%0 Journal Article %A Jakob W. Kist %A Bart de Keizer %A Manfred van der Vlies %A Adrienne H. Brouwers %A Dyde A. Huysmans %A Friso M. van der Zant %A Rick Hermsen %A Marcel P.M. Stokkel %A Otto S. Hoekstra %A Wouter V. Vogel %T 124I PET/CT to Predict the Outcome of Blind 131I Treatment in Patients with Biochemical Recurrence of Differentiated Thyroid Cancer: Results of a Multicenter Diagnostic Cohort Study (THYROPET) %D 2016 %R 10.2967/jnumed.115.168138 %J Journal of Nuclear Medicine %P 701-707 %V 57 %N 5 %X Patients with suspected recurrence from differentiated thyroid carcinoma, based on an increased thyroglobulin (Tg) level and negative neck ultrasound (US), pose a clinical dilemma. Because standard imaging has a low yield identifying potential recurrence, blind 131I treatment is often applied. However, a tumor-negative 131I whole-body scintigraphy (WBS) prevails in 38%–50% of patients. We performed a prospective multicenter observational cohort study to test the hypothesis that 124I PET/CT can identify the patients with a tumor-negative posttherapy 131I WBS. Methods: Our study was designed to include 100 patients with detectable Tg and a negative neck US, who were planned for blind 131I therapy. All patients underwent 124I PET/CT after administration of recombinant human thyroid-stimulating hormone. Subsequently, after 4–6 wk of thyroid hormone withdrawal patients were treated with 5.5–7.4 GBq of 131I, followed by WBS a week later. The primary endpoint was the number of 131I therapies that could have been omitted using the predicted outcome of the 124I PET/CT, operationalized as the concordance of tumor detection by 124I PET/CT, using post-131I therapy WBS as the reference test. The study would be terminated if 3 patients had a negative 124I PET/CT and a positive posttherapy 131I scan. Results: After inclusion of 17 patients, we terminated the study preliminarily because the stopping rule had been met. Median Tg level at 131I therapy was 28 μg/L (interquartile range, 129). Eight posttherapy WBS were negative (47%), all of which were correctly predicted by negative 124I PET/CT. Nine posttherapy WBS showed iodine-avid tumor, of which 4 also had positive 124I PET/CT findings. Sensitivity, specificity, negative predictive value, and positive predictive value of 124I PET/CT were 44% (confidence interval [CI], 14%–79%), 100% (CI, 63%–100%), 62% (CI, 32%–86%), and 100% (CI, 40%–100%), respectively. Implementation of 124I PET in this setting would have led to 47% (8/17) less futile 131I treatments, but 29% of patients (5/17) would have been denied potentially effective therapy. Conclusion: In patients with biochemical evidence of recurrent differentiated thyroid carcinoma and a tumor-negative neck US, the high false-negative rate of 124I PET/CT after recombinant human thyroid-stimulating hormone 124I PET/CT as implemented in this study precludes its use as a scouting procedure to prevent futile blind 131I therapy. %U https://jnm.snmjournals.org/content/jnumed/57/5/701.full.pdf