Comparison of outcomes after (123)I versus (131)I pre-ablation imaging before radioiodine ablation in differentiated thyroid carcinoma.

UNLABELLED Detection of residual tissue after thyroidectomy for papillary or follicular thyroid carcinoma may be performed using diagnostic imaging with either (123)I or (131)I. The former is often preferred to avoid "stunning"-defined as a reduction in uptake of the therapeutic dose of (131)I caused by some form of cell damage from the diagnostic dosage of the radionuclide. Stunning could potentially reduce the therapeutic efficacy of (131)I given to ablate a post-thyroidectomy remnant. This study examines the outcomes of ablative (131)I therapy after diagnostic studies with either (123)I or (131)I to determine if the diagnostic dosages of these radionuclides used in our Thyroid Cancer Center reduce the efficacy of (131)I 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 microIU/mL. They were divided prospectively into 2 groups. Group 1 had diagnostic imaging with 14.8 MBq of (123)I followed by thyroid remnant ablation with 3.7 GBq of (131)I. Group 2 had empiric ablation with the same 3.7-GBq (131)I dosage, but the preceding diagnostic scan was performed with 74 MBq of (131)I. Comparisons of equivalence of the 2 population samples and of the post-ablation outcomes were evaluated by chi(2) 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 post-thyroidectomy ablation rate (group 1, 81%; group 2, 74%; P > 0.05). CONCLUSION If thyroid remnant stunning occurs due to 74 MBq (131)I used as a diagnostic agent before (131)I ablation, it has no significant clinical correlate, as it yields the same ablation rate as that which occurs after 14.8 MBq of (123)I used for imaging.