Atypical meningioma, light micrograph. Meningioma is the most common primary intracranial tumour in adults. Most meningiomas are WHO Grade I and behave in a benign fashion. The remaining 20% are WHO Grade II or III with atypical to anaplastic (malignant) features and an aggressive clinical course. Atypical meningioma are WHO Grade II tumours. Two major criteria come into consideration in order to upgrade meningiomas; (1) Mitotic index (greater than or equal to 4/10 HPF); and (2) At least 3 of the 5 following parameters: sheeting architecture, small cell formation, hypercellularity, macronucleoli, spontaneous necrosis. This picture illustrates mitotic figures and sheeting architecture.