PET in the Diagnosis of Neuroendocrine Tumors

Abstract: For general oncological imaging, positron emission tomography (PET) using [18F]fluoro‐deoxy‐glucose (FDG) has evolved as a powerful functional imaging modality. Unfortunately, FDG‐PET has not been as advantageous for imaging gastropancreatic neuroendocrine tumors, and only tumors with high proliferative activity and low differentiation have shown an increased FDG uptake. Therefore, the 11C‐labeled amine precursors L‐dihydroxyphenylalanine and 5‐hydroxy‐L‐tryptophan (5‐HTP) were developed for PET imaging of these tumors. Because of the higher tumor uptake of the latter tracer in a study of patients with endocrine pancreatic tumors, 11C‐5‐HTP was chosen for further evaluation. In comparative studies of patients with carcinoids and endocrine pancreatic tumors, 5‐HTP‐PET proved better than CT and somatostatin receptor scintigraphy for tumor visualization, and many small, previously overlooked lesions were diagnosed by 11C‐5‐HTP‐PET. The strong correlation found during medical treatment between the changes in the transport rate constant at repeated PET and those of U‐HIAA indicates the possible use of 11C‐5‐HTP‐PET also for therapy monitoring. By premedication of patients with Carbidopa orally before PET examination, in order to block the aromatic amino acid decarboxylase enzyme, the decarboxylation rate of 11C‐5‐HTP was decreased, leading to a higher tumor uptake and a considerably lower urinary radioactivity concentration.