Abstract:Rationale:A 30-year-old man presented to the emergency department (ED) with respiratory failure and acute respiratory distress syndrome (ARDS).Patient concerns: Imaging demonstrated diffuse cystic lung lesions.Diagnoses: After serial investigations, a lung wedge biopsy showed adenocarcinoma with CDX2 positivity and TTF-1 negativity, and 18F-FDG PET/CT revealed intense uptake in the proximal stomach (cardia) with additional hypermetabolic pulmonary lesions, consistent with metastatic gastric adenocarcinoma.Interventions: Serial investigations were performed including lung biopsy and PET/CT to establish the diagnosis.Outcomes: The findings confirmed metastatic gastric adenocarcinoma presenting with diffuse cystic pulmonary lesions.Lessons: This case highlights an atypical cystic pulmonary presentation of gastric cancer that complicated diagnosis and management.