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ABSTRACT: Objective: Insulinoma is the most frequent cause of hyperinsulinemic hypoglycemia in adults, but its preoperative localization may be challenging.Methods: We report on a patient with a remarkably long history of persistent hypoglycemia due to an insulinoma, which was finally localized after 18 years using glucagon-like peptide-1 receptor (GLP-1R) imaging.Results: The patient was admitted in 1995 for hypoglycemia. A fasting test documented inadequate insulin suppression. She had given birth to a baby who was small for gestational age a few weeks earlier. The patient underwent surgery for a pancreatic tail nodule, which histologically proved to be normal tissue. Hypoglycemia recurred postoperatively. Diazoxide initially controlled symptoms, but over the years, the patient had recurrent hypoglycemic seizures and coma. The patient underwent repeated pancreatic magnetic resonance imaging (MRI), indium-111 (111In)-octreotide scintigraphy, galladium-68 (68Ga)-DOTATATE-positron emission tomography (PET) scan, 18F-DOPA-PET scan and arterial calcium stimulation with hepatic venous sampling; all were negative. Finally, a SPECT-CT with indium-111 diethylene triamine pentaacetic acid (111In-DTPA)-exendin-4, a radioligand with high affinity to the GLP-1R, showed focally increased tracer uptake in the pancreas tail. Endoscopic ultrasound (US) and fine-needle aspiration confirmed the endocrine origin of the tumor. The patient underwent laparoscopic tail pancreatectomy and was cured after 18 years of hyper-insulinemic hypoglycemia. Ex vivo studies of the insulinoma confirmed elevated insulin secretion at low glucose concentrations and abundant GLP-1R expression.Conclusion: GLP-1R imaging is a highly valuable tool in the localization of insulinoma when other conventional methods fail to identify the tumor. This new imaging tool will enable timely localization and cure for insulinoma patients.Abbreviations: ASVS selective arterial calcium stimulation and hepatic venous sampling 11C carbon 11 CT computed tomography DTPA diethylene triamine penta-acetic acid 18F-DOPA 18F-3,4-dihydroxyphenylalanine GLP-1R glucagonlike peptide-1 receptor 111In indium-111 KATP channels adenosine triphosphate-sensitive potassium channels MRI magnetic resonance imaging PET positron emission tomography SPECT single-photon-emission computed tomography SST2R somatostatin receptor type 2 SST5R somatostatin receptor type 5 US ultrasound