Meningioma

Meningiomas are extra-axial tumours and represent the most common tumour of the meninges. They are a non-glial neoplasm that originates from the meningocytes or arachnoid cap cells of the meninges, and are located anywhere that meninges are found, and in some places where only rest cells are presumed to be located. They Although they usually easily diagnosed, and are typically benign with a low rate of recurrence rate but rarelyfollowing surgery, there are a large number of histological variants with variable imaging features and in some instances more aggressive biological behaviour. 

A broad division of meningiomas is into primary intradural (which may or may not have secondary extradural extension) and primary extradural (rare) ¹⁸.  They can also be malignantclassified according to location (e.g. spinal, intraosseous, intraventricular etc.), by histological variants (e.g. clear cell, rhabdoid etc.) and by aetiology (e.g. radiation induced etc.). 

Typical meningiomas appear as dural based masses isointense to grey matter on both T1 and T2 weighted imaging, and demonstrate vivid contrast enhancement on both MRI and CT. There are Some of the aforementioned variants can, however, many variants some of which can vary vary dramatically in their imaging appearance.

A broad division of meningiomas is into primary intradural (which may or may not have secondary extradural extension) and primary extradural ¹⁸.  

This article is a general discussion of meningioma focusing on typical primary intradural meningiomas and focuses on the imaging findings of intracranial disease. For For spinal diseaseand primary extradural tumours refer to spinal meningioma and primary extradural meningioma articles respectively. Many of the  histological variants are also discussed separately. 

Epidemiology

Meningiomas are more common in women, with a ratio of 2:1 intracranially and 4:1 in the spine. Atypical and malignant meningiomas are slightly more common in males. They are uncommon in patients before the age of 40 and should raise suspicion of neurofibromatosis type 2 (NF2) when found in young patients.

Clinical presentation

Many small meningiomas are found incidentally and are entirely asymptomatic. Often they cause concern as they are mistakenly deemed to be the cause of vague symptoms, most frequently headaches. Larger tumours or those with adjacent oedema or abutting particularly sensitive structures can present with a variety of symptoms. Most common presentations include ⁸:

• headache: 36%
• paresis: 22%
• change in mental status: 21%
• focal neurological deficits

Meningiomas may also become clinically apparent due to complications dependent onmass effect their location including:

• convexity/parasagittalsupratentorial: 85-90% ⁸
  • parasagittal, convexities: 45%
    • seizures and hemiparesis
  • basisphenoidsphenoid ridge: 15-20%
  • olfactory groove / planum sphenoidale: 10%
    • anosmia (usually not recognised)
  • juxtasellar: 5-10%
    • visual field defectdefects
    • cavernous sinus: cranial nerve deficit(s)deficits
• frontalinfratentorial: anosmia (although often become very large before becoming symptomatic) 5-10% 
  • obstructive hydrocephalus
  • dural venous sinus invasion/dural venous sinus thrombosiscranial nerve deficits
• miscellaneous intradural: <5%
  • intraventricular meningioma
  • optic nerve meningioma (usually this occurs gradually and even occlusion is asymptomatic,
  • pineal gland
    • Parinaud syndrome
    • obstructive hydrocephalus

Occasionally transosseous or intraosseous involvement with collateral veins having time to enlarge)

It should be noted that although dural venous sinus invasion and occlusion is not infrequent, as this usually this occurs very gradually most are asymptomatic, as collateral veins have had time to enlarge. 

Pathology

Meningiomas are thought to arise from meningocytes or arachnoid cap cells, which themselves arise from pluripotential mesenchymal progenitor cells, which accounts for sometimes unusual location of primary extradural tumours ^18-19. 

Although the majority of tumours are sporadic, they are also seen in the setting of previous cranial irradiation and of course in patients with neurofibromatosis type II (NF2) (Merlin gene on Chromosome 22). Additionally meningiomas demonstrate oestrogen and progesterone sensitivity and may grow during pregnancy.

Grading

Grading of meningiomas follows the WHO classification for CNS tumours  and includes both usual histological features ( e.g. mitotic index) as well as a number of histological subtypes, some of which have been associated with more aggressive behaviour ^{7, 11, 23}:

• grade I: 'benign" (70%)
  • transitional (40%): mixed histology, typically containing meningothelial and fibrous components
  • meningothelial meningioma (17%)
  • fibrous meningioma (7%)
  • microcystic meningioma
  • psammomatous meningioma
  • angiomatous meningioma *
  • secretory meningioma
  • metaplastic meningioma
  • lymphoplasmacyte-rich meningioma
• grade II: "atypical" (30%)
  • clear cell meningioma
  • chordoid meningioma
  • atypical by histological criteria (29%) ¹¹
    • 4 to 19 mitoses per ten high-power fields
    • infiltration into brain parenchyma **
    • 3 or more of the following 5 histologic features: necrosis, sheet-like growth, small cell change, increased cellularity, prominent nucleoli
• grade III: "anaplastic" or "malignant" (~1%)
  • rhabdoid meningioma
  • papillary meningioma
  • anaplastic by histological criteria ¹¹
    • obvious malignant features similar to those seen in melanoma, carcinoma, or high grade sarcoma
    • 20 or more mitosis per ten high-power fields

* Haemangiopericytomas were, until 1993, considered angiomatous meningiomas, but are not classified as a separate entity; "Other neoplasms related to the meninges" according to WHO 2007 classification of CNS tumours. 

** It is important to note, when reading older literature, that in the WHO 2007 classification, infiltration into brain parenchyma of an otherwise "benign" grade I tumour was sufficient to designate it a grade II tumour. As such, incidence of grade II tumours increased to ~30% ¹¹.

Macroscopic

In general, there are two main macroscopic forms easily recognized on image studies:

• globose: rounded, well defined dural masses, likened to the appearance of a fried egg seen in profile (the most common presentation)
• en plaque: extensive regions of dural thickening

Histological subtypes

The cut surface reflects the various histologies encountered, ranging from very soft to extremely firm in fibrous or calcified tumors. They are also divided histologically into ^3,8usually light tan in colouring, although again this will depend on histological subtypes. 

Radiographic features

In addition to histological variants, many of which have 'atypical' imaging appearances, a number of 'special examples' of meningiomas are best discussed separately. These include:

• meningothelial
• fibroblastic: abundant reticulum and 'stout' collagen
• transitional: whorl formation
• syncytial: poorly formed polygonal cells arranged in lobules
• angioblastic: now classified separately as a haemangiopericytoma
• clear cell: high rate of local recurrence ⁶
• psammomatous
• microcystic¹²
• secretory
• chordoid
• lymphoplasmacyte-rich
• metaplastic
• papillary: has a high rate of local recurrence ⁸
• rhabdoid: aggressive and have a very poor prognosis
• mixed type

Variants

• intraosseous meningioma: sclerotic or lucent
• rarely degeneration into:
  • cysticburnt out meningioma
  • cystic meningiomas
  • intraosseous meningioma
  • meningioma with sarcomatous degeneration
  • meningioma with fatty degeneration
• intraventricular meningioma
• optic nerve sheath meningioma (WHO II): have an increased mitotic rate, corresponds to ~7% of all meningiomas ⁴ and have a greater tendency to recur
  • clear cell meningioma: have up to a 60% recurrence rate and occur in younger patients ⁶
• malignant meningioma (WHO III)
  • uncommon accounting for ~2.5% of all meningiomas ⁴ and demonstrates intraparenchymal invasion, rapid growth, and a high mitotic rate or sarcomatous degeneration
  • like atypical meningiomas they too demonstrate restricted diffusion on DWI
  • they are thought to originally be standard meningiomas which undergo malignant degeneration
  • papillary subtype appears to do so more frequently than other
• radiation induced meningioma
  • more frequently multiple, and typically occur ~35 years after radiotherapy
  • meningiomas are a much more frequent complication of radiotherapy compared to sarcomas or gliomas
• microcystic meningioma: rare, and are typically very high on T2 weighted imaging and are more commonly associated with atypical features and adjacent brain oedema ¹²

Grading

Generally follows the WHO classification for CNS tumours^7,11:

• WHO I: meningioma ~88-95 %
• WHO II: atypical meningioma (atypical, clear cell, chordoid) ~ 5-6%
• WHO III: malignant meningioma (rhabdoid, anaplastic, papillary) ~1%
• WHO IV: meningioma with sarcomatous degeneration, extremely rare ¹¹

There is also a Simpson grade forThe remainder of this section focuses on more typical imaging appearances of runofthemill meningiomas.

Plain radiography

Plain films no longer have a role in the diagnosis or management of meningiomas. Historically a number of features were observed, including:

• enlarged meningeal artery grooves
• hyperostosis or lytic regions
• calcification
• displacement of calcified pineal gland / choroid plexus due to mass effect

CT

CT is often the first modality employed to investigate neurological signs or symptoms, and often is the modality which detects an incidental lesion:

• non-contrast CT
  • 60% slightly hyperdense to normal brain, the rest are more isodense
  • 20-30% have some calcification ⁸
• post-contrast CT
  • 72% brightly and homogeneously contrast enhance ⁸, less frequent in
  • malignant or cystic variants demonstrate more heterogeneity / less intense enhancement {ref needed}
• hyperostosis (5%) ²³
  • typical for meningiomas that abut the base of the skull
  • need to distinguish reactive hyperostosis from:
    • direct skull vault invasion (eventually involves the outer table too)by adjacent meningioma
    • primary intraosseous meningioma
• enlargement of the paranasal sinuses (pneumosinus dilatans) has also been suggested to be associated with anterior cranial fossa meningiomas ²⁰
• lytic / destructive regions: are seen particularly in higher grade lesions
• pneumosinus dilatanstumours, but should make one suspect alternative pathology (e.g. haemangiopericytoma or metastasis) {ref needed}

MRI

As is the case with most other intracranial pathology, MRI is the investigation of choice for the diagnosis and characterisation of meningiomas. When appearance and location is typical, the diagnosis can be made with a very high degree of certainty. In manysome instances, however, the appearances are atypical and careful interpretation is needed to make a correct preoperative diagnosis.

Meningiomas typically appear as extra-axial masses with a broad dural base. They are usually homogeneous and well circumscribed, although many variants are encountered.

Signal characteristics of typical meningiomas include:

• T1
  • isointense: ~60 to grey matter (60-90%) ^{3,8, 13}
  • somewhat hypointense: 10-40% compared to grey matter (10-40%): particularly fibrous, psammomatous variants
• T1 C+ (Gd): usually intense and homogeneous enhancement
• T2
  • isointense: to grey matter (~50%) ^3,8,13
  • hyperintense: 35 to grey matter (35-40%)
    • usually correlates with soft texturestexture and hypervascular tumours ¹³
    • very hyperintense lesions may represent the microcystic variant ¹²
  • hypointense: 10 to grey matter (10-15%): compared to grey matter and usually correlates with harder texture and more fibrous and calcified contents
• DWI/ADC: atypical and malignant subtypes may show greater than expected restricted diffusion although recent work suggests that this is not useful in prospectively predicting histological grade ^15-16
• MR spectroscopy: Usually it does not play a significant role in diagnosis but can help distinguish meningiomas from mimics. Features include:
  • increase in alanine (1.3-1.5 ppm)
  • increased glutamine/glutamate
  • increased choline (Cho): cellular tumour
  • absent or significantly reduced N-acetylaspartate (NAA): non-neuronal origin
  • absent or significantly reduced creatine (Cr)
• MR perfusion: it has good correlation between volume transfer constant (k-trans) and histological grade {ref needed}

HelpfulA number of helpful imaging signs includehave been described, including:

• CSF vascular cleft sign, which is not specific for meningioma, but helps establish the mass to be extra-axial; loss of this can be seen in grade II and grade III which may suggest brain parenchyma invasion
• dural tail seen seen in 60-72%²(note that a dural tail is also seen in other processes)
• sunburst or or spokewheel appearance appearance of the vessels

Meningiomas

Oedema

Oedema canmay be seenpresent adjacent to some meningiomas, variable in amount and correlates with size, rapid growth, location (convexity and parasagittal > elsewhere), and invasion in the case of malignant meningiomas. The underlying mechanisms for this uncertain and likely relates to a number of mechanisms including: {ref needed}

Angiography (DSA)

• Catheter angiography is rarely now of diagnostic use, but rather is performed for preoperative embolisation to reduce intraoperative blood loss. This is especially useful for skull base tumours, or those thought to be particularly vascular (e.g. microcystic variants or those with very large vessels). Particles are favoured typically 7-9 days prior to surgery. {ref needed}

  Meningiomas can have dual blood supply. The majority of tumours are predominantly supplied by meningeal vessels; these are responsible for the sunburst or spokewheel pattern observed on MRI / DSA. Some tumours also have significant pial supply to the periphery of the tumour. {ref needed}

  A well known angiographic sign of meningiomas is the mother-in-law sign:, in which the tumour contrast blush "comes early, stays late, and is very dense", tumour blush

• dual blood supply from both
  • pial (ICA) supplies periphery
  • meningeal vessels (ECA) supplies core
• spoke wheel appearance
• dense venous filling
• preoperative embolisation: especially skull base, particles are favoured; 7-9 days prior to surgery

Treatment and prognosis

Treatment is usually with surgical excision. If only incomplete resection is possible (especially at the base of skull) then external-beam radiation therapy can be used ⁸.

The Simpson grade correlated the degree of surgical resection completeness with with symptomatic recurrence.

Recurrence rate varies with grade and length of follow-up ^{8, 21}

• WHOgrade I: meningioma, 6.9 = 7-25%
• WHOgrade II: atypical meningioma, 34.6 = 29-52%
• WHOgrade III: malignant meningioma, 72.7%

• 5-year follow up: 5%
• 10-year follow up: 5%
• 32-year follow up: 5 = 50-94%

Metastatic disease is rare, but has been reported ⁸.

History and etymology

The term "meningioma" was first introduced by Harvey Cushing, neurosurgeon, in 1922 ^9,23.

Differential diagnosis

The differential diagnosis largely depends on location, and generally includes other dural masses as well as some location specific entities. 

The main dural masses to consider include: 

• haemangiopericytoma
  • more aggressive often destroying bone
  • extensive peripheral vascularity 
  • more microlobulation
• dural metastases (e.g. breast cancer)
• for other less common masses differentials see  dural masses

Specific location differentials include:

• cerebellopontine angle
  • acoustic schwannoma
• parasellar region
  • pituitary macroadenoma
  • craniopharyngioma
• elsewherebase of skull
  • haemangiopericytoma
  • granuloma
    • sarcoidosis
    • tuberculosis
  • idiopathic hypertrophic pachymeningitis
  • extramedullary haematopoeisis
  • chondrosarcoma
  • chordoma

In the setting of hyperostosis consider:

• Paget's disease
• fibrous dysplasia
• sclerotic metastases (e.g. prostate and breast carcinoma)