Predicting the efficacy of first iodine-131 treatment in differentiated thyroid carcinoma

The purpose of this study was to search for predictors of (131)I first ablative treatment efficacy in patients with postsurgical remnants after total thyroidectomy for nonmedullary differentiated thyroid carcinoma with no known metastasis.

Thirty-seven patients were retrospectively studied. None presented antithyroglobulin antibodies. All patients received 111 MBq of (131)I for diagnostic purpose and, 9 days later, 3.7 GBq of (131)I for ablative therapy (IAT). To assess the efficacy of treatment, all patients were studied with (131)I and with thyroglobulin (Tg) plasma assays 6-15 mo later. Treatment was considered successful if no abnormal uptake was seen on whole-body scan and if the Tg plasma level was lower than 1 ng/ml.

Ablative treatment was found to be successful in 17 patients [IAT(+)] and unsuccessful in 20 [IAT(-)]. There was no significant difference between the two groups for clinical and histological data, size of thyroid remnants on a 1:1 dot scan and TSH level just before treatment. Although Tg levels were not different in the two groups before scanning dose administration (D0), Tg levels were higher in IAT(-) group 9 days later, just before radioiodine treatment administration (D9) and, in contrast, Tg levels were higher in the IAT(+) group 5 days after treatment administration (D14). Tg percentage change between D9 and D14 was significantly higher in the IAT(+) group and, with an optimal cutoff value of 750%, this parameter would have been able to predict successful treatment in 9 of 10 cases and unsuccessful treatment in 18 of 21 cases. Conversely, Tg percentage change between D0 and D9 was significantly higher in the IAT(-) group and of 11 patients with more than 100% change, 10 belonged to this group.

The increase in Tg during the first (131)I ablative treatment could be a good predictor of treatment efficacy for patients with nonmetastatic differentiated thyroid carcinoma. Conversely, the increase in Tg observed after the administration of the scanning dose of (131)I just before ablative therapy is associated with a more frequent incomplete ablation, perhaps reflecting a stunning effect on the thyroid remnants.