Microcystic 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. Microcystic meningioma belongs to the Grade I category of meningiomas according to the WHO classification. Meningothelial cells still retain their classic ample eosinophilic cytoplasm or can also appear vacuolated. This particular arrangement is referred as a 'spider-web' appearance, and may be due to secretory activity by the tumour, cystic degeneration, ischemia or penetration of CSF into the tumour.