Metaplastic 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. Metaplastic meningiomas are Grade I meningiomas that, as the name implies, contain focal mesenchymal differentiation (bone, cartilage, adipose or xanthomatous change). This picture illustrates predominantly central areas of xanthomatous differentiation accompanied by syncytia of meningothelial cells.