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Introduction
General preoperative and surgical treatment in
meningiomas.
Histological aspects in meningiomas.
Supratentorial meningiomas in
general.
Parasagittal meningiomas.
Falx meningiomas.
Convexital meningiomas
Olfactory groove meningiomas.
Tuberculum sellae meningiomas.
Sphenoid wing meningiomas.
Optic sheath meningiomas.
Middle fossa meningiomas.
Cavernous sinus meningiomas.
Intraventricular meningiomas.
Malignant meningiomas.
Peritorcular meningiomas.
Tentorial meningiomas.
Infratentorial
meningiomas.
Meningiomas outside the
nervous system.
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Meningiomas. so named by Harvey Cushing in 1922. are common tumors that arise from cells of the meninges. A large amount of information on the biology and clinical features of meningiomas has accumulated in the almost nine decades since Cushing and Eisenhardt published their classification and morphologic descriptions of these tumors.
Histogenesis
Meningiomas are thought to originate from arachnoidal cap cells (cells forming the outer lining of the arachnoid membrane). related cells such as arachnoidal fibroblasts. or perhaps the precursor cell of the meninges the meningoblast. Whether the arachnoidal cap cell is derived from the neural crest or the mesoderm is itself still controversial. Many of these tumors show fibroblastic differentiation and some show other mesenchymal features such as chondromatous foci. xanthomatous foci. and bone formation. Irrespective of this problem. it is accurate to consider the meningiomas as tumors derived from the arachnoidal cells. The similarities in ultrastructural features between normal meningeal cap cells and the tumor cells of meningiomas. as well as the tendency of both to form whorls. are well known. The deeper cells of the arachnoid show some changes from the typical characteristics of the cap cells. especially a decrease in cell junctions and in the number of cell processes formed. These cells are mixed with the more typical fibroblasts near the arachnoid space. Two types of meningiomas. the transitional and the fibroblastic types. mimic the features of the fibroblasts at the light microscopic level. In addition to these mesenchymal features. arachnoid cap cells. by covering a surface in a continuous layer that is separated from the underlying connective tissue by a basal lamina. also mimic features of epithelium. Likewise some meningiomas. in particular the secretory type, exhibit epithelial features, such as the formation of glands and secretion. The pluripotential nature of arachnoid cap cells or their precursors is reflected in the remarkable histologic diversity of meningiomas.
Epidemiology
Meningiomas constitute about 15 percent of primary brain tumors and 25 percent of spinal cord tumors. However, because most meningiomas are benign and completely resectable, the mortality figures are much lower: only about 6 percent of brain tumors causing death are meningiomas. Although reports from most countries give similar figures.
Meningiomas are tumors of adults, with the main age of incidence ranging between 20 and 60 years. The peak incidence is around the age of 40. A female preponderance is seen, especially in the spinal cord meningiomas. Most meningiomas are solitary, but multiple meningiomas can occur. alone or in association with neurofibromatosis. Meningiomas in children account for less than 2 percent of meningiomas and less than 2 percent of intracranial tumors of childhood. The tumors can occur in any of the more typical locations. but intraventricular and spinal epidural tumors are especially associated with childhood. Meningiomas are more common in boys than girls. and childhood tumors have a higher incidence of malignancy as compared to tumors in adults.
Although meningiomas can potentially occur at any site in the meninges. certain intracranial locations are more common than others: parasagittal. attached to the falx. over the cerebral convexities. in the olfactory groove. on the tuberculum sellae, along the sphenoid ridge. in the cerebellopontine angle. along the clivus, and at the foramen magnum. Spinal meningiomas are most frequently located in the thoracic region. Meningiomas may also be found in the orbit. and rarely. in extracalvarial or ectopic locations.
Gross Features
Most meningiomas are well-demarcated. round or oval. frequently lobulated tumors attached to the dura. However, in a minority of tumors a dural attachment cannot be shown. The benign meningiomas tend to compress the brain but not invade it. An occasional meningioma. called meningioma-enplaque. has a flattened appearance that conforms to the curves of the brain and the inside of the skull. A delicate compressed layer of arachnoidal cells intervenes between the tumor and the brain, but often this is highly attenuated in places. On cut surfaces the typical meningioma is a grayish pink homogenous tumor with a faint whorled appearance. Occasionally. yellow flecks representing collections of fat-containing (xanthoma) cells may be seen. Grossly visible cystic degeneration and hemorrhage are infrequent. Most meningiomas. especially the transitional and fibroblastic types, are firm to hard in consistency. The presence of numerous psammoma bodies and calcification may impart a gritty feeling when the tumor is sliced for frozen-section diagnosis. A significant number of meningiomas invade surrounding tissues. including the dura, adjacent bone. and extracranial musculature. soft tissue of the orbit. and the paranasal sinuses. This feature. because it makes complete excision of the tumor difficult, is associated with an increased incidence of local recurrence after operation. However, most meningiomas are completely resectable. In the spinal canal. meningiomas are usually round or oval small tumors attached to the dura and compressing the adjacent spinal cord. Sometimes they may be en-plaque and partly or totally encircle the spinal cord. Although they may invade epidural soft tissue or intervertebral foramina, they do not show the typical dumbbell shape of the spinal nerve sheath tumors. Occasional tumors in this site tend to invade bone and adjacent soft tissues. but the majority of spinal cord meningiomas are slow growing and well circumscribed.
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