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.
Keywords: bone metastasis, differentiated thyroid carcinoma, FDG, PET, prognosis