PREDICTING THE EFFICACY OF FIRST I-131 TREATMENT IN DIFFERENTIATED THYROID-CARCINOMA

The purpose of this study was to search for predictors of I-131 first ablative treatment efficacy in patients with postsurgical remnants aft er total thyroidectomy for nonmedullary differentiated thyroid carcino ma with no known metastasis. Methods: Thirty-seven patients were retro spectively studied. None presented antithyroglobulin antibodies. All p atients received 111 MBq of I-131 for diagnostic purpose and, 9 days l ater, 3.7 GBq of I-131 for ablative therapy (IAT). To assess the effic acy of treatment, all patients were studied with I-131 and with thyrog lobulin (Tg) plasma assays 6-15 mo later. Treatment was considered suc cessful if no abnormal uptake was seen on whole-body scan and if the T g plasma level was lower than 1 ng/ml. Results: 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 (D 0), Tg levels were higher in IAT(-) group 9 days later, just before ra dioiodine treatment administration (D9) and, in contrast, Tg levels we re higher in the IAT(+) group 5 days after treatment administration (D 14). Tg percentage change between D9 and D14 was significantly higher in the IAT(+) group and, with an optimal cutoff value of 750%, this pa rameter would have been able to predict successful treatment in 9 of 1 0 cases and unsuccessful treatment in 18 of 21 cases, Conversely, Tg p ercentage change between DO and D9 was significantly higher in the IAT (-) group and of 11 patients with more than 100% change, 10 belonged t o this group. Conclusion: The increase in Tg during the first I-131 ab lative treatment could be a good predictor of treatment efficacy for p atients with nonmetastatic differentiated thyroid carcinoma. Conversel y, the increase in Tg observed after the administration of the scannin g dose of I-131 just before ablative therapy is associated with a more frequent incomplete ablation, perhaps reflecting a stunning effect on the thyroid remnants.