Variable Tracer Avidity and Interlesional Heterogeneity on 18 F-FDG, 68 Ga-DOTATATE, and 68 Ga-DOTA-FAPi-04 PET/CT Imaging in a Single Individual Patient With Progressive Metastatic Medullary Thyroid Carcinoma.

In medullary thyroid carcinoma (MTC), distant metastases have few therapeutic options with low success rates and substantial toxicity in many patients, warranting exploration of alternate systemic treatments with fewer adverse effects. The fibroblast activation protein inhibitor (FAPI)-based PET/CT opens new avenues for several cancers, including MTC. A case of MTC with varying tracer avidity and interlesional heterogeneity on 18 F-FDG, 68 Ga-DOTATATE, and 68 Ga-DOTA-FAPI-04 PET/CT imaging is presented. 68 Ga-DOTA-FAPI-04 PET/CT exhibited superior efficacy in terms of visualization of metastatic soft tissue, lymph nodal, and skeletal lesions, with enhanced target-to-background ratio compared with the other two, facilitating the detection of these lesions, thereby demonstrating the potential for theranostic translation.