Extreme Carcinoembryonic Antigen Elevation Revealing Clinically Silent Colonic Metastases From Gastric Adenocarcinoma: A Case Report

Gastric adenocarcinoma rarely metastasizes to the colon, and extreme carcinoembryonic antigen (CEA) elevation exceeding 1,000 ng/mL occurs in approximately 1.3% of gastric cancers, where it is strongly associated with lymphatic dissemination and poorly differentiated histology. We report the case of a 47-year-old man with progressive constitutional symptoms in whom markedly elevated CEA prompted a comprehensive diagnostic workup, ultimately revealing stage IV gastric adenocarcinoma with clinically silent colonic metastases. Computed tomography demonstrated numerous pulmonary nodules, mediastinal and hilar lymphadenopathy, and a 3.5-cm periceliac mass with diffuse retroperitoneal and retrocrural adenopathy, without hepatic lesions or peritoneal implants. Initial CEA exceeded 600 ng/mL and subsequently rose to 1,170 ng/mL. Notably, endoscopy performed four years earlier had revealed only mild chronic erosive gastritis without Helicobacter pylori infection or intestinal metaplasia, and colonic biopsies were normal. Initial esophagogastroduodenoscopy during the current presentation showed gastric ulcers without a discrete mass, with nondiagnostic biopsies. Fine-needle aspiration of the Virchow node confirmed adenocarcinoma, but the primary site remained indeterminate. Positron emission tomography demonstrated hypermetabolic activity in the stomach (standardized uptake value 9) and left supraclavicular node (standardized uptake value 11), and liquid biopsy (Guardant360) confirmed an upper gastrointestinal primary. Follow-up colonoscopy revealed a friable submucosal mass in the transverse colon and multiple hemorrhagic ulcerated lesions in the descending colon, confirmed as metastatic gastric adenocarcinoma. A systematic review identified only 26 reported cases of colonic metastasis from gastric cancer. This case demonstrates that extreme CEA elevation (>1,000 ng/mL) should prompt comprehensive metabolic imaging and repeat targeted endoscopy even when initial biopsies are negative, and underscores the aggressive natural history of gastric adenocarcinoma, while acknowledging that an apparently benign prior endoscopy may reflect unsampled early disease rather than true de novo progression within years.