Meningiomas rarely may behave in an aggressive or malignant fashion. Certain histologic features can help to identify atypical or anaplastic meningiomas.
Criteria for atypical meningioma (WHO grade II): one or more of the following:
(1) high mitotic index, with >= 4 mitoses per 10 high power field (or >= 2.5 per square mm)
(2) presence of 3 or 4 of the following:
(2a) sheeting architecture
(2b) hypercellularity
(2c) macronucleoli
(2d) small cell formation
(3) brain invasion
Criteria for anaplastic meningioma (WHO grade III or IV): either of the following:
(1) excessive mitotic rate, with >= 20 mitoses per 10 high power fields (or >= 12.5 per square mm)
(2) focal or diffuse loss of meningothelial differentiation at the light microscopic level resulting in an appearance resembling sarcoma, carcinoma, and/or melanoma
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