The Impact of Thyroid Stunning on Radioactive Iodine Ablation Compared to Other Risk Factors

Document Type : Original Paper, Endocrine

Author

Nuclear medicine units, NEMROCK , Cairo University, Egypt.

Abstract

Purpose: To evaluate the impact of postoperative diagnostic I131 whole body scan (WBS) on thyroid ablation compared to other risk factors. Patient and Methods: A prospective study of 336 patients with differentiated thyroid carcinoma was included after near-total thyroidectomy. They were classified into 2 major groups: Group 1 (183 patients) received ablative dose after performing diagnostic I131 WBS and group 2 (153 patients) received ablative dose without prior I131 WBS. In patients who received diagnostic I131 WBS, ablative dose was planned and given within 1 -6 weeks after performing the diagnostic study. Follow up WBS and stimulated serum Tg for all patients were done 6 months after initial ablative dose, image interpretation was made qualitatively and our criteria for successful remnant ablation defined as absence of any significant RAI uptake at the thyroid bed and stimulated serum Tg level less than 2 ng/mL. Accordingly two categories were identified either complete or incomplete ablation. Results: Successful complete ablation was reported in 237 patients representing 70.5% of the whole patient population, while incomplete ablation was found in the remaining 99 patients (29.5%). A statistically significant difference in ablation outcome was found between patients who had performed I131 WBS and those who received ablation directly without prior diagnostic imaging in favor of the latter group. Successful complete ablation was reported in 54.1% and in 90.2% of patients in group 1,4 and failure of complete ablation was encountered in 45.9% and 9.8% in patients who performed DxWBS and those who received I131 ablation without prior imaging respectively (p value < 0.001). Comparing this factor (post operative I131 WBS before I131 ablation), which may lead to stunning with affect ablation outcome. We found that this was the major independent variable that significantly affected response rate with OR of 4.6 (95% CI 2.2-9.7), whereas there is no statistically significant difference in success of thyroid remnant ablation regarding the age (less than or more than 45 years old), gender (male or female), tumor histology (papillary, minimally invasive follicular and widely invasive follicular cancer thyroid), associated pathology (Colloid/ multinodular goitre and Hashimoto’s thyroiditis), tumor multifocality, presence or absence of minimal extrathyroidal extension and vascular invasion. 
Conclusion: Post-operative diagnostic I131 WBS will significantly affect the success of I131 ablation if the ablative dose is planned within 1 -6 weeks after the diagnostic study.

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