A quantitive study about thyroid stunning after diagnostic whole-body scanning with 74 MBq 131I in patients with differentiated thyroid carcinoma

Aim. The aim of this paper was to explore by a quantitative method whether a 74 MBq I-131 scanning activity produces a stunning effect in patients with differentiated thyroid carcinoma (DTC). Methods. We included 70 patients with DTC who had their first radioiodine treatment for ablation of thyroid remnants and/or metastases. All the patients received 1850 similar to 7400 MBq I-131. Before ablation, 34 patients (group A) performed a diagnostic scan (Dscan) 24 hours after the administration of 74 MBq I-131; 36 patients (group B) received I-131 therapy without a previous Dscan. A therapeutic scan (Tscan) was performed after the ablation. The fractional concentrations of I-131 in remnants or functional metastases were quantified on Dscan and Tscan, and were expressed as Dx and Tx respectively. The level of significance was set at 0.05. Results. For group A, 67 foci were found both on Dscan and Tscan, the mean Dx and Tx was 26.13 +/- 37.98 and 7.46 +/- 10.63 (P=0.000), respectively. For group B, 70 foci were found on Tscan, the mean Tx was 15.23 +/- 17.23, which was higher than group A significantly (P=0.002). Conclusion. 74 MBq I-131 for diagnostic scan can decrease the uptake of I-131 by thyroid remnants or metastases, the thyroid stunning exists.