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)

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 I-131 treatment is often applied. However, a tumor-negative I-131 whole-body scintigraphy (WBS) prevails in 38%-50% of patients. We performed a prospective multicenter observational cohort study to test the hypothesis that I-124 PET/CT can identify the patients with a tumor-negative posttherapy I-131 WBS. Methods: Our study was designed to include 100 patients with detectable Tg and a negative neck US, who were planned for blind I-131 therapy. All patients underwent I-124 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 I-131, followed by WBS a week later. The primary endpoint was the number of I-131 therapies that could have been omitted using the predicted outcome of the I-124 PET/CT, operationalized as the concordance of tumor detection by I-124 PET/CT, using post-I-131 therapy WBS as the reference test. The study would be terminated if 3 patients had a negative I-124 PET/CT and a positive posttherapy I-131 scan. Results: After inclusion of 17 patients, we terminated the study preliminarily because the stopping rule had been met. Median Tg level at I-131 therapy was 28 mu g/L (interquartile range, 129). Eight posttherapy WBS were negative (47%), all of which were correctly predicted by negative I-124 PET/CT. Nine posttherapy WBS showed iodine-avid tumor, of which 4 also had positive I-124 PET/CT findings. Sensitivity, specificity, negative predictive value, and positive predictive value of I-124 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 I-124 PET in this setting would have led to 47% (8/17) less futile I-131 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 I-124 PET/CT after recombinant human thyroid stimulating hormone I-124 PET/CT as implemented in this study precludes its use as a scouting procedure to prevent futile blind I-131 therapy.