S6353 CA 19-9-Producing Gastric Adenocarcinoma With Peritoneal Metastases: An Elusive Diagnosis

Introduction: Carbohydrate antigen 19-9 (CA 19-9) is a tetrasaccharide produced by exocrine epithelial cells in the esophagus, stomach, pancreas, and biliary ducts. This marker is currently used to diagnose and prognosticate pancreatic cancer. CA 19-9-producing gastric adenocarcinoma (CA 19-9 GA) is exceedingly rare and most cases originate in East Asia. Herein, we describe a unique case of CA 19-9 GA with potential familial clustering. Case Description/Methods: A 33-year-old man with a family history of gastric cancer presented to the emergency department complaining of epigastric pain for 1 month. The abdominal pain worsened with food intake was associated nausea, vomiting, a 40-pound weight loss. On exam, the patient appeared jaundiced with icteric sclera, and the abdomen was diffusely tender to palpation. Triage labs: aspartate aminotransferase, 286 unit/L, alanine transaminase, 369 unit/L, alkaline phosphatase, 834 unit/L, and CA 19-9, 739 U/mL. Diagnostic imaging showed a distended gallbladder with a large amount of sludge and small ascitic fluid in the right upper quadrant. Magnetic resonance cholangiopancreatography showed a complex collection in the right anterior abdominal wall with no evidence of ductal dilatation. Esophagogastroduodenoscopy (EGD) showed erythema and erosions in the gastric antrum and irregular polypoid gastric mucosa, concerning for an infiltrative disorder. Surgical pathology revealed a poorly differentiated invasive adenocarcinoma of the stomach. Diagnostic laparoscopy revealed diffuse peritoneal metastases and a large gastric mass along the lesser curvature of the stomach. The patient was diagnosed with CA 19-9 GA with peritoneal metastases, and completed 4 cycles of palliative chemotherapy before being transitioned to hospice care due to worsening clinical status. Discussion: CA 19-9 GA is a rare and aggressive subtype of gastric cancer with early metastasis and poor prognosis. Currently, there are no standardized treatment regimens for the advanced stages of the disease. CA 19-9 is most recognized as a biomarker for pancreaticobiliary malignancies, but its elevation in non-pancreatic cancers, remains an underrecognized phenomenon. In this case, markedly elevated serum CA 19-9 levels initially raised concern for pancreatic carcinoma; however, imaging and histopathologic findings ultimately revealed gastric cancer with diffuse peritoneal carcinomatosis. The diagnostic challenge lay in the nonspecific clinical presentation and misleading biomarker profile, underscoring the importance of a broad differential diagnosis when interpreting tumor markers.