The word "meningioma" was first used by Cushing in 1922 to describe a tumor originating from the meninges (Cushing 1922). In 1938 Cushing and Eisenhardt, in a classic monograph, described a classification system for these tumors (Cushing and Eisenhardt 1962). Meningiomas originate from the arachnoidal cap cell, a meningothelial cell in the arachnoidal membrane. They generally arise where arachnoidal villi are numerous (Kleihues et al 1993). Meningiomas were classified by their site of origin, and this system is still used. The common sites of origin and incidence rates are shown in Table 1 (Cushing and Eisenhardt 1962; MacCarty and Taylor 1979; Rohringer et al 1989). The 2007 World Health Organization pathologic classification system subdivides meningioma into 11 histologic subtypes. The first 3 subtypes are (1) meningothelial, (2) fibrous, and (3) transitional; these subtypes refer to the predominant histologic background, transitional being a combination of the other 2 histologic subtypes. The other 9 histologic variants often occur in a background of meningothelial or fibrous cells (Kleihues et al 1993).
Meningiomas are classified as benign, atypical, or malignant. Benign meningiomas are not encapsulated; they grow invaginating, but demarcated, from the brain. They grow with finger-like projections, and penetrate surrounding mesenchymal tissue, including bone. They may produce both an osteoblastic and a lytic reaction (Kleihues et al 1993). Meningiomas immunostain with vimentin, desmoplakin, and epithelial membrane antigen. They have a grade 1 biological behavior. Meningiomas grow in 3 primary histologic patterns: (1) meningothelial, (2) fibroblastic, or (3) transitional, a combination of meningothelial and fibrous. Meningothelial meningiomas consist of lobules of cells with oval pale nuclei, with chromatin marginated around the nucleus. The cell has an ill-defined cellular membrane, and nuclear and cytoplasmic invaginations often produce pseudoinclusions (Kleihues et al 1993). Fibroblastic meningiomas have parallel interlacing bundles of spindle-shaped cells with abundant collagen and reticulin between cells. Whorl formation and psammoma bodies are infrequent in these 2 histologic pattern types. Transitional meningiomas have a mixed pattern of both meningothelial and fibroblastic features. They more often contain whorls or psammoma bodies. The other 9 meningioma subtypes are psammomatous, papillary, angiomatous, microcystic, secretory, clear cell, chordoid, lymphoplasmacyte-rich, and metaplastic.
WHO grade II meningiomas include atypical, chordoid, and clear cell meningiomas (Bollag et al 2010). Both grade II and III WHO classifications of meningiomas require brain invasion as a criterion (Campbell et al 2009). WHO grade II meningiomas make up 5% to 7% of all meningiomas (McGovern et al 2010). Some pathologists feel that a meningioma should be called “malignant” only when there is frank brain invasion, although brain invasion has been documented in benign, atypical, and anaplastic meningiomas and felt to be an additional criteria for malignant classification (Perry et al 1999). Atypical meningiomas are diagnosed based on increased mitotic index of equal to or greater than 4 mitoses per 10 high-power fields or 3 or more of the following features: increased cellularity, small cells with high nuclear:cytoplasmic ratio, prominent nucleoli, uninterrupted patternless or sheet-like growth, and foci of "spontaneous" or "geographic necrosis" (Campbell et al 2009).
Clear-cell meningiomas make up only 0.2% of all meningiomas. This type usually behaves aggressively and can metastasize to the CSF. Clear-cell meningiomas often occur in patients of younger age and occur more frequently in the spinal and cerebellar pontine region. On histologic examination, clear-cell tumor cells are arranged in sheets of glycogen-containing polygonal cells with small, round monomorphic nuclei and abundant clear cytoplasm. Recurrence rate of clear-cell meningiomas is 46% to 80% (Tong-tong et al 2010).
WHO grade III meningiomas make up 1.0% to 2.8% of all meningiomas. These include anaplastic, rhabdoid, and papillary types (Bollag et al 2010). Malignant meningiomas have further increase in mitoses and cellularity with conspicuous necrosis (Maier et al 1992). Anaplastic or malignant meningiomas by definition must have equal to or greater than 20 mitoses per 10 high-power fields (Campbell et al 2009). Atypical and malignant meningiomas have a much higher recurrence rate after resection than do benign meningiomas. Recurrence rates were 6.9% for benign meningiomas, 34.6% for atypical meningiomas, and 72.7% for malignant meningiomas (Maier et al 1992).
Papillary and rhabdoid meningiomas are rare variants and have an aggressive clinical course and higher rates of recurrence, metastases, and mortality (Campbell et al 2009). Papillary meningiomas generally occur in the pediatric population (Bollag et al 2010). Their cell processes terminate in papilla on blood vessels, with tapering of their processes to form pseudorosettes. Rhabdoid meningioma is a new pathologic variant of malignant meningioma with peritumoral edema, bone involvement, and significant cystic components on MRI (Kim et al 2007).
Hemangiopericytoma is an aggressive mesenchymally derived tumor with oval nuclei with scant cytoplasm. There is dense intercellular reticulin staining. Tumor cells can be fibroblastic, myxoid, or pericytic. These tumors, in contrast to meningiomas, do not stain with epithelial membrane antigen. They have a grade 2 or 3 biological behavior, and need to be distinguished from benign meningiomas because of their high rate of recurrence (68.2%) and metastases (Maier et al 1992; Kleihues et al 1993).