Adjuvant thyroid remnant ablation in patients with differentiated thyroid carcinoma confined to the thyroid: a comparison of ablation success with different activities of radioiodine (I-131).

Objective: To assess efficiency of various I-131 activities on thyroid remnant ablation in thyroid cancer patients. The significance of patients' characteristics, pathologic features and levels of Tg were analyzed.

Patients And Methods: This study included 259 consecutive differentiated thyroid cancer patients, with disease confined to the thyroid, treated with I-131 after total thyroidectomy. Patients were divided into the three groups: 80 patients receiving low [1110-1850 MBq (30-50 mCi)], 121 intermediate [2775 MBq (75 mCi)] and 58 high [3700 MBq (100 mCi)] postoperative I-131 activities. Six to eight months after the application of radioiodine, measurements of TSH, Tg, anti-Tg antibodies (in hypothyroid state) together with ultrasound exam and whole-body scintigraphy were performed.

Results: The ablation was significantly more effective (after the first application) in patients receiving 100 mCi of I-131-89.7% than in patients receiving lower activities (P = 0.016). There was no significant difference in ablation rate between the 30-50 mCi (77.5%) and 75 mCi (70.2%) groups. In the group receiving 30-50 mCi, patients with solitary tumors had significantly higher ablation rate (P = 0.038). In patients receiving 75 mCi ablation rates were higher among older patients (P = 0.005), with infiltration of the single lobe (P = 0.005), and with solitary tumor (P = 0.012). The rates of successful ablation after the second application of I-131 (after 12-16 months) amounted to 96, 97 and 96% in the 30-50, 75 and 100 mCi groups, respectively. The activity of I-131 and age were independent factors for thyroid ablation failure after the first application of I-131 (model of binary logistic regression).

Conclusion: The results of remnant ablation were satisfactory with all activities applied. Although after the first application of I-131 the activity of 100 mCi is significantly more effective in thyroid ablation than the administration of 30-50 mCi and 75 mCi, the ablation rates between all the three groups are similar (almost equal) after the second application. Thus, the activity to be administered may depend on patients' characteristics and a detailed consideration of the merits and demerits of each I-131 activity.