18F-DOPA and Other Radiopharmaceuticals for Imaging Unknown Primary Neuroendocrine Tumors

Neuroendocrine tumors (NETs) are a heterogeneous and complex group of neoplasms that have various biologic, chemical, and physical behaviors, mainly depending on the degree of differentiation and the location (1). The simplest clinical classification is between functioning tumors, for which a specific syndrome related to hypersecretion of peptides, biogenic amines, or hormones is observed, and nonfunctioning, for which no specific symptoms related to tumor production are present (2). Although considered relatively rare entities, NETs are increasing in incidence, with about 70% of them deriving from the gastroenteropancreatic system (1,3).