Initial staging of differentiated thyroid carcinoma: Continued utility of posttherapy ¹³¹I whole-body scintigraphy

Purpose: To retrospectively compare pretherapy iodine 123 (¹²³I) and posttherapy iodine 131 (¹³¹I) sodium iodide wholebody scintigraphy of patients with newly diagnosed differentiated thyroid cancer to determine if there is significant and clinically relevant discordance of nonphysiologic iodide-avid foci (IAFs) between the two examinations. Materials and Methods: This study was approved by the Institutional Review Board, the requirement for informed consent was waived, and the study complied with HIPAA. The authors identified 108 patients (88 women, 20 men; age range, 16-86 years; mean, 47.5 years; 45 patients younger than 45 years, 63 patients 45 years and older) who previously had undergone total or near-total thyroidectomy for differentiated thyroid carcinoma. Each patient had undergone a pretherapy ¹²³I whole-body scan followed by a posttherapy ¹³¹I whole-body scan. The number and location of IAFs were recorded on both scans. Data were compared by using a Wilcoxon signed rank test for paired data and assessed clinical relevance based on changes in tumor staging. Results: Posttherapy ¹³¹I whole-body scans revealed additional IAFs outside the thyroid bed not detected on pretherapy ¹²³I scans in 21 (19%, P < .001) of 108 patients. Nineteen (90%) of these 21 had IAFs in new locations (P < .001), with tumor upstaging of 11 (59%, 10% of total) of those 19 patients; six (55%, 6% of total) of those 11 had scintigraphic patterns consistent with unsuspected metastatic disease. Concordant scintigraphic patterns were observed in 87 (81%) of 108. Conclusion: In patients with newly diagnosed differentiated thyroid cancer who had undergone thyroidectomy and ¹³¹I ablation, posttherapy ¹³¹I whole-body scintigraphy revealed new IAFs in 18% and clinical upstaging occurred in 10% of patients compared with pretherapy ¹²³I whole-body scintigraphy. Therefore, posttherapy ¹³¹I whole-body scintigraphy provides incremental clinically relevant information as it helps to establish the true extent of IAFs and may contribute to altering of staging.