Anaplastic 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. Anaplastic meningioma, also known as malignant meningioma, is a Grade III tumour according to the WHO that presents with either of two criteria: (1) greater than or equal to 20 mitoses per 10 HPF and/or (2) frank anaplasia (sarcoma, carcinoma or melanoma-like histology) as shown in this image.