Intracranial Metastasis of a Primary Mediastinal Seminoma Mimicking a Convexity Meningioma: A Case Report

Convexity germ cell tumors are exceedingly rare and may closely mimic meningiomas, particularly in young men. This case report presents a rare convexity germ cell tumor characterized by dural invasion and skull destruction. An 18-year-old man presented with headache and nausea. Neuroimaging demonstrated a right frontoparietal extra-axial mass accompanied by adjacent skull hyperostosis and significant cerebral edema. Based on these radiological findings, a convexity meningioma was strongly suspected, and a craniotomy was performed for tumor resection. Unexpectedly, histopathological examination revealed a germ cell tumor with diffuse infiltration of the dura and bone, characterized by a high Ki-67 labeling index and immunoreactivity for placental alkaline phosphatase. Subsequent systemic evaluation identified a mass in the anterior mediastinum, leading to a final diagnosis of intracranial metastasis from a primary mediastinal seminoma. The patient was referred for systemic chemotherapy and achieved a complete response. Germ cell tumorscannot be excluded in young male patients, even when lesions arise on the convexity and exhibit imaging features typical of meningiomas. Accordingly, germ cell tumors should be included in the differential diagnosis, and tumor marker evaluation and nuclear medicine studies should be proactively performed.