferior response to radioiodine therapy in differentiated patients with extrathyroid extension.

OBJECTIVES: Radioactive iodine (RAI) therapy is recommended for differentiated thyroid cancer (DTC) patients with microscopic extrathyroid extension (ETE). Patients with a low preablative stimulated thyroglobulin (ps-Tg) level have a more favorable prognosis. It remains uncertain whether low-activity RAI could have the same efficacy in patients with low ps-Tg. This study aimed to evaluate the effectiveness of low-activity RAI therapy in low-level ps-Tg DTC with ETE.

PATIENTS AND METHODS: The inclusion criteria for this retrospective study were as follows: (a) age 18 years or older, (b) DTC after total or near-total thyroidectomy of DTC, (c) no distant metastasis after thyroidectomy and before RAI therapy, (d) American Joint Committee on Cancer pT3 stage (with microscopic ETE) with any American Joint Committee on Cancer N stage, (e) ps-Tg level of 5 ng/ml or less, and (f) after the first time of RAI therapy. The response of patients was assessed at 20-24 months after RAI therapy and considered an excellent response (ER) or non-ER.

RESULTS: A total of 132 patients were included, 69 patients in low activity (1100 MBq) and 63 in high activity (≥3700 MBq). ER was observed in 86.9% of patients in the low-activity group (60/69), which did not differ from the high-activity group (P=0.165) at 20-24 months' assessment. For patients with different N statuses (N0, N1a, N1b), a noninferior response could also be achieved under low-activity RAI therapy compared with the high-activity group (P=1.000, 0.286, 0.722, respectively).

CONCLUSION: Low-activity RAI therapy is not inferior to high-activity therapy in achieving an ER in microscopic ETE DTC with ps-Tg less than 5 ng/ml irrespective of the lymph node metastases.