Answer: Atypical meningioma

Histology: In areas of this lesion, the thick, fibrous dura can be seen. The tumor is hypercellular with cells growing in a sheet-like architecture. Focally, whorls and lobules can be seen. The nuclei are round to oval with coarse chromatin and prominent nucleoli. Brisk mitotic activity can be seen throughout. Areas of necrosis are also seen within this tumor.

Discussion: The tumor is associated with thick, fibrous dura, suggesting that it is dural-based. However, it is a hypercellular lesion and the whorled appearance and bland cytology seen in a typical meningioma are not present in this case. This is an example of an atypical meningioma, which is a tumor that shows hypercellularity, sheet-like growth pattern, focal areas of necrosis and cytologic atypia, high nuclear-to-cytoplasmic ratios, coarse chromatin, and prominent nucleoli. Brisk mitotic activity is also seen in atypical meningiomas. Areas suggestive for brain invasion can also be seen. All of these features distinguish an atypical meningioma from the usual meningioma. However, an atypical meningioma will focally show features that suggest the meningothelial origin of the tumor, such as focal whorls and lobules and nuclei showing pseudo-inclusions. Immunohistochemical stains can help differentiate atypical meningiomas from a true sarcoma, metastatic carcinoma or glial neoplasms of the brain. Meningiomas are EMA positive and negative for GFAP. Although they can show focal cytokeratin reactivity, strong reactivity favors the diagnosis of metastatic carcinoma.