Gastric metastasis of primary lung adenocarcinoma: a case report and review of the literature

Gastric metastasis from primary lung adenocarcinoma is rare. This case highlights the diagnostic challenges when lung cancer presents with gastrointestinal involvement as part of advanced metastatic disease. A 59-year-old female presented with right upper arm swelling and pain for over 4 months. Imaging revealed a right upper lobe pulmonary nodule and multiple metastases, including thickening of the upper gastric body wall. Biopsy of the arm lesion showed poorly differentiated adenocarcinoma. Initial clinical and imaging findings suggested possible primary gastric cancer with metastases, and chemotherapy was started. Subsequent immunohistochemistry (TTF-1+, Napsin A+, CDX2−) on gastric and lung biopsies confirmed metastatic lung adenocarcinoma. An EGFR exon 21 L858R mutation was identified. Treatment was switched to targeted therapy with icotinib. The patient remains in good condition under regular follow-up. Accurate differentiation of gastric metastasis from primary gastric adenocarcinoma using immunohistochemistry and molecular profiling is essential for guiding targeted therapy in patients with advanced (stage IV) lung cancer.