SUN-313 Insulinoma: Secreting but Not Appearing

Introduction: The diagnosis of an insulinoma requires clinical suspicion and confirmatory biochemical testing. Insulinomas are localized using computed tomography (CT), magnetic resonance imaging (MRI), endoscopic ultrasonography and if needed selective pancreatic arterial calcium injections with measurements of hepatic venous insulin levels (1). Clinical Case: A 71 year old female with no significant past medical history was referred for evaluation of hypoglycemia. She was noted to have frequent episodes of symptomatic fasting and post-prandial hypoglycemia for two years prior to presentation. Symptoms associated with hypoglycemia included: weakness, diaphoresis, and peri-oral numbness which resolved after treatment with high carbohydrate foods or juice. On several occasions severe hypoglycemia required emergency services assistance. Endocrine evaluation at an outside clinic during an episode of hypoglycemia found a serum glucose of 30 mg/dL, elevated insulin level (20 U/ml, n<3.0 U/mL), C-peptide (4.3 ng/mL, n<0.6 ng/mL), and proinsulin (130 pmol/L, n<5.0 pmol/L) levels. All other laboratory work was normal. CT scan of the abdomen localized a lesion to the tail of the pancreas, but based on radiographic appearance differential diagnosis included a splenule. She underwent an endoscopic ultrasound and magnetic resonance cholangiopancreatography of the abdomen both of which were unable to characterize the lesion. Given high clinical suspicion, an intra-arterial calcium stimulation test was conducted, with inconclusive results. A gallium-68 Dotatate scan showed Dotatate uptake, without further distinction. Given the challenge of identifying the lesion, the decision was made in collaboration with radiology to complete a Tectnetium-99m sulfur colloid single-photon emission computed tomography (SPECT)/CT scan. This imaging modality shows accumulation of sulfur colloid in normal splenic tissue and is expected to show no radiotracer uptake in pancreatic tissue (2). The scan showed no significant accumulation in the area of concern presenting high suspicion for a neuroendocrine tumor. Endocrine Surgery completed a partial pancreatectomy with removal of a 1.4 cm tumor. Pathology was consistent with insulinoma. Post-operatively the patient had complete resolution of her hypoglycemia. Conclusion: This case highlights a diagnostic challenge which required interdisciplinary collaboration to use alternative imaging to localize an insulinoma preoperatively. References: (1) Cryer, PE, Axelrod L, Grossman, AB, et al. Evaluation and Management of Adult Hypoglycemic Disorders: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinool Metab. 2009; 94;709-728. (2) Castellani, M, Cappellini, MD, Cappelletti, M, et al. Tc-99m Sulphur Colloid Scintigraphy in the Assessment of Splenic Tissue after Splenectomy. Clinical Radiology. 2001;56;596-98.