Meningioma

Incidence

Meningiomas account for about one fifth of all primary CNS tumors.

Origin

Meningiomas originate from special meningothelial cells of the arachnoid membrane or the arachnoid granulations. These cells are found in the skull, the spinal canal, the choroid plexus and the arachnoid around cranial nerves. Most meningiomas are attached to the dura in areas where arachnoid villi are numerous.

Macroscopic appearance and general characteristics (image 2)

Most meningiomas are spherical masses, with broad implantation on the underlying dura mater.

Carpet-like growth over the dural surface ("meningioma en plaque") or pedunculated masses are the exception.

Meningiomas are not invasive, with evidence of a well-delineated interface between the tumor and the brain.

Most meningiomas have a firm consistency. In case of calcification the tumor can even be very hard. Cysts within meningioma can be due to central necrosis, however peripheral true cysts can be secondary to reactive change, while pseudocysts are formed by entrapped CSF spaces.

Edema can be focal or can involve an entire hemisphere. The latter is probably vasogenic in origin, due to venous mechanical obstruction or tumor hypervascularity.

Meningiomas are typically highly vascularized. In the typical case dural supply from the middle meningeal artery, meningeal branches of the tentorium or the falx, or posterior meningeal arteries is seen. At the interface with the brain, the insertion of the meningioma can be supplied by pial branches. The meningeal supply typically gives rise to a central spoke wheel of hypertrophic arteries.

Meningiomas are classically accompanied by hyperostosis (images 3 - 4), more rarely by bony erosion (image 5). The hyperostosis is probably due to osteoblastic activity, but the medullary spaces of the overlying bone often are filled with tumor cells. Meningiomas are typically surrounded by a collar of thickened dura mater.

There is a peak incidence in the fifth and sixth decades. There is a 2:1 preponderance in females.

Multiple meningiomas are seen in 1 to 9% of MR studies and can be associated with neurofibromatosis type 2.

Meningiomas correspond histologically to Grade I. Anaplastic meningioma is extremely rare.

Microscopic appearance

Histologically, menigiomas constitute a heterogeneous group.

The most frequent are:

1. Meningothelial meningiomas: composed of solid lobules of meningothelial cells (image 6).

2. Fibrous meningioma: with spindle shaped cells and abundant collagen (image 7).

3. Transitional meningioma (image 8): with a mixture of the features of meningotheliomatous and fibrous meningioma. The tumors show a tendency to concentric whorl formation.

4. Psammomatous meningioma: in which calcified psammoma bodies are abundant (image 9).

5. Angiomatous meningiomas: with prominent vascular channels.

Other types (neurocystic, secretory, clear-cell, chordoid, lympho-plasmocyte-rich meningioma, metaplastic meningioma) are rare.

Most meningiomas are benign and grade I. The local recurrence rate is 20 to 30%. Anaplastic (malignant) meningioma is extremely rare.

Location of meningiomas

- Convexity meningiomas (25%) arising from the dura that overlies the cerebral hemispheres (image 10).

- The falx meningioma or parasagittal meningiomas (25%), arising from the falx cerebri (image 11).

- Sphenoid ridge meningiomas (20%) originating from the lesser wing, around the anterior clinoid process or the sphenoid ridge (image 12).

- Olfactory groove meningiomas (5%), arising from the dura of the cavernous sinus, the sellar diaphragm or the tuberculum sellae (image 13).

- Meningiomas of the petrous bone, the clivus, the tentorium or the foramen magnum (10%) (images 14 - 15).

- Intraventricular meningiomas (2%) (image 16).

Imaging features

- CT scan

On non-contrast CT meningioma appears as a well circumscribed rounded mass, with broad implantation on the dural surface. Most lesions are homogeneously slightly hypodense to brain.

Calcification is seen in approximately 20%. Total calcification is possible.

Hyperostosis is frequent.

On contrast enhanced CT, enhancement is typically homogeneous and intense.

- MRI

Classically it is stated that meningiomas on MRI are isointense to cortical grey matter both on T1 and T2 weighted spin-echo sequences (image 17). For this reason, in the early publications on MRI, it was said that meningiomas could easily be missed on non-enhanced images.

On T1 weighted images, even with up-to-date equipment, it is known that meningiomas are almost always isointense to grey matter (94%). A minority of the lesions are hypo- or hyperintense.

On T2 weighted spin-echo images, however about 45% of the lesions are relatively isointense. 44% of the lesions are hyperintense to grey matter with about 5% demonstrating very high tumor intensity (image 18). Only 10% of the lesions are hypo-intense (image 19).

Although some authors have found a correlation between the MR appearance and the histological subtype of the meningiomas, we could not confirm this. A high proportion of calcification and fibrous change most likely demonstrate lower signal intensities on T2 weighted images than do highly cellular or vascular meningiomas.

The presence of moderate to severe edema in 65% of syncitial meningioma confirms previous reports that there is a partial correlation between the degree of edema and the histological subtype.

Inhomogeneities within meningiomas can be due to calcification, with focal or diffuse hypointensity on T1 and T2 weighted images.

Intra- or peritumoral signal voids can reflect the hypervascularity of the tumor or its prominent venous drainage.

Cysts, either true cysts or pseudocysts are characterized by long T1 and long T2. In case of blockage of CSF drainage secondary to arachnoid adhesions around the tumor, the arachnoid lining can enhance and simulate peripheral cysts (image 20). The same is true for central necrosis.

In case of lipoblastic changes, short T1 and short T2 areas are seen within the meningioma (image 21).

Most meningiomas are surrounded by a low intensity rim, indicating its extra-axial location by the demonstration of an interface with the normal brain.

Hyperostosis is said to be less well demonstrated by MR. Nevertheless, thickening of the bone in the vicinity of a meningioma can be adequately visualized. After injection of gadolinium-chelates enhancement may be seen within the bone, indicating bone invasion.

After intravenous injection of Gd-chelates the enhancement of meningiomas is intense and homogeneous with the exception of focal calcifications, cystic or necrotic changes. A rather constant sign is the adjacent dural thickening and enhancement, the so-called dural tail (images 22 - 23). This probably represents reactive change, rather than tumor invasion. Although the sign is seen in most meningiomas, it has also been described in other benign lesions, such as sarcoidosis, acoustic neurinoma, chloroma and malignant lesions, such as lymphoma, metastases and extra-axial malignant tumors.

- Angiography (image 24)

Meningiomas are typically hypervascular, fed by dural vessels with a typical sunburst of enlarged dural feeders and an intense tumor blush.

The periphery of the lesion can be supplied by pial branches of the internal carotid artery.

Differential diagnosis

Differential diagnosis can be difficult from dural metastases and even peripheral parenchymal metastases. Peripherally located gliomas can mimic meningioma by the presence of a dural tail. Differential diagnoses includes hemangiopericytoma, hemangioma and fibrous tumor. In the cerebellopontine angle differentiation has to be made from schwannoma.

Case reports

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Meningioma

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