Preablation Imaging Before Radioiodine Ablation in Differentiated Thyroid Carcinoma

Detection of residual tissue after thyroidectomy for papillary or follicular thyroid carcinoma may be performed using diagnostic imaging with either 123I or 131I. The former is often preferred to avoid ‘‘stunning’’—defined as a reduction in uptake of the therapeutic dose of 131I caused by some form of cell damage from the diagnostic dosage of the radionuclide. Stunning could potentially reduce the therapeutic efficacy of 131I given to ablate a postthyroidectomy remnant. This study examines the outcomes of ablative 131I therapy after diagnostic studies with either 123I or 131I to determine if the diagnostic dosages of these radionuclides used in our Thyroid Cancer Center reduce the efficacy of 131I given for remnant ablation. Methods: Fifty patients with nonmetastatic papillary or follicular carcinoma of the thyroid received total thyroidectomy; this was followed by thyroid hormone withdrawal to achieve a serum thyroid-stimulating hormone level in excess of 30 mIU/mL. They were divided prospectively into 2 groups. Group 1 had diagnostic imaging with 14.8 MBq of 123I followed by thyroid remnant ablation with 3.7 GBq of 131I. Group 2 had empiric ablation with the same 3.7-GBq 131I dosage, but the preceding diagnostic scan was performed with 74 MBq of 131I. Comparisons of equivalence of the 2 population samples and of the postablation outcomes were evaluated by x2 analysis. Successful ablation required a negative follow-up thyroid scan 6–8 mo after ablation and also an undetectable serum thyroglobulin level in the absence of antithyroglobulin antibodies. Results: There was no significant difference between the 2 groups demographically, in tumor burden or stage, or in the postthyroidectomy ablation rate (group 1, 81%; group 2, 74%; P . 0.05). Conclusion: If thyroid remnant stunning occurs due to 74 MBq 131I used as a diagnostic agent before 131I ablation, it has no significant clinical correlate, as it yields the same ablation rate as that which occurs after 14.8 MBq of 123I used for imaging.