[ ¹⁸ F]-FDG and [ ⁶⁸ Ga]Ga-FAPI-04 PET/CT in Radioiodine-refractory Differentiated Thyroid Cancer: A Comparative Pictorial Review along with Insights into Potential for Theranostics and Nonspecific Uptake Patterns on [ <sup>68</

Radioiodine-refractory differentiated thyroid cancer (RAIR-DTC) poses challenges to oncologists and nuclear medicine physicians owing to its poor disease-specific survival rates and limited therapeutic options. In this present pictorial review, we present qualitative visual representation coupled with semiquantitative standardised uptake value findings in patients of RAIR-DTC, evaluated with dual-tracer positron emission tomography–computed tomography (PET/CT), namely [ 18 F]-FDG-PET/CT and [ 68 Ga]Ga-fibroblast activation protein inhibitor (FAPI)-04 PET/CT. Whole-body PET/CT imaging was performed using standard protocols after intravenous injection of 185–259 MBq of [ 18 F]-FDG and 74–111 MBq of [ 68 Ga]Ga-FAPI-04 on two separate days within a time period of 2 weeks. A relatively small fraction of patients in the current series portrayed good fibroblast activation protein (FAP) expression, which offers a potential therapeutic avenue to FAP-targeted radioligand therapy. In addition, a few nonspecific uptake patterns encountered on [ 68 Ga]Ga-FAPI-04 PET-CT studies are also highlighted, which will enable the readers’ insight into false-positive uptakes of FAPI. At the end of this review, we discuss the feasibility of FAPI theranostics in the examined patient population based upon our findings and portray a clinical context on FAPI’s role in the RAIR-DTC scenario.