Detection of thyroid remnant/metastasis without stunning: An ongoing dilemma

The use of I-131 in doses of several mCi for scans can stun the thyrocytes and thyroid cancer cells, whereas the usual dose (300 mu Ci) of I-123 does not. We compared the diagnostic accuracy of the I-123 (300 mu Ci) scans and that of I-131 (3-10 mCi) scans in 155 patients. The diagnostic accuracy of a I-123 scan in detecting functioning thyroid remnant/metastasis was 89.5% (77/86 scans) and that of a I-131 scan was 92.9% (39/42) in 6 week-postoperative patients (p = 0.750). For radioablation therapy follow-up patients, the diagnostic accuracy of I-123 in determining presence or absence of functioning remnant or metastasis was 69.4% (25/36) and that of I-131 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 I-123 and after I-131 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 I-123 scan was essentially equal to that of the I-131 scan, and the success rate of radioablation therapy appears to be better after an I-123 scan; and 2) for postablation follow-up surveys, the I-131 scan appears to be better but carries the risk of stunning the functioning cells.