Meningiomas are primary brain tumours and are documented as forming 10-20% of all intracranial tumours. They are typically benign (>90%), originating from the cranial meninges (the membrane-like structures that surround the brain and spinal cord), which consist of three layers: the dura, arachnoid and pia mater (figure 1). They are extra-axial tumours which arise from the arachnoid cells on the inner surface of the dura mater and may come from any part of the meninges over the surface of the brain. Meningiomas can occur virtually anywhere in the cranial cavity, may be multiple, and typically have a definite dural attachment.
Epidemiology and aetiology
Meningiomas are basically adult tumours, with peak occurrence between 40 and 60 years of age, and the highest incidence being in middle-aged women – there is a 2:1 female-to-male ratio in Caucasians(#1). Some well-known predisposing factors in the development of meningiomas are exposure to radiation and genetic disorders. In the paediatric population, they account for 1% to 3% of all paediatric intracranial tumours(#2) with many resulting from neurofibromatosis (especially NF-2).
Meningiomas compress the brain from outside and are slow growing, therefore a significant percentage are discovered incidentally (<10% are symptomatic). The common presenting symptoms of many primary brain tumours are raised intracranial pressure, epilepsy and a progressive neurological deficit, but because meningiomas can occur virtually anywhere in the cranial cavity, a broad range of symptoms may be observed, including: headaches and focal seizures (convexity meningiomas), nausea and gait disturbances (tentorial meningiomas with secondary hydrocephalus), and visual disturbances and exophthalmos (sphenoid wing meningiomas).
A large percentage of symptomatic meningiomas show perifocal oedema at diagnosis. Malignant meningiomas are almost invariably associated with peri focal oedema.
The World Health Organisation (WHO) divides meningiomas into three basic categories based on histology:
• Grade I common or typical ‘benign’ meningioma – have a low risk of recurrence and aggressive growth
• Grade II atypical meningioma
• Grade III anaplastic (malignant) meningioma(#3)(#4).
Grade I has a recurrence rate of 7%, while grades II and III are associated with a higher risk of recurrence (38% and 78% respectively) and shorter survival times.
The prognosis for the patient is determined as much by the location and resectability as it is by histological type(#5). Tumour location is one of the most important features regarding therapy because it is a determining factor in predicting the success of surgical intervention.
Detection is important because, since the great majority of meningiomas are benign, it is the only tumour that can be completely and reliably removed by surgery. This is possible because there is a plane of cleavage between the brain tissue and tumour and, if the tumour is reasonably accessible and unattached to the dural venous sinuses, eg, sagittal sinus, then complete removal of the tumour is possible, thereby providing a good prognosis for the patient, although some neurological deficit may be present following surgery.
Most meningiomas are extra-axial dural-based lesions, with 90% supra-tentorial. The percentage of appearance in the cranial cavity is shown in table 1.
Table 1: Tumour location of meningiomas.
|2%||Other (intraventicular/pineal region/optic nerve sheath)|
Computed tomography (CT)
CT scanning is the primary method of investigation in the diagnosis of meningiomas because patients generally present with non-specific symptoms and the majority are discovered incidentally. A pre- and post-contrast enhanced CT scan will anatomically locate the presence of the majority of these brain tumours. The images will also demonstrate if the tumour is confined or if there are any effects of the tumour on surrounding structures (midline shift), cerebral oedema, ventricular dilatation or calcification.
Meningiomas typically appear as circumscribed round or smoothly lobulated masses that abut the dural surface. On non-contrast CT, they appear as a homogeneous, slightly hyperdense mass in relation to the brain parenchyma. This is due to dense cellularity and a relatively small amount of intra-cellular water and the presence of calcified psammoma (sandlike) bodies within the tumour.
Characterisation of tumours can be aided by the pattern of enhancement. Following administration of intravenous contrast material, intense rapid and homogenous enhancement in approximately 80-90% of tumours is observed (figure 4). Contrast enhancement does not result from a defect in the blood brain barrier (which is absent because meningiomas are extra-axial) but is based on tumour vascularity and contrast extravasation into the interstitium.
Calcification can be seen in 20-25% of tumours and can be diffuse or focal. Lesions that are significantly calcified on unenhanced CT show little to no density changes after contrast administration. Meningiomas may lead to hyperostosis of the calvarium underlying the dural base. These characteristic changes may be noted in the adjacent bone and can be demonstrated using high resolution CT and bone windows. Hyperostosis has been found in 18-45% of cases (figure 5). CT can help to evaluate bony involvement and the presence of calcifications, which can be seen in 30% of benign meningiomas, but are rare in malignant meningiomas.
Although benign tumours can have associated oedema, it is much more common in malignant meningiomas (figure 6). Several imaging features such as cystic changes, intracranial haemorrhage, focal or diffuse irregular contour, poorly defined margins and ring enhancement are considered unusual or atypical.
Magnetic resonance imaging (MRI)
MRI is considered to be the gold standard when undertaken pre- and post-gadolinium contrast. MRI is better for defining the relation to the meninges and showing peri-tumoral meningeal enhancement. On MRI, meningiomas are typically isodense, dural-based masses that often show homogeneous enhancement. T1 images are often hypo- or isointense; T2 images often reveal isointense or slightly hyperintense masses, and gadolinium increases the signal intensity of meningiomas. Fast fluid attenuation inversion recovery (FLAIR) sequences (figure 7) may be useful for defining the exact extent of any oedema present and when examining the brain-meningioma interface.
The presence of a dural tail helps in the diagnosis of a meningioma (figure 8 ). MRI is of value in showing anatomical relationships and in demonstrating vessel patency. The traditional advantage of multiplanar capacity is now rivalled with MDCT.
Although the imaging appearance of meningiomas is characteristic, some mimic benign tumours such as schwanomas and others resemble malignant neoplasms such as anaplastic astrocytoma. Similarly, the imaging appearance of metastases can be identical to meningiomas and the meninges can also give rise to other tumours, most notably melanin containing lesions(#6) eg, malignant melanoma (figure 9) .
Meningiomas can reoccur after a complete resection and a low grade tumour does not exclude recurrence. Recurrence-free survival rate after a complete resection is approximately 90% or more at five years, 80% at ten years and 70% at 15 years. MRI should be used in the routine follow-up of meningiomas because it will demonstrate contrast enhancement of the recurrence, although dural enhancement is frequently present as a normal post-operative finding. For patients not suitable for MR, a follow-up CT can detect dural changes only to the extent that a definite soft tissue mass is present.
CT will remain the mainstay in the screening of patients presenting with many symptoms, due to availability and speed. MRI will provide follow- up imaging for surgical planning and is the preferred modality for follow- up and detecting recurrence. Cross-sectional imaging can identify, locate meningiomas and can suggest a differential diagnosis. However, histological diagnosis following surgery remains the gold standard.
About the Authors
George Michalopoulos is an MSc student at the University of Bradford. Linda Clarkson is lecturer and postgraduate CT course co-ordinator at the University of Bradford.
Jennifer Beer, PG Cert in cranial CT reporting 2008, University of Bradford.
1. Perry A, Louis DN, Scheithauer BW, Budka H, Von Deimling A, Meningeal Tumours. Louis DN, Ohgaki H, Wiestler OD, Cavenee WK: World Health Organization Classification of Tumours of the Central Nervous System Lyon, IARC, 2007. 2. Osborne A.G. Diagnostic Neuroradiology; A Text Atlas. (1994) Mosby 3. Drevelegas A. Imaging of brain tumours with histological correlations. (2002) Springer. 4. Tsitouridis I, Meningioma – Radiological examination –Imaging problems. (2004) Dodoni. 5. Hosten N, Liebig T. CT of the Head and Spine. (2002). Thieme. 6. Mawhinney RR, Buckler JH, Holland IM, Worthington BS. The Value of Magnetic Resonance Imaging in the Diagnosis of Intracranial Meningiomas. Radiol, 1986;37(5):429-39