Clinical outcomes of low-dose and high-dose postoperative radioiodine therapy in patients with intermediate-risk differentiated thyroid cancer.

Purpose: Recent studies have suggested that a low dose (LD) of radioiodine (RAI) is sufficient to treat differentiated thyroid cancer (DTC) even in patients with intermediate risk. However, these studies evaluated the efficacy of RAI therapy, irrespective of the results of the whole-body scan (WBS). The aim of the present study was to evaluate the response to LD and high-dose (HD) RAI therapy using two different criteria (with and without WBS results) and the reclassification system according to the revised 2015 guidelines of the American Thyroid Association in Korean intermediate-risk DTC patients. In addition, we evaluated the long-term clinical outcomes of treatment with LD and HD RAI.
Materials and Methods: In total, 204 intermediate-risk DTC patients who underwent postoperative RAI therapy at two tertiary referral hospitals from 2003 to 2004 were enrolled in the present retrospective study. One hundred and twenty-four patients were treated with 3.7 and 5.55 GBq (HD) of RAI in one center and 80 patients were treated with 1.11 GBq (LD) in the other center. The success rate of RAI therapy was assessed with or without the inclusion of WBS results in the analysis. In addition, the response to therapy during the first 2 years of follow-up after the initial RAI therapy was categorized according to the reclassification system of 2015 American Thyroid Association guidelines as excellent response, indeterminate response, biochemical incomplete response, or structural incomplete response. Recurrence was defined as a newly detected cytologically or pathologically confirmed lesion.
Results: There were no significant differences between the success rates of the HD and LD groups irrespective of the inclusion of WBS results in the analysis (with WBS: 54.84 vs. 45.0%, P=0.23; without WBS: 60.48 vs. 62.5%, P=0.77). The response to HD and LD RAI therapy was excellent in 54.84 and 45.0% of the patients, respectively; indeterminate in 34.68 and 30.0% of the patients, respectively; biochemical incomplete in 4.03 and 13.75% of the patients, respectively; and structural incomplete in 6.45% and in 11.25% of the patients, respectively (P=0.04). In particular, the biochemical or structural incomplete response rate was lower in patients treated with HD than in patients treated with LD (HD, 10.48%; LD, 25.0%, P=0.01). At the last follow-up (HD, median 11 years; LD, median 10 years), patients who achieved an excellent response showed no evidence of disease. After the initial RAI therapy, eight patients in the HD group and 18 patients in the LD group who achieved either indeterminate response or biochemical incomplete response received additional RAI therapy. Seven patients (indeterminate response in five patients; biochemical incomplete response in two patients) in the HD group and seven patients (indeterminate response in five patients; biochemical incomplete response in two patients) in the LD group showed recurrences.
Conclusion: LD RAI therapy after thyroidectomy appears to be insufficient in Korean DTC patients with intermediate risk. The patients in the LD group predominantly showed biochemical or structural incomplete response to initial RAI therapy and additional RAI therapy was required.