The effects of radioactive iodine in thyroid remnant ablation and treatment of well differentiated thyroid carcinoma

Although the use of radioactive iodine (I-131) in the treatment of thyroid cancer is well established, treatment dose is not well standardized. In order to deduce the appropriate dose for thyroid remnant ablation and the effect of I-131 in the treatment of distant metastases, data for 544 patients with papillary or follicular thyroid cancer were retrospectively reviewed. All patients received surgical treatment followed by post-operative I-131. If remnants were present in the 0.2 GBq I-131 diagnostic scan, 1.1-3.7 GBq I-131 were administered for ablation. For the treatment of distant metastases 3.7-5.6 GBq were used. Of 318 patients receiving I-131 for thyroid remnant ablation, 290 were successfully ablated. After one dose of 1.1 GBq I-131, 82% (159/194) of thyroid remnants were ablated. During the follow-up period, two of 14 Stage IV patients with lung or mediastinal metastases at the time of operation achieved complete clinical remission. Factors identified as influencing response to I-131 therapy included age, clinical stage, survival, recurrence, extent of surgery and the 1 month post-operative serum thyroglobulin (Tg) level. In conclusion 1.1 GBq I-131 was adequate for thyroid remnant ablation unless distant metastases were present. Radioactive I-131 has a role in the treatment of well differentiated thyroid carcinoma with pulmonary metastases but seems to be less effective for treatment of bone metastases.