Answer: Meningioma

Histology: The tumor is composed of fascicles of spindle-shaped cells enmeshed in dense bands of collagen. The entire specimen consists of tumor with no normal brain structures identified. Tumor cell nuclei are oval to spindled. Rare meningothelial whorls are present. The MIB-1 index ranges from 4-8%, but mitoses are rare (< 4 per 10 high powered fields). A few calcifications are present. Discussion: This lesion is a typical fibrous meningioma in an unusual location. Intraventricular meningiomas are most common in the lateral ventricles, but can also occur in the third and fourth ventricles. It has been estimated that 0.5% to 2% of all intracranial meningiomas arise in the ventricles. Intraventricular meningiomas are not intrinsically more aggressive than other meningiomas but can be difficult to resect. While this lesion contains a significant number of MIB-1 positive nuclei, the 4 mitoses per ten high-powered fields required for a diagnosis of atypical meningioma using the current WHO criteria are not present. The differential diagnosis includes spindled glial neoplasms and metastases to the choroid plexus. High-grade astrocytic tumors such as glioblastomas and gliosarcomas infiltrate through the brain and would not develop as a bulky intraventricular lesions without involvement of the surrounding cortex. In addition, gliosarcomas and metastatic fibrosarcomas are generally more mitotically active than the pictured tumor and would not be widely EMA immunopositive. Molecular analyses strongly suggest that gliosarcomas are monoclonal, with both the glial and mesenchymal components arising from a common (presumably glial) progenitor. Choroid plexus carcinomas occur in infants and are not spindled.