Multiple Unusual Distant Metastasis in TENIS: Comparative Evaluation of [18F]FDG and [68Ga]-FAPI-04 PET-CT Uptake Patterns at Metastatic Sites

Abstract A relatively uncommon occurrence of multiple unusual distant metastasis in a patient with thyroglobulin-elevated negative iodine scintigraphy (TENIS) (subcutis, skeletal muscle, kidney, and pericardium) is presented. A 65-year-old man with papillary thyroid carcinoma post two cycles of radioactive iodine therapy on subsequent follow-up showed elevated serum thyroglobulin (>300 ng/mL) and negative iodine scintigraphy (TENIS). [18F]-fluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT) for disease restaging revealed extensive metastatic disease involving cervical and mediastinal nodes, bones, liver, subcutis, skeletal muscle, kidney, and pericardium. [68Ga]-fibroblast activation protein inhibitor (FAPI)-04 PET/CT was done to evaluate for fibroblast activation protein (FAP) expression, and the feasibility of targeted radionuclide therapy showed nil to very low FAP expression at the metastatic sites, except for the few cervical and mediastinal nodes, subcutaneous nodule in the left arm, and L3 vertebral lesion. In addition to illustrate the rare presentation in TENIS, this case demonstrates the superiority of [18F]-FDG PET/CT over FAPI PET/CT in this patient. However, FAPI-PET/CT may be used as a theranostic tool for assessing radionuclide therapy prospects in selected cases.