Fibrous 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. Fibrous meningioma also known as 'fibroblastic', is a grade I meningioma according to the WHO classification, and is characterized by spindle cells arranged in intersecting fascicles, which are longer than those seen in meningothelial meningiomas. Fibroblastic meningiomas can have variable interspersed collagen and reticulin fibres.