Predictive Value of ¹⁸F-FDG PET in Patients with Advanced Medullary Thyroid Carcinoma Treated with Vandetanib

Therapeutic options in advanced medullary thyroid carcinoma (MTC) have markedly improved since the introduction of tyrosine kinase inhibitors (TKIs). We aimed to assess the role of metabolic imaging using ¹⁸F-FDG PET/CT shortly before and 3 mo after initiation of TKI treatment. Methods: Eighteen patients with advanced and progressive MTC scheduled for vandetanib treatment underwent baseline ¹⁸F-FDG PET/CT before and 3 mo after TKI treatment initiation. During follow-up, CT scans were obtained every 3 mo and analyzed according to RECIST. The predictive value for estimating progression-free survival (PFS) and overall survival (OS) was examined by investigating the ¹⁸F-FDG SUV_mean/max of the metabolically most active lesion, as well as by analyzing clinical parameters (tumor marker doubling times [calcitonin, carcinoembryonic antigen], prior therapies, rearranged-during-transfection mutational status, and disease type). Results: Within a median follow-up of 5.2 y, 9 patients experienced disease progression after a median interval of 2.1 y, whereas the remainder had ongoing disease control (5 with a partial response and 4 with stable disease). Eight of the 9 patients with progressive disease died from MTC after a median of 3.5 y after TKI initiation. A pretherapeutic SUV_mean of more than 4.0 predicted a significantly shorter PFS (1.9 y vs. 5.2 y, P = 0.04). Furthermore, sustained high ¹⁸F-FDG uptake at 3 mo with a SUV_mean of more than 2.8 tended to portend an unfavorable prognosis, with a PFS of 1.9 y (vs. 3.5 y, P = 0.3). Prolonged carcinoembryonic antigen doubling times were significantly correlated with longer PFS (r = 0.7) and OS (r = 0.76, P < 0.01). None of the other clinical parameters had prognostic significance. Conclusion: Pretherapeutic ¹⁸F-FDG PET/CT provides prognostic information in patients with advanced MTC scheduled for treatment with the TKI vandetanib. A low tumor metabolism with an SUV_mean of less than 4.0 before treatment predicts a longer PFS.