A Case of Pulmonary Pleomorphic Carcinoma With Brain Metastasis: A Diagnostic Pitfall in Distinguishing Glioblastoma

Introduction: Pulmonary pleomorphic carcinoma is a rare, highly aggressive subtype of lung cancer with frequent early multiple-organ metastases including to the brain. However, brain metastases that precede detection of a primary lung lesion are exceedingly rare. We report a case of pulmonary pleomorphic carcinoma with brain metastasis that was initially diagnosed and treated as glioblastoma. Case Presentation: A 69-year-old man presented with left-sided spatial neglect and abnormal behaviors. Brain computed tomography revealed a right temporal-subcortical hemorrhage. The patient subsequently underwent right temporal craniotomy and hematoma evacuation surgery. Histopathological examination revealed markedly large atypical cells with necrosis. Immunohistochemistry revealed focal GFAP positivity. As no primary lesion was detected on systemic computed tomography, the patient was diagnosed with glioblastoma, IDH-wildtype, and temozolomide was initiated. Without a significant recurrence of the brain lesion, the patient’s condition deteriorated. Follow-up computed tomography revealed new mass lesions in the lungs and small intestine. The patient died of peritonitis secondary to small intestinal perforation. Autopsy revealed a large mass in the right lung that extended to the mediastinum and small daughter lesions in multiple organs, including the small intestinal mesentery. The known brain tumor and trunk tumors showed similar histology and strong immunohistochemical positivity for cytokeratin 7. Conclusion: This case illustrates that metastatic pulmonary pleomorphic carcinoma may mimic glioblastoma both radiologically and histopathologically, particularly when the primary tumor remains undetected. When brain metastasis cannot be confidently excluded during the evaluation of a brain tumor, re-evaluation of histopathological and neuroimaging findings, together with repeated systemic evaluation at short intervals, is important for establishing an accurate diagnosis.