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Meningiomas are extra-axial tumors and represent the most common tumor 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.
Although they are usually easily diagnosed and are typically indolent with a low rate of recurrence following surgery, there are 15 subtypes with variable imaging features and, in some instances, more aggressive biological behavior and higher grade.
Typical meningiomas appear as dural-based masses isointense to grey matter on both T1 and T2 weighted imaging enhancing vividly on both MRI and CT. Some of the subtypes can vary dramatically in their imaging appearance.
This article is a general discussion of meningioma focusing on typical primary intradural meningiomas and the imaging findings of intracranial disease.
When describing meningiomas a variety of terms can be used to more accurately describe these common tumors.
Most commonly they are either classified according to the histological subtype (e.g. rhabdoid or papillary etc.), location (e.g. skull base, spinal, intraosseous, intraventricular, etc.), and by etiology (e.g. radiation-induced, etc.).
A broad division of meningiomas into primary intradural (which may or may not have a secondary extradural extension) and primary extradural is also used, although the latter is rare accounting for only 1-2% of cases 27. Ectopic primary meningiomas include tumors riding in the head and neck, orbit, nose, paranasal sinus, oropharynx and even more remotely (e.g. lung).
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 when found in young patients.
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 tumors or those with adjacent edema 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%
Meningiomas may also become clinically apparent due to mass effect depending on their location:
- supratentorial: 85-90% 8
- infratentorial: 5-10%
- obstructive hydrocephalus
- cranial nerve deficits
- miscellaneous intradural: <5%
Although dural venous sinus invasion and occlusion does occur, it usually occurs very gradually. Therefore most cases of venous invasion are asymptomatic as collateral veins have had time to enlarge.
Meningiomas are thought to arise from meningocytes or arachnoid cap cells, which themselves arise from pluripotent mesenchymal progenitor cells, which accounts for the unusual location of primary extradural tumors 18,19.
Although the majority of tumors are sporadic, they are also seen in the setting of previous cranial irradiation and of course in patients with neurofibromatosis type II (Merlin gene on Chromosome 22). Additionally, meningiomas demonstrate estrogen and progesterone sensitivity and may grow during pregnancy.
In the 5th Edition (2021) WHO classification of CNS tumors 15 subtypes of meningioma are recognized, including:
- angiomatous meningioma
- chordoid meningioma: grade 2
- clear cell meningioma: grade 2
- fibrous meningioma (7%)
- lymphoplasmacytic-rich meningioma
- meningothelial meningioma (17%)
- metaplastic meningioma
- microcystic meningioma
- papillary meningioma: usually more aggressive behavior
- psammomatous meningioma
- rhabdoid meningioma: usually more aggressive behavior
- secretory meningioma
- transitional meningioma (40%): mixed histology, typically containing meningothelial and fibrous components
Meningiomas are graded from grade 1 to 3 based on histological features (e.g. mitotic index) some histological subtypes (e.g. chordoid meningiomas and clear cell meningiomas) and molecular features (see below) 7,11,23,33.
An important change in the 5th Edition (2021) WHO classification of CNS tumors is that the identification of some histological subtypes (e.g. papillary meningiomas and rhabdoid meningiomas) no longer is sufficient to denote a higher grade 33. Also, as is the case with other tumor groups, the term "anaplastic" as well as "atypical" and "malignant" have been retired in favor simply of grades 1 - 3 33.
Brain invasion as a stand-alone feature remains controversial. In recent prior editions of the WHO classification (e.g. 2016) if a meningioma (regardless of histology) demonstrated brain invasion it was designated as grade 2 as it was believed to denote a poorer prognosis with higher likelihood or recurrence 7. The current (5th Edition) has backed away from this dogmatic recommendation 33,34. Nonetheless, overt brain invasion remains a concerning feature.
In general, there are two main macroscopic forms easily recognized in imaging 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
The cut surface reflects the various histologies encountered, ranging from very soft to extremely firm in fibrous or calcified tumors. They are usually light tan in coloring, although again this will depend on histological subtypes.
Increasingly molecular markers are being incorporated into the diagnosis and grading of meningioma subtypes 33.
- SMARCE1 mutations: clear cell subtype
- BAP1 mutations: papillary and rhabdoid subtypes
- KLF4/TRAF7 mutations: secretory subtype
- TERT promoter mutation: grade 3
- homozygous deletion of CDKN2A/B: grade 3
- H3K27me3 loss of nuclear expression: worse prognosis
- methylome profiling: prognostic subtyping
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:
- burnt out meningioma
- cystic meningiomas
- intraosseous meningioma
- intraventricular meningioma
- optic nerve sheath meningioma
- radiation-induced meningioma
The remainder of this section focuses on more typical imaging appearances of run-of-the-mill meningiomas.
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
- displacement of calcified pineal gland/choroid plexus due to mass effect
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
- malignant or cystic variants demonstrate more heterogeneity/less intense enhancement
hyperostosis (5%) 23
- typical for meningiomas that abut the base of the skull
- need to distinguish reactive hyperostosis from:
- direct skull vault invasion 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 tumors but should make one suspect alternative pathology (e.g. hemangiopericytoma or metastasis) ref
As is the case with most other intracranial pathology, MRI is the investigation of choice for the diagnosis and characterization of meningiomas. When appearance and location are typical, the diagnosis can be made with a very high degree of certainty. In some 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. It seems that the signal intensity of meningiomas on T2-weighted images correlates with the histological subtypes 29.
Signal characteristics of typical meningiomas include:
- T1 C+ (Gd): usually intense and homogeneous enhancement
- usually isointense to grey matter (~50%) 3,8,13
- hyperintense to grey matter (35-40%)
- hypointense to grey matter (10-15%): compared to grey matter and usually correlates with harder texture and more fibrous and calcified contents
- DWI/ADC: grade 2 and 3 tumors may show greater than expected restricted diffusion although this is not universally useful in prospectively predicting histological grade 15,16
- MR spectroscopy: usually does not play a significant role in diagnosis but can help distinguish meningiomas from mimics. Features include:
- MR perfusion: good correlation between volume transfer constant (k-trans) and histological grade 28
MR tractography: allows the identification of white matter tracts adjacent to the meningioma
- this may aid in preoperative planning for meningioma resection by allowing planning of a safer access route that would result in less residual functional iatrogenic deficits 30
Helpful imaging signs
A number of helpful imaging signs have been described, including:
- CSF 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 is seen in 60-72% 2 (note that a dural tail is also seen in other processes)
- sunburst or spoke-wheel appearance of the vessels
- white matter buckling sign
- arterial narrowing
- typically seen in meningiomas which encase arteries
- useful sign in parasellar tumors, in distinguishing a meningioma from a pituitary macroadenoma; the latter typically does not narrow vessels
More than half of the meningiomas demonstrate a variable amount of vasogenic edema in adjacent brain parenchyma 24. Correlation between age, gender, tumor size, rapid growth, location (convexity and parasagittal > elsewhere), histologic type, and invasion in the case of malignant meningiomas have been suggested in literature but not yet confirmed. Although in general, the presence of severe adjacent edema is considered more compatible with aggressive meningiomas, in some histologically benign types such as secretory type, edema can be disproportionately larger than the small tumor size.
The underlying mechanism is most likely multifactorial however it has been shown that there is a strong association between the presence and severity of the peritumoral vasogenic edema (i.e. edema index) and expression of the vascular endothelial growth factor (VEGF) or expression of CEA and CK 17,25.
List of some of the proposed underlying mechanisms are:
- venous stasis/occlusion/thrombosis
- compressive ischemia
- aggressive growth/invasion
- parasitization of pial vessels
- histologic subtype: secretory meningioma 25
- vascular endothelial growth factor (VEGF): produced within the meningioma that enters the adjacent parenchyma
- expression of CEA and CK
Catheter angiography is rarely now of diagnostic use but rather is performed for preoperative embolization to reduce intraoperative blood loss and alleviate resection of a tumor. This is especially useful for skull base tumors, or those thought to be particularly vascular (e.g. microcystic variants or those with very large vessels). Particles are favored typically 7-9 days prior to surgery although they are not free of complication, particularly one study showed a high prevalence of complications associated with particles smaller than 45-150 μm, so risks and benefits should be thoroughly assessed 26.
Meningiomas can have a dual blood supply. The majority of tumors are predominantly supplied by meningeal vessels; these are responsible for the sunburst or spoke-wheel pattern observed on MRI/DSA. Some tumors also have a significant pial supply to the periphery of a tumor.
A well known angiographic sign of meningiomas is the mother-in-law sign, in which the tumor contrast blush "comes early, stays late, and is very dense".
Treatment and prognosis
Treatment is usually with surgical excision. If only incomplete resection is possible (especially at the base of the skull) then external-beam radiation therapy (or even brachytherapy) can be used 8,31. Radiation has been shown to improve local control and prolongs overall survival 35.
No widespread chemotherapeutic/systemic therapy has been proven to be efficacious although some mTOR inhibitor and antiangiogenic treatments show promise 36.
The Simpson grade correlates the degree of surgical resection completeness with symptomatic recurrence rate which also varies with grade and length of follow-up 8,21. Metastatic disease is rare but has been reported 8.
History and etymology
The term "meningioma" was first introduced by Harvey Cushing, a renowned American neurosurgeon, in 1922 9,23.
The differential diagnosis generally includes other dural masses as well as some location-specific entities.
The main dural masses to consider include:
- solitary fibrous tumors of the dura
- more aggressive often destroying bone
- extensive peripheral vascularity
- more microlobulation
- dural metastases (e.g. breast cancer)
- for other less common differentials see dural masses
Specific location differentials include:
- cerebellopontine angle
- pituitary region
- base of the skull
In the setting of hyperostosis consider:
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