A Case of Duodenal Metastasis From a Poorly Differentiated Lung Carcinoma.

Lung cancer is one of the leading causes of cancer-related mortality worldwide and commonly metastasizes to the brain, bones, liver, and adrenal glands, whereas gastrointestinal (GI) involvement is uncommon and rarely diagnosed during a patient's lifetime. We report a rare case of poorly differentiated carcinoma of the lung with duodenal metastasis. The patient was a 73-year-old man who presented with abdominal pain and melena, prompting esophagogastroduodenoscopy (EGD), which revealed ulcerated duodenal lesions. Histopathologic examination of the duodenal biopsy demonstrated a poorly differentiated carcinoma morphologically similar to the known lung tumor. Immunohistochemical analysis showed positivity for pancytokeratin (AE1/AE3) and PD-L1, with negative staining for TTF-1, CK7, CDX2, CK5/6, p63, neuroendocrine markers, and lymphoid markers, supporting metastatic disease when considered alongside the patient's known primary lung malignancy and clinicopathologic findings. As the lesions were neither actively bleeding nor causing obstruction, no endoscopic or surgical intervention was required. Despite initiation of oncologic therapy, the patient's condition rapidly deteriorated, and he died during the same hospitalization. The interval from diagnosis of the primary lung tumor to identification of GI involvement was only two months, followed by a further survival of 28 days after identification of duodenal involvement. This report underscores the importance of maintaining a high index of suspicion for GI metastases in patients with lung cancer who develop GI symptoms, as timely recognition may facilitate appropriate diagnostic evaluation and management.