Detection of Thyroid Remnant/Metastasis Without Stunning: An Ongoing Dilemma

Abstract

The use of 131I doses of several mCi for scans can stun the thyrocytes and thyroid cancer cells, whereas the usual dose (300 microCi) of 123I does not. We compared the diagnostic accuracy of the 123I (300 microCi) scans and that of 131I (3-10 mCi) scans in 155 patients. The diagnostic accuracy of a 123I scan in detecting functioning thyroid remnant/metastasis was 89.5% (77/86 scans) and that of a 131I scan was 92.9% (39/42) in 6 week-postoperative patients (p = 0.750). For radioablation therapy follow-up patients, the diagnostic accuracy of 123I in determining presence or absence of functioning remnant or metastasis was 69.4% (25/36) and that of 131I was 92.5% (49/53) with a p value of 0.079. The success rates for complete ablation of functioning tissue after radioiodine therapy administered after diagnostic 123I and after 131I were 72% (34/47) and 56% (24/43), respectively, with a p value of 0.125. Our study indicates the following: 1) for the first postoperative evaluation, the diagnostic accuracy of the 123I scan was essentially equal to that of the 131I scan, and the success rate of radioablation therapy appears to be better than 123I scan; and 2) for postablation follow-up surveys, the 131I scan appears to be better but carries the risk of stunning the functioning cells.

Impact of Post-Thyroidectomy I-123 Versus I-131 Whole Body Scan on the Outcome of Radioactive Iodine (I-131) Ablation for Differentiated Thyroid Carcinoma

“…We found a statistically significant higher successful I-131 ablation rate among patients studied with I-123 WBS compared to those studied with I-131 WBS before radioiodine ablation. Our results were consistent with those reported by Park et al, 1997 (27) who compared the successful ablation rate of functioning thyroid tissue, one year following I-131 therapy administered after diagnostic I-123 WBS in 47 patients and after I-131 WBS in 43 patients. Using the one-year follow-up WBS, without a Tg level, they reported that total ablation was obtained in 72% of patients who received (300 µCi) 11.1 MBq of I-123 as diagnostic dosages before ablation and in 56% of those who received (3-10 mCi) 111-370 MBq of I-131 as diagnostic dosages.…”

University of the Philippines – Philippine General Hospital Revised Clinical Practice Guidelines for the Management of Well-Differentiated Thyroid Carcinoma of Follicular Cell Origin

“…This occurs most prominently with higher I-131 activities (5-10 mCi) and with increasing time between the diagnostic dose and therapy. [46][47] Although stunning is not visually appreciated at activities of 1 -3 mCi, the accuracy of low dose scans has been questioned, particularly in patients who have had previous RAI therapy. reasons: 1) optimal I-123 activity and time to scan after I-123 administration is not known; 2) I-123 is expensive and not universally available; 3) the short half-life (t1/2 = 13 hours) of I-123 makes handling this isotope more difficult; 4) stunning may also occur, although to a lesser degree than I-131.…”

Down-regulation of the Sodium/Iodide Symporter Explains 131I-Induced Thyroid Stunning

“…Numerous studies have reported that diagnostic exposure to 131 I may diminish subsequent uptake of the therapeutic dose, a phenomenon known as thyroid stunning, which potentially may compromise the therapeutic efficiency and outcome of radioiodine treatment (reviewed in refs. [2][3][4][5]. The stunning mechanism has for long been obscure mainly because of lack of experimental data.…”