Utility of FDG PET at the Initial Radioiodine Therapy in Differentiated Thyroid Cancer

Background/Aim: Differentiated thyroid cancer (DTC) has a good prognosis, except in the case of patients with radioiodine therapy (RIT)-refractory cancer. However, since DTC is essentially a slowly progressing cancer, it is usually judged to be a DTC with a poor prognosis after multiple RITs and yearly follow-up with echo, computed tomography (CT), and serum thyroglobulin values. This study investigated whether fluorodeoxyglucose-positron emission tomography/CT (FDG PET/CT) combined with initial RIT could identify early-stage patients with poor prognosis. Patients and Methods: We evaluated 100 patients with high-risk DTC who underwent total thyroidectomy and received RIT at our institution. We analyzed the clinical outcomes of patients and ¹⁸F-FDG accumulation using univariate and multivariate Cox proportional hazards regression models. Results: The 10-year overall survival (OS) was 87.9%, with no significant difference in OS between ¹⁸F-FDG accumulation at pre-total or near-total thyroidectomy (NTT) (p=0.180) and ¹³¹I accumulation at initial RIT (p=0.577). However, ¹⁸F-FDG positive patients had a significantly worse prognosis than negative patients (p=0.005) at initial RIT. Conclusion: ¹⁸F-FDG PET/CT plays an important role in both the diagnosis and prognostic prediction of RIT refractory disease in DTC patients. ¹⁸F-FDG PET/CT can be a useful tool particularly at the time of initial RIT since the ¹⁸F-FDG accumulation enables the screening of high-risk DTC with poor prognosis at a very early time stage.