Can I-124 PET/CT Predict Pathological Uptake of Therapeutic Dosages of Radioiodine (I-131) in Differentiated Thyroid Carcinoma?

Abstract

Introduction: The aim of the study was to assess the role of I-124 PET/CT in the prediction of uptake of a therapeutic dosage I-131 during follow-up of patients with differentiated thyroid cancer (DTC). Methods: The results of 34 I-124 PET/CT scans performed in our hospital between 2007 and 2012 were retrospectively evaluated. All scans were made in patients under follow up, replacing the diagnostic I-131 or I-123 scintigraphy. In all cases thyroglobulin (Tg) was stimulated (by recombinant thyroid stimulating hormone (rhTSH) or thyroid hormone withdrawal). A dosage of 40 MBq I-124 was used, with scans at 24 hours and 96 hours after administration. Results were compared to subsequent I-131 post-treatment scans (6 cases) and a combination of follow up, stimulated Tg and other imaging tools results available to assess presence of recurrence. Results: Recurrence of DTC was found in 14/34 cases. I-124 PET/CT correctly detected recurrence in 2 cases, with false negative results in 12 cases. In 1 case a false positive I-124 PET/CT result was recorded. 19 true negative results were found. For I-124 PET/CT this meant a sensitivity of 14% and a specificity of 95%. Positive predictive value was 67%, negative predictive value 61%. Post-treatment I-131 uptake (6 cases) was correctly predicted in 1 case, with false negative results in 4 cases and 1 true negative result. Conclusions: In this study I-124 PET/CT did not reliably detect recurrent differentiated thyroid carcinoma. More importantly it failed to predict I-131 uptake on post-treatment scintigraphy in a significant number of cases, which would lead to under-treatment.