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 (DTC), based on an increased thyroglobulin (Tg) level and negative neck ultrasound (US), pose a clinical dilemma. Since 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 post-therapy 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 rhTSH stimulation. Subsequently, after 4-6 weeks of thyroid hormone withdrawal patients were treated with 5.5-7.4 GBq 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 three patients had a negative ¹²⁴I PET/CT and a positive post-therapy ¹³¹I. Results: After inclusion of 17 patients we terminated the study preliminarily, as the stopping rule had been met. Median Tg-level at ¹³¹I therapy was 28 μg/L (interquartile range: 129). Eight post-therapy WBS were negative (47%), all of which correctly predicted by negative ¹²⁴I PET/CT. Nine post-therapy WBS showed iodine avid tumor, of which four also had positive ¹²⁴I PET/CT findings. Sensitivity, specificity, negative predictive value and positive predictive value of ¹²⁴I PET/CT were 44% (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 DTC and a tumor negative neck ultrasound, the high false negative rate of rhTSH stimulated ¹²⁴I PET/CT as implemented in this study precludes its use as a scouting procedure to prevent futile blind ¹³¹I therapy.

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