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

Background 18F-FDG PET and 131I 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 18F-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 131I and 18F-FDG uptake. BMs with positive 18F-FDG uptake had a less favorable response to RAI therapy in comparison to those with negative 18F-FDG uptake (p = 0.018), even in 131I-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 131I-avid but non-FDG-avid BMs, patients presenting with 18F-FDG-avid BMs had shorter PFI, whether with 131I uptake (p = 0.002) or without (p = 0.002). Conclusion More than half of BMs (58%) from DTC show simultaneous 18F-FDG and 131I uptakes which are contrary to the flip-flop phenomenon (131I negative and 18F-FDG positive). Regardless of 131I uptake, 18F-FDG uptake of BMs portends a less favorable therapeutic response and poorer prognosis for patients with DTC.