A Large Endodermal Cyst with Xanthogranuloma at the Frontal Skull Base, Slowly Recurring with Hemorrhage

A 45-year-old man with a past history of the removal of a degenerated hematoma two times presented with general convulsion. Computed tomography (CT) showed a high-density lobular mass growing from the right frontal skull base and occupying the right frontal lobe. Magnetic resonance imaging (MRI) demonstrated a homogeneously hyperintense mass on T₁-weighted imaging and a homogeneously hypointense mass on T₂- and T₂*-weighted imaging. We removed the lesion, which intraoperatively showed a blackish-brown and jellylike mass with machine oil-like fluid. There was a thin and elastic membrane at the boundary between the mass and degenerated brain tissue, and we removed as much of the membrane as possible. On light microscopy, most parts of the mass consisted of a degenerated hematoma with xanthogranuloma, while the thin and elastic membrane revealed one or two layers of non-ciliated columnar epithelial cells based on thin fibrous tissues with microvessels. Immunohistochemical staining showed that these epithelial cells contained periodic acid-Schiff (PAS)-positive granules, and were positive for cytokeratin (CAM5.2), carcinoembryonic antigen (CEA), and epithelial membrane antigen (EMA). Ultrastructual examination showed numerous microvilli at the surface of non-ciliated cells, and an interdigitation-like, dense adhesion structure. On the basis of pathological findings, the patient was considered to have a large endodermal cyst (EC) at the frontal skull base, probably derived from Seessel's pouch. We speculate that EC developed inflammatory changes with xanthogranuloma, which caused further damage to the blood vessels and continuous hemorrhage.