Interlesional Heterogeneity Between [ ⁶⁸ Ga]Ga-DOTATATE and [ ⁶⁸ Ga]Ga-FAPI-04 PET/CT After [ ¹⁷⁷ Lu]Lu-DOTATATE PRRT in Well-Differentiated NET with Progressive Disease

Theranostics is the main treatment strategy for well-differentiated neuroendocrine tumors (NETs), using somatostatin receptor (SSTR)–based PET/CT and radiopharmaceutical therapy. SSTR-based PET/CT is an important imaging tool to detect progressive disease after peptide receptor radionuclide therapy, but it sometimes fails to detect disease because of downregulation of SSTRs or changes in grade. Recently introduced fibroblast activation protein inhibitor (FAPI) PET/CT may serve as a promising imaging modality for high-proliferative NETs. A unique case of a pancreatic NET (grade II) is illustrated, which showed progressive liver and skeletal metastatic disease after peptide receptor radionuclide therapy and an increase in grade (Ki-67 proliferation index, 40%–50%) with variable tracer avidity and interlesional heterogeneity on [⁶⁸Ga]Ga-DOTATATE (no SSTR uptake in liver lesions) and [⁶⁸Ga]Ga-FAPI-04 PET/CT (intensely tracer-avid liver lesions and non–tracer-avid skeletal lesions). We found that [⁶⁸Ga]Ga-FAPI-04 PET/CT may be used as a complementary imaging modality to detect FAPI-avid lesions and be considered as another theranostic pair with SSTR-based radiopharmaceutical therapy in patients with NETs.