Abstract: Background and purpose: Radioactive iodine-refractory differentiated thyroid cancer (RAIR-DTC) is a hot topic in recent years. The growth rates of different lesions and their responses to treatment vary a lot due to the heterogeneity of tumor cells. This study aimed to explore the value of ¹⁸F-FDG PET/CT in RAIR-DTC after the therapy of apatinib. Methods: RAIR-DTC patients undergoing apatinib therapy in Peking Union Medical College Hospital were enrolled. ¹⁸F-FDG PET/CT was performed at baseline, 4 weeks and 8 weeks after apatinib therapy. Results: A total of 7 patients including 49 lung metastases were analyzed after apatinib therapy, with SUV_max varying from 0.8 to 23.2. The results showed that the SUV_max level of lesions at baseline was significantly associated with the changes in SUV_max at 4 and 8 weeks after the treatment (SUV_max-4w/SUV_max-baseline, SUVmax-8w/SUV_max-baseline) (P<0.000 1, P<0.000 1). The rate of lesion diameter change (CT4w/b, CT8w/b) was not correlated with baseline SUV_max at 4 weeks and 8 weeks (P=0.666 4, P=0.478 7). The 4-week change in SUVmax (SUV_max-4w/SUV_max-baseline) was significantly associated with CT4w/b and CT8w/b (P=0.033 3, P=0.004 8). For lesions with baseline SUV_max greater than 5, SUV_max-baseline had a linear relationship with CT4w/b and CT8w/b (P=0.008 2, P=0.016 9). Conclusion: ¹⁸F-FDG PET/CT has great value in evaluating the heterogeneity, selecting target lesions and predicting the response in patients with RAIR-DTC after apatinib therapy.

Key words: Radioactive iodine-refractory differentiated thyroid cancer, ¹⁸F-FDG PET/CT, Apatinib, Efficacy evaluation