A Clinical Trial of Optimal Time Interval Between Ablation and Diagnostic Activity When a Pretherapy RAI Scanning Is Performed on Patients With Differentiated Thyroid Carcinoma

This article investigates the association of the time interval between the diagnostic dose and ablation with the stunning effect, when a 74 MBq (131)I pretherapy scanning was performed on patients with differentiated thyroid carcinoma (DTC); the patients who were diagnosed as DTC and would be performed radioiodine (RAI) ablation of thyroid remnants or metastases were recruited during January 2011 and May 2012 in our hospital. Thirty-seven patients with DTC who had the RAI ablation of thyroid remnants or metastases for the first time were recruited. All the patients received a dose of 1850 to 7400 MBq of (131)I for ablation and a diagnostic scan was performed 24 hours after the administration of 74 MBq (131)I before ablation. A posttherapy scan was performed 2 to 7 days after the ablation. The patients were broken down into 3 groups (G1, G2, and G3) according to the interval time between the diagnostic dose and therapy (1–3, 4–7, and >7 days). The fractional concentrations of (131)I in remnants or functional metastases were quantified and expressed as therapeutic/diagnostic (Rx/Dx). The level of significance was set at 0.05. Sixty-seven foci were found both on pretherapy and posttherapy scans, the mean ratio of Rx/Dx was 0.43 ± 0.29, and the ratio of 49 foci (73.13%) was <0.6. The ratios in G1, G2, and G3 were 0.46 ± 0.29, 0.29 ± 0.18, and 0.55 ± 0.33, respectively. The differences between G1 and G2, and G2 and G3 were statistically significant (t = 2.40, P = 0.021 and t = 3.28, P = 0.002), whereas the difference between G1 and G3 was not significant (t = 1.01, P = 0.319). By a diagnostic scan of 74 MBq (131)I, stunning prominently occurs with a time of 4 to 7 days between the diagnostic dose and ablation. We recommend that for less stunning effect, RAI ablation should be performed within 3 days or postponed until 1 week after the diagnostic dose administrated.