Persistent fecal occult blood due to the small intestinal metastasis of pleomorphic lung carcinoma

The gastrointestinal tract is one of the locations that lung cancers cause metastasis. A 70-year-old male underwent right lower lobectomy while presenting fecal occult blood with a preoperative colonoscopy showing colon polyps as the cause. The pathological diagnosis was pleomorphic carcinoma of the lung, with stage pT3N0M0. Seven months after the lung surgery, the patient presented with sudden-onset abdominal pain and severe anemia. Computed tomography scanning revealed a large mass in the abdominal cavity, and subsequent intestinal endoscopy demonstrated jejunum tumors. Partial jejunum resection was successfully performed. The patient developed multiple peritoneal nodules suggesting metastatic tumors but well responded to an immune checkpoint inhibitor. It can be challenging to diagnose gastrointestinal metastasis in routine radiography; therefore, endoscopic examination, including the small intestine, might be an important option when a lung cancer patient with advanced clinical stage presents with abdominal symptoms, including fecal occult blood.