S5222 Unmasking Gastric Adenocarcinoma: Diagnostic Yield of EUS-Guided Fine Needle Biopsy in a Patient With Negative Conventional Workup

Introduction: Gastric adenocarcinoma can present insidiously, particularly when initial endoscopy and biopsies are non-revealing. This case highlights the diagnostic utility of endoscopic ultrasound-guided fine needle biopsy (EUS-FNB) in detecting infiltrative malignancy. Case Description: A 54-year-old man with a recent cholecystectomy presented with epigastric pain, nausea, vomiting, and weight loss. Liver enzymes and total bilirubin were normal. Initial upper endoscopy and colonoscopy with biopsies of the stomach and duodenum were negative. Computed tomography abdomen revealed distal gastric wall thickening and mild ascites. A gastric emptying study was normal. Due to persistent symptoms, repeat esophagogastroduodenoscopy showed diffuse moderate inflammation with congestion, erythema, and granularity throughout the stomach, and a clean-based antral ulcer (Forrest III). Biopsies again showed only reactive changes. Given high suspicion for malignancy, EUS was performed, revealing diffuse thickening (11 mm) of the gastric wall with loss of normal wall layer differentiation. EUS-FNB using a 19-gauge needle was performed. Histopathology demonstrated poorly differentiated adenocarcinoma with positive caudal type homeobox 2 (CDX2) staining, confirming gastrointestinal origin. Discussion: This case illustrates the limitations of mucosal biopsies in detecting subepithelial gastric cancer. Despite repeated negative biopsies, persistent symptoms and imaging findings warranted further evaluation. EUS not only revealed structural abnormalities but enabled definitive diagnosis through tissue acquisition. The use of CDX2 immunostaining supported the gastric origin of the adenocarcinoma. Infiltrative gastric cancer may elude standard endoscopic biopsies. EUS-FNB is a crucial diagnostic tool in patients with concerning clinical features and inconclusive initial workup, facilitating timely diagnosis and treatment.