Tumors of the meninges comprise a large and diverse group of neoplasms that mostly have menin-gothelial or mesenchymal, non-meningothelial origins (see Table 1.9) [24,25]. The most common primary tumor of this group is the meningioma (18-20 per cent of intracranial tumors), which has meningothelial cell origins and is composed of neoplastic arachnoidal cap cells of the arachnoidal villi and granulations. Meningiomas can occur anywhere within the intra-cranial cavity, but favor the sagittal area along the superior longitudinal sinus, over the lateral cerebral convexities, at the tuberculum sellae and parasellar region, the sphenoidal ridge, and along the olfactory grooves. Numerous histologic variants of menin-gioma are described and recognized by the WHO (see Table 1.9). However, the biological behavior of most of these variants does not impact on the clinical behavior of the tumor. Meningioma sub-types that have a more indolent nature and low risk for
TABLE 1.9 WHO Classification: Tumors of the Meninges
Tumors of meningothelial cells
Meningioma Meningothelial Fibrous (fibroblastic) Transitional (mixed) Psammomatous Angiomatous Microcystic Secretory
Anaplastic meningioma Mesenchymal, non-meningothelial tumors Lipoma Angiolipoma Hibernoma
Liposarcoma (intracranial) Solitary fibrous tumor Fibrosarcoma
Malignant fibrous histiocytoma
Primary melanocytic lesions
aggressive growth or recurrence are classified as WHO grade I, and include the meningothelial, fibrous/fibroblastic, transitional (mixed), secretory, psammomatous, angiomatous, microcystic, lympho-plasmocyte-rich, and metaplastic variants [52,53]. Of this group, the meningothelial, fibrous, and transitional variants are most frequently diagnosed. The histological features common to most low-grade meningiomas are the presence of whorls (tightly wound, rounded collections of cells), psammoma bodies (concentrically laminated mineral deposits that often begin in the center of whorls), intranuclear pseudoinclusions (areas in which pink cytoplasm protrudes into a nucleus to produce a hollowed-out appearance), and occasional pleomorphic nuclei and mitoses (see Fig. 1.16) [52,53]. Meningothelial meningiomas are composed of lobules of typical meningioma cells, with minimal whorl formation. The tumor cells are uniform in shape, with oval nuclei that may show central clearing. Fibrous variants have spindle-shaped cells resembling fibroblasts that form parallel and interlacing bundles within a matrix of collagen and reticulin. More typical menin-gothelial type tumor cells may be intermixed with the spindle cells. Whorl formation and psammoma bodies are uncommon. The transitional meningioma have features between those of the meningothelial and fibrous variants. Regions of typical meningothe-lial cells are present, and may display lobular and fascicular patterns. In other areas, spindle cells may be more prominent. Whorls and psammoma bodies are very common in transitional tumors.
Meningioma sub-types that are more likely to display aggressive clinical behavior and to recur are classified by the WHO as grade II (atypical, clear cell, chordoid) and grade III (rhabdoid, papillary, anaplastic) [24,25,52,53]. On histological examination, all of the grade II tumors are likely to demonstrate increased cellularity, more frequent mitoses, diffuse or sheetlike growth, nuclear pleomorphism and atypia, and evidence for micronecrosis. The atypical meningiomas are similar to low-grade meningothelial tumors, with additional features including increased mitotic activity, small cells with high nuclear/ cytoplasmic ratio, high cellularity, and prominent nucleoli. Clear cell tumors are composed of polygonal cells with a clear, glycogen-rich cytoplasm that is PAS positive. The chordoid meningioma contains regions that are similar to chordoma, with trabeculae of eosinophilic, vacuolated cells in a myxoid background. Chordoid regions are interspersed with more typical areas of meningioma. Grade III tumors such as anaplastic meningioma, show features consistent with frank malignancy, including a high mitotic rate, advanced cytological atypia, nuclear pleomorphism, and necrosis (see Fig. 1.17). Invasion of underlying brain is frequently noted in grade III meningiomas, but can also occur in lower grade variants. Proliferation studies using Ki-67 demonstrate labeling indices ranging from 8 to 15 per cent.
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