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Meningiomas are the most common benign tumors of the brain (95% of benign tumors). They arise from the dura mater (outer most layer of the meninges to which the arachnoid mater connects) in the meninges of the brain or the spinal cord and represent about 15% of all primary brain tumors. They are more common in females than in males (2:1) and has a peak incidence in the sixth and seventh decades. Most cases are sporadic while some are familial. There has been some evidence that persons who have undergone radiation to the scalp are more at risk for developing meningiomas. The most frequent genetic mutations involved in meningiomas are inactivation mutations in the neurofibromatosis 2 gene (merlin) on chromosome 22q.
=Pathogenesis= Meningiomas arise from arachnoidal cells, most of which are near the vicinity of the venous sinuses, and this is the site of greatest prevalence for meningioma formation. They are most frequently attached to the dura over the supperior parasagittal surface of frontal and parietal lobes, along the sphenoid ridge, in the olfactory grooves, the sylvian region, superior cerebellum along the falx cerebri, cerebellopontine angle, and the spinal cord. The tumor is usually gray, well-circumscribed, and takes on the form of space it occupies. They are usually dome-shaped, with the base lying on the dura.
Histologically, the cells are relatively uniform, with a tendency to encircle one another, forming whorls and psammoma bodies (laminated calcific concretions). They have a tendency to calcify and are highly vascularized.
=Clinical manifestations= Small tumors (e.g., < 2.0 cm) are usually incidental findings at autopsy without having caused symptoms. Larger tumors can cause symptoms depending on the size and location.
- Focal seizures may be caused by meningiomas that overlie the cerebrum
- Progressive spastic weakness in legs and incontinence may be caused by tumors that overlie the parasagittal frontoparietal region.
- Sylvian tumors may cause a myriad of motor, sensory, aphasic, and seizure symptoms depending on the location.
- Increased intracranial pressure eventually occurs, but is less frequent that in gliomas
=Diagnosis= Meningiomas are readily visualized with contrast CT, MRI with gadolinium, and arteriography, all attributed to the fact that meningiomas are highly calcified and vascularized. CSF protein is usually elevated if lumbar puncture is attempted.
=Treatment=
- Surgical resection -- Meningiomas can usually be surgically resected with permanent cure if the tumor is superficial on the dural surface and easily accessible. For incompletely accessible tumors, recurrence is likely. These regions include the medial sphenoid bone, parasellar region, or anterior brainstem. If invasion of the adjacent bone occurs, total removal is nearly impossible. Malignant transformation is rare.
- Radiation therapy include gamma-knife or proton beam treatment may be pursued in cases of inoperable or unresectable tumors, or if tumor shows malignant transformation. Focused radiation may also be helpful for small tumors at the base of the skull.
- Conventional chemotherapy is likely not effective. Antiprogestin agents have been used but with variable results. Recent evidence that hydroxyurea has the capacity to shrink unresectable or recurrent meningiomas is being further evaluated.
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