S3386 An Unusual Case of Gallbladder Small Cell Neuroendocrine Carcinoma Presenting as Acute Calculous Cholecystitis

Introduction: Small cell neuroendocrine carcinoma (SCNEC) of the gallbladder is an extremely rare and aggressive malignancy with a poor prognosis. We present a case of gallbladder SCNEC initially presenting as acute calculous cholecystitis, followed by rapid metastatic progression and tumor-induced pancreatitis. To the best of our knowledge, this is the first reported case of its kind. Case Description/Methods: A 60-year-old man presented with right upper quadrant abdominal pain radiating to the back, consistent with acute calculous cholecystitis. Laboratory studies revealed elevated liver enzymes and bilirubin. Imaging showed gallbladder wall thickening and cholelithiasis without masses or metastases. The patient underwent laparoscopic cholecystectomy; the gallbladder was gangrenous with multiple small cholesterol stones. Histopathology unexpectedly revealed SCNEC of the gallbladder. A follow-up computed tomography scan within 7 days showed new bilateral pulmonary nodules, mediastinal lymphadenopathy, and adrenal and pancreatic masses absent on preoperative imaging. Mediastinal lymph node biopsy confirmed metastatic small cell carcinoma. Immunohistochemistry revealed a Ki-67 index of 70%. The patient developed pancreatitis with lipase >6000 U/L; imaging demonstrated a pancreatic mass without gallstones or duct obstruction, consistent with tumor-induced pancreatitis. Liver enzymes normalized after surgery. Systemic chemotherapy was initiated. Discussion: This case underscores the aggressive nature of gallbladder SCNEC and its rare initial presentation as acute calculous cholecystitis. The rapid metastatic spread and tumor-induced pancreatitis observed emphasize the need for early diagnosis and management. To our knowledge, tumor-induced pancreatitis in metastatic gallbladder SCNEC has not been previously described.