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WHO I:
Meningiomas, with low risk of recurrence and/or low risk
of aggressive growth
WHO II:
Atypical
meningiomas, with increased mitotic activity or three or more
of the following features: increased cellularity, small cells
with high nucleus-to-cytoplasm ratio, prominent nucleoli, uninterrupted
patternless or sheetlike growth, and foci of spontaneous or geographic
necrosis
WHO III:
Anaplastic
(malignant) meningiomas: exhibit frank histologic features of
malignancy far in excess of the abnormalities present in atypical
meningiomas
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