S3313 SIADH as a Rare Presentation of Gallbladder Small Cell Carcinoma: A Case Report

Introduction: Gallbladder Small Cell Carcinoma (GB SCC) is a form of small cell neuroendocrine tumor (NET). Literature of this disease is limited to a small number of published cases. While NETs can be linked to paraneoplastic syndromes, such as Syndrome of Inappropriate Antidiuretic Hormone (SIADH), it is rarely the presenting feature. This case reports an atypical presentation of GB SCC. Case Description/Methods: A 75-year-old man with a history of hypertension, hyperlipidemia, COPD, cholelithiasis, and chronic Hepatitis C presented with abdominal bloating, early satiety, and associated poor oral intake. Labs showed sodium 115 mEq/L, aspartate aminotransferase 38 U/L, alanine aminotransferase 24 U/L, alkaline phosphatase 57 U/L, total bilirubin 0.9 mg/dL. Serum studies revealed serum osmolarity 241 mOsm/kg, urine osmolarity 328 mOsm/kg, urine sodium 52 mEq/L, urine creatinine 74 mEq/L. The patient was placed on a fluid restriction, which improved sodium levels, suggesting SIADH. Due to persistent bloating, an abdominal computed tomography was obtained showing a large heterogeneous mass in the gallbladder extending to the right hepatic lobe. Oncology was consulted, and a staging Positron Emission Tomography (PET) scan showed hypermetabolism only in the gallbladder. Core needle biopsy confirmed small cell carcinoma (pan-CK positive, synaptophysin positive, chromogranin positive, and CD56 positive). The patient began chemotherapy but chose to transfer care out of state for family support. Discussion: Small cell NETs are rare, aggressive tumors that occur in organs lined with neuroendocrine cells such as lungs, gastrointestinal tract, gallbladder, pancreas, or cervix. GB SCC particularly rare, accounting for 0.2% of all NETs and 1-5% of gallbladder cancers. It is a fast-spreading, highly aggressive tumor, often found in advanced stages with metastasis and poor prognosis. Treatment typically involves surgery with chemotherapy, though therapeutic options and established protocols are scarce. Symptoms typically arise from mass effect on the biliary system, including obstructive jaundice, abdominal pain, and weight loss. However, NETs may present with paraneoplastic syndromes, like SIADH, as was the case here. The patient’s advanced GB SCC was discovered after abnormal serum chemistries indicated SIADH. This highlights the importance of broad differential diagnoses, as earlier detection can significantly affect clinical outcome and prognosis.