FDG PET Predicts the Effects of 131I and Prognosis for Patients with Bone Metastases from Differentiated Thyroid Carcinoma

Background: F-18-FDG PET and I-131 scans are important in the detection of metastases from differentiated thyroid carcinoma (DTC). The relationship of FDG and radioiodine (RAI) metabolism in bone metastases (BMs) from DTC and its prognostic value on RAI treatment is not clear. Methods: The retrospective study included DTC patients with BMs from two medical centers, who underwent F-18-FDG PET/CT scans and RAI therapy. Therapeutic response was evaluated by serum thyroglobulin (Tg) levels and anatomical imaging changes. Results: The analyses were performed on 30 patients with 72 BMs. Forty-two (42/72, 58%) lesions displayed simultaneous I-131 and F-18-FDG uptake. BMs with positive F-18-FDG uptake had a less favorable response to RAI therapy in comparison to those with negative F-18-FDG uptake (p = 0.018), even in I-131-avid lesions (p = 0.033). Sixteen (53%) patients had disease progression with a median PFI of 26 months (range: 3 to 89 months). Compared to those with I-131-avid but non-FDG-avid BMs, patients presenting with F-18-FDG-avid BMs had shorter PFI, whether with I-131 uptake (p = 0.002) or without (p = 0.002). Conclusion: More than half of BMs (58%) from DTC show simultaneous F-18-FDG and I-131 uptakes which are contrary to the flip-flop phenomenon (I-131 negative and F-18-FDG positive). Regardless of I-131 uptake, F-18-FDG uptake of BMs portends a less favorable therapeutic response and poorer prognosis for patients with DTC.