The success of 131I ablation in thyroid cancer patients is significantly reduced after a diagnostic activity of 40 MBq 131I

Objective: Dosimetry studies have shown that activities of I-131 as small as 10-20 MBq may cause a stunning effect. A result of this stunning effect may be a lower success rate of the ablative I-131 therapy for differentiated thyroid carcinoma (DTC). The aim of this study was to determine whether pre-therapeutic uptake measurement with 40 MBq I-131 causes a lower success rate of ablation. Design: retrospective chart review study. Patients, methods: In two hospitals the ablation protocols differed in one respect only: in the one hospital no diagnostic I-131 was applied before ablation (group 1, n = 48), whereas in the other hospital a 24-h uptake-measurement with 40 MBq I-131 was performed (group 2, n = 51). Included were all DTC patients without distant metastases who had undergone I'll ablation between July 2002 and December 2005, and who had returned for I-131 follow-up. Successful ablation was defined as absence of pathological I-131 uptake on diagnostic whole-body scintigraphy and undetectable thyroglobulin-levels under TSH stimulation. Results: Overall, ablation was successful in 31/48 patients (65%) in group 1 and in 17/51 patients (33%) in group 2 (p=0.002). Multivariate analysis showed that pre-therapeutic uptake measurement using 40 MBq I-131 was an independant determinant for success of ablation (p = 0.002). Conclusions: After applying a diagnostic activity of 40 MBq I-131 before ablation, the success rate of ablation is severely reduced. Consequently, the routine application of I-131 for diagnostic scintigraphy or uptake measurement prior to I-131 ablation is best avoided.