Table 1.

Histological criteria for meningiomas

Benign meningioma (WHO Grade I)

Lacks criteria for atypical and anaplastic meningiomas

Any histology without clear cells, chordoid, papillary, or rhabdoid

Atypical meningioma (WHO Grade II)

Mitotic counts: 4 or more per 10 high-power fields

or

At least three of the following five findings:

Increased cellularity

Small cells with high N/C ratio

Prominent nucleoli

Uninterrupted patternless or sheet-like growth

Spontaneous or geographic necrosis

Anaplastic meningioma (WHO Grade III)

Frank anaplasia (carcinoma, melanoma, or high grade sarcoma-like histology)

Mitotic counts: 20 or more per 10 high-power fields

N/C: nuclear/cytoplasmic ratio, WHO: World Health Organization.