Meningioma

the most common nonglial primary intracranial tumour (15-20% of all primary brain tumours). They arise from specialized meningothelial cells mainly in arachnoid granulations although they may develop from any meningeal structure or from ectopic cell rests of meningeal derivation. A genetic predisposition mediated by chromosome 22 is recognized as well as a sex hormone influence. These tumours mostly affect adults with a strong female predilection (2:1 over males) and are rare in childhood; when they do occur in this age group they are frequently associated with NF2 (see neurofibromatosis) and have multiple locations. Sporadic multiple meningiomas has been also reported in the older age population with varying incidence (1-8%).

The classic histological classification includes: meningotheliomatous (syncytial), fibrous, transitional and angioblastic (or better "angiomatous") meningioma. A more recent classification by the World Health Organization (WHO) divides meningiomas into: typical "benign", atypical and anaplastic, depending on proliferative activity and aggressiveness. Gross pathology shows two basic configurations: an oval or lobulated mass with a broad-based attachment to the dura ("globular"); or a flattened, linear lesion extending along the dura ("en plaque"). Several factors produce tumour heterogeneity: tumoral vascularity, cystic foci of degeneration developing from coalescence of microcystic component and calcifications. The marked prevalence of each factor determines "angioblastic", "cystic" and "calcific" meningiomas respectively. Occasionally trapped arachnoid spaces are present at the margins of the tumour. Meningiomas frequently cause hyperostosis of the inner table and can extensively penetrate the skull, causing marked thickening of the bone and osteoblastic reaction. Bone erosion is rare. Invasion of adjacent dural sinuses is also frequent. Brain oedema develops in approximately 50% of meningiomas.

Most meningiomas are extra-axial dural-based lesions (except intraventricular, optic nerve sheath and pineal lesions). Ninety percent are supratentorial. They typically grow along intradural venous sinuses, at the confluence of dural septa (falcotentorial junction) or at the confluence of cranial sutures (pterion). Their preferential locations are as follows: parasagittal or attached to the sagittal sinus (25%); the convexity (20%); sphenoid wings (15-20%) called clinoidal if located in the inner middle third of the sphenoid ridge and pterional those in the outer portion; parasellar (5-10%) including tuberculum sellae, cavernous sinus, dorsum and diaphragma sellae; olfactory grooves (210%). Ten percent are infratentorial and arise from the petrous bone in the cerebellopontine angle, the clivus, the tentorium and the tentorial incisura. Rarely meningiomas are intraventricular, from the choroid plexus or the tela choroidea, or arise from the optic nerve sheath and in the pineal region.

Symptomatology is variable depending on the size and location of tumours.

Imaging

Hyperostosis and rarely bone erosion are findings on plain films of meningiomas. CT and MR are essential for showing the highly specific signs of an extra-axial origin (dural-based margin, bony hyperostosis and/or invasion, anatomical interfaces between the tumour and the brain surface such as pial vascular structures, CSF clefts, and dural margins) and for establishing differential diagnoses. CT without and with contrast detects about 85-95% of meningiomas, respectively.

CT shows a well marginated oval or lobulated mass, attached to the dura at an obtuse angle.

Most lesions are homogeneously hyperdense, some isodense and a few hypodense (lipoblastic and xanthomatous variants). Calcifications are diffuse or focal (20–25%) but some meningioma are diffusely calcific. Cysts appear as hypodense nonenhancing areas. Variable peripheral oedema is seen. Marked enhancement follows contrast injection. Hyperostosis is easily detectable.

MRI (Fig. 1) (Fig. 2) is superior at evaluating interfaces separating the tumour from the brain: displaced pial vessels and draining veins appear as punctate and curvilinear signal voids on all sequences; CSF clefts parallel the signal of subarachnoid spaces; the dural interface appears as a low-intensity rim on all imaging sequences. Most meningiomas are iso- or mildly hypointense relative to grey matter on T1-weighted images. A more variable and inhomogeneous signal is shown on T2-weighted images, due to intrinsic tumour vascularity (serpiginous or "sunburst" hypointensities within the mass on both the T1- and T2-weighted images), calcifications (irregular regions of hypointensity on both T1- and T2-weighted sequences) and cystic foci.

The degree of oedema is variable. Intratumoral calcifications are less well defined with MRI than with CT. Nearly all meningiomas strongly enhance after contrast injection, unless in cystic and calcific portions. Contrast is essential for delineating the full extension of en plaque lesions and infiltration around the cranial nerves. Sixty percent of meningiomas have a "dural tail" of enhancement.

Venous sinus invasion, most common in parasagittal meningiomas, is recognizable as partial or complete obliteration of the sinus flow void. MR venous angiography well documents the thrombosed sinus. Arteries encased by parasellar and cavernous sinus meningiomas have a reduced diameter but remain hypointense on all imaging sequences.

Transdural invasion can be demonstrated through the tentorium cerebelli, falx cerebri or cavernous sinus. Bony hyperostosis is also well demonstrated with MRI as are primarily intraosseous meningiomas. Angiography shows a tumoral blush with supplies from meningeal arteries and pial branches of the anterior, middle and posterior cerebral arteries.

GS-NC

NC