**4.1 F-18 Fluorodeoxyglucose PET/CT**

Fluorodeoxyglucose (FDG) is a glucose analogue. This is taken into the cell and phosphorylated by the same mechanism as glucose. In thyroid cancer, "Flip-flop phenomenone" is reported (Feine et al., 1996; Khan N. et al., 2003). It means the alternating uptake pattern of I-131 and FDG by the differentiated papillary or follicular thyroid cancer, which thought to be related to the differentiation of tumor (Figure 11 and 12). Bertagna et al. reported that F-18 FDG PET/CT positive results correlated with the serum thyroglobulin level in patients with negative I-131 whole body scan and high serum thyroglobulin level. They also reported that F-18 FDG PET/CT showed highest accuracy when the patient's thyroglobulin level was higher than 21 ng/mL (Bertagna et al., 2009). In addition to that, they also revealed that the levothyroxine therapy regimen does not influence F-18 FDG PET/CT results. Yoshio et al. evaluated 55 cases with differentiated thyroid cancer with F-18 FDG PET/CT and reported that FDG-avid lesions are resistant to radioactive iodine therapy with or without I-131 uptake (Yoshio et al., 2011).

Fig. 11. Flip-flop phenomenone. A 44-year-old male underwent total thyroidectomy due to thyroid papillary carcinoma. I-123 DxWBS (A) shows several hot uptakes in the anterior neck, whereas F-18 FDG PET/CT (B) is negative.
