A Case of an Invisible Pancreatic Neuroendocrine Tumor Decoded by Endoscopic Ultrasound.

Endoscopic ultrasound (EUS) is a highly sensitive modality of imaging for detecting small pancreatic masses that may be missed on other cross-sectional imaging. We report a 61-year-old woman who presented with recurrent symptomatic, predominantly postprandial, hypoglycemic episodes, which were confirmed on continuous glucose monitoring. Pertinent medical history included a remote use of tirzepatide, a dual glucagon-like peptide-1 (GLP-1)/glucose-dependent insulinotropic polypeptide (GIP) receptor agonist, for weight loss. Mixed-meal testing performed four months after discontinuation of tirzepatide demonstrated hypoglycemia with inappropriately normal insulin and C-peptide levels and a high proinsulin level. Contrast-enhanced computed tomography (CT) of the abdomen and pelvis was negative for pancreatic masses. Due to high clinical suspicion for an insulinoma, EUS was pursued, and a 12-mm hypoechoic mass was identified in the pancreatic body. EUS-guided fine needle biopsy demonstrated a well-differentiated, World Health Organization (WHO) grade 1 pancreatic neuroendocrine tumor with immunostaining consistent with insulinoma. Later, she underwent pancreatic tumor enucleation with complete resolution of symptoms postoperatively. This case highlights the diagnostic utility of EUS in patients with biochemically suspected insulinoma and negative cross-sectional imaging. EUS not only improves mass detection but also facilitates tissue acquisition, supporting timely surgical management of small pancreatic neuroendocrine tumors.