Meningioma comprises about one fourth of all primary tumors of the central nervous system (CNS). It is the most common primary intracranial neoplasm and the most diversified in histologic patterns among all primary tumors of the CNS. Meningiomas, as defined by the World Health Organization (WHO), are "meningothelial (arachnoid) cell neoplasms, typically attached to the inner surface of the dura mater," and these tumors fall into WHO grades I, II, and III. 
Meningothelial (arachnoidal) cells are believed to be the cell of origin of meningiomas. Based on studies in birds, the telencephalic leptomeninges arise from the neural crest (neuroectoderm) and the posterior brain, and the spinal cord arises from the mesoderm.  The arachnoid cells have several proposed functions, including acting as a structural barrier with cellular wrapping/ensheathing, acting as a conduit for cerebrospinal fluid (CSF) drainage/absorption into dural sinuses/veins (arachnoid villi), epithelial-like/secretory functions, monocytelike functions, trophic support and byproduct detoxification for glial and neuronal cells, and participation in reactive/reparative processes. 
The histologic patterns and biologic spectrum of meningiomas partially reflect the aforementioned biologic functions and embryogenesis. Both non-neoplastic meningothelial cells and meningiomas possess mixed features of epithelial and mesenchymal cells. In meningiomas, one feature may be dominant over the other, and this phenomenon partly contributes to the rich diversification of histologic patterns in these tumors.
Although some of the observed variants, such as chondroid meningiomas, clear-cell meningiomas, papillary meningiomas, and rhabdoid meningiomas, are associated with unfavorable prognoses, the other histologic patterns are not indicators of unfavorable biologic behavior. Some of the uncommon histologic patterns, such as the lymphoplasmacyte-rich meningioma, may raise concern for lymphoproliferative diseases. Anaplastic meningiomas often shed most of the obvious features of meningiomas, and their diagnosis may be difficult.
With all the above features, it is not difficult to imagine that some meningiomas can be true imposters, histologically mimicking other entities. Although the diagnosis is often straightforward, it can also be challenging.
In the United States, meningiomas comprise about 32% of all primary intracranial tumors,  with an annual incidence of 5.2 per 100,000 population.  These tumors are twice as common in women, and there is a regional variation. The overall incidence of meningioma in Norway is 1.5 per 100,000 population among men and 2.8 in women.  In comparison, the incidence in Italy is 13 per 100,000 population.  Patients with multiple meningiomas generally comprise less than 10% of cases. Most meningiomas are benign. In general, atypical meningiomas and anaplastic meningiomas comprise less than 10% of all meningiomas. 
Meningioma is essentially a tumor of adulthood, with a peak incidence in the sixth decade of life.  There is a definite predilection for women, with an average male-to-female ratio of 1:2.2.  Tumors arising in the spinal cord have a distinctly high incidence in women. Atypical and anaplastic meningiomas, however, show a male predominance.  Childhood meningiomas occur more often in males. [8, 9] Meningiomas associated with neurofibromatosis type 2 (NF2) tend to occur in younger individuals and with equal distribution between males and females.
With the exception that papillary meningiomas are more common in children, meningiomas are rather uncommon in children and almost never occur in infants. When these tumors occur in children, however, they are more often infratentorial, intraventricular, or intraparenchymal than in adults. They also tend to be more aggressive, with an increased frequency of recurrence. [10, 11, 12] Up to 25% of childhood cases of meningiomas are associated with neurofibromatosis type 1 (NF1) or NF2. Tumors associated with NF2 may also have a more aggressive course. 
With the close resemblance between meningiomas and meningothelial cells, these cells are believed to be the cell of origin of meningiomas. This would also explain the high incidence of meningiomas around the sagittal sinus, which has a high concentration of meningothelial cells.
Although meningiomas commonly arise in locations where meningothelial cells are found, the embryonic origin of intraventricular and pulmonary meningiomas are intriguing. Intraventricular meningiomas probably arise from the meningothelial cells of the tela choroidea, where there is an arachnoidal invagination into the stromal base of the choroid plexus. During early embryonic development, the tela choroidea represents a portion of the pia and, together with the adjacent ependyma, forms the roof of the diencephalon. Minute meningothelial pulmonary nodules (MMPNs) [24, 25] are small perivascular nodules that are histologically and ultrastructurally identical to meningothelial cells of the CNS. These nodules probably represent the origin of primary meningiomas in the lung. Developmental lesions such as heterotopia of meningothelial cells in the scalp are believed to be the origin of meningiomas under the scalp and skin of the head. 
Similar to other tumors, the risk factors for the development of meningiomas can be divided into those with clearly defined genetic etiology and those that are attributed to environmental and other nongenetic factors. The etiology of most meningiomas, however, remains unclear.
As noted above, meningiomas are more common in women, and the female-to-male ratio has increased over the past several decades. [5, 27] Rapid growth of meningiomas in pregnant women has also been well documented. [28, 29] These increases suggest that the more widespread use of hormonal contraceptives may contribute to the tumorigenesis of meningiomas. In tumors obtained from the first operation, 88%, 40%, and 39% of meningiomas are positive for progesterone, estrogen, and androgen receptors, respectively. WHO grade I meningiomas have been reported to have significantly higher incidences of estrogen, progesterone, and androgen receptors than higher-grade meningiomas. However, differences in sex hormone receptor expression alone may not explain the observed increase in incidence in women. 
NF2 gene (merlin) on chromosome 22  and the related cytoskeleton element gene DAL-1/4.1B (EPB41L3) [32, 33] on chromosome 18 are the two genes known to be associated with meningiomas. Genetically, meningiomas can be subdivided into 3 major genetic groups. The sporadic type, the familial type not related to NF2, and the familial type related to NF2. The sporadic type and familial types not related to NF2 may have different genetic mechanisms. [34, 35] There is a strong relationship between NF2 and the development of both meningiomas and meningioangiomatosis. [31, 36] These patients often develop multiple meningiomas. In contrast, patients with NF1 are not at an increased risk for the development of meningiomas.
Radiation at low, moderate, and high doses is a well-described risk factor for the development of meningiomas. [37, 38, 39] The implicated radiation exposure range spans from dental radiography, low-dose irradiation to the scalp for tinea capitis, radiation therapy for tumors in the head and neck region, and exposure to atomic explosions in Hiroshima and Nagasaki. The mean interval for tumor development is 35, 26, and 19-24 years, respectively.  Radiation-induced meningiomas tend to occur in younger patients and are more likely to be atypical or aggressive, multifocal, and highly proliferative. [40, 41, 42]
Meningiomas are most commonly dural-based tumors in the brain and, less commonly, the spinal cord. Rare cases occur as intraventricular and pulmonary tumors. Most meningiomas are intracranial extra-axial tumors. About half of these tumors occur in the falcine and parasagittal locations, [17, 43, 19] and they are often firmly affixed to the sagittal sinus. The majority of the remainder occur in the skull base. Posterior fossa and, in particular, intraventricular meningiomas are relatively uncommon. However, meningiomas in children are more often infratentorial, intraventricular, or intraparenchymal than those in adults. Up to 25% of childhood cases of meningiomas are associated with NF2. [8, 11]
Most of the spinal meningiomas are firmly affixed to the dura, subdural and laterally situated in close relationship with the spinal nerve roots. The thoracic spine is the most common site, followed by the cervical spine. Meningiomas are rarely seen in the lumbar region. There is a striking female predominance in spinal meningiomas.
Meningiomas outside the CNS have been described in multiple areas, including the scalp  and skin and connective tissue covering the head and neck,  lung, [45, 46] mediastinum,  peripheral nerve plexus and ganglion,  salivary gland,  mandible, and other odd locations. Primary meningiomas arising in the bone are rare and must be differentiated from meningioma with osseous invasion.
Clinical Features and Imaging
Meningiomas are slow-growing tumors, and smaller ones often remain asymptomatic throughout life. For the larger and symptomatic tumors, symptoms result from local compression and peritumoral edema. Headache and newly onset seizures are the most common initial manifestations. For the rare tumors that arise in the ventricles, hydrocephalus is often part of the clinical picture.  The local manifestations result from local compression and irritation of the brain and spinal cord, which lead to focal neurologic symptoms and signs.
Tumors that arise in the cranial base have a strong tendency to invade the surrounding osseous and nonosseous tissue, and they can be surgically challenging. [14, 15] Invasion of the cranial base and adjacent structures could cause a spectrum of manifestations, ranging from cranial nerve palsy, symptoms related to involvement of the sinuses and the orbit,  dental complaints,  and masses in the forehead.  Cranial base meningiomas are more likely to recur, but this probably reflects the difficulty in total surgical resection rather than the biologic nature of these tumors.  Tumors in the anterior visual pathway, such as along the optic nerve, are more likely to recur.  Extracranial meningiomas can also be seen in the sinonasal tract [20, 21] and should be distinguished from extensions from intracranial primaries.
The 5-year recurrence rates of meningiomas, atypical meningiomas, and anaplastic meningiomas are 3%, 38%-40%, and 78%, respectively. [7, 22] The overall 5-year survival rates for WHO grade I, II, and III meningiomas are approximately 95%, 80%, and 20%, respectively.  In one study, the median survival of patients with anaplastic meningioma was less than 2 years.  Gross total resection appears to be one of the most significant factors for favorable prognosis. 
Most, but not all, meningiomas are hyperdense on CT scans, and about a quarter of all cases demonstrate calcifications. Occasionally, meningiomas invade the cranial vault and cause characteristic hyperostosis. Large meningiomas may mold the overlying bone.
Typically, meningiomas are almost isodense with the gray matter on T1-weighted MRIs (see image below). These tumors vary from being hypointense to hyperintense on T2-weighed images.
Contrast-enhanced MRI is the most sensitive method for detecting meningiomas. Meningiomas enhance strongly and often homogeneously. About half of patients have an area of dural enhancement, or so-called "dural tail." Such enhancement is common in all dural-based processes and is not entirely specific for meningiomas. Histologically, the dural tails may be composed entirely of hypervascular, presumably reactive tissue, but not meningioma tumor cells.
On T2-weighted MRI, a crest of CSF, so-called "CSF crest" (see image below) is often seen around the tumor. The presence of the crest is a good indicator of the extra-axial location of these tumors.
Meningiomas often attach to, encase, or compress adjacent arteries and sinuses. The nature of tumor relationship with the regional vasculature is an important surgical consideration and is best assessed with MRI. Meningiomas are also typically associated with peritumoral edema that is well demonstrated on fluid attenuation inversion recovery (FLAIR) images or T2-weighted MRIs.  Occasionally, such edema may be disproportionally large relative to the size of the tumor and may suggest an invasive tumor or a metastatic carcinoma or melanoma. Secretory meningiomas are well known for inducing a disproportionately large amount of peritumoral edema.
Benign meningiomas are round, bosselated, or lobulated well-demarcated dural-based nodules (see image below).  The great majority of them are dural based. Depending on their collagen content, the consistency of these tumors ranges from rubbery to firm.
Atypical meningiomas and anaplastic meningiomas are usually larger. The cut surfaces of benign meningiomas are usually granular to homogeneous. Cases with a substantial amount of psammoma bodies are gritty to the knife cut. Yellow areas resulting from the accumulation of lipid-rich cells may be present. Ossifications may occur. Cysts and spontaneous hemorrhages are uncommon. Necrosis may be present in high-grade tumors. The size of meningiomas varies significantly and ranges from a few millimeters to several centimeters in size. The smallest ones are often asymptomatic, but they can be detected with powerful MRI.
Benign meningiomas can be easily separated from the underlying brain parenchyma without disruption of the cortex. This feature is well reflected by the well-circumscribed appearance and the CSF crest on imaging. The underlying cortex is often compressed and appears gliotic. When invasion is present, the neurosurgeon may have difficulty in establishing a clear cleavage plane between the tumor and the underlying cortex. In contrast, cranial-base meningiomas often invade the surrounding osseous and fibrous structures. Meningiomas that involve the orbit may represent primary tumors arising within the orbit, as well as intracranial meningiomas with extension into the orbit.
En plaque meningiomas refer to tumors with a carpetlike growth pattern along the dura that lead to dural thickening closely apposed to the inner surface of the overlying skull. This pattern is most common in the sphenoid wing and other cranial base locations. Owing to their location and growth pattern, manifestations often refer to the ophthalmic and cranial nerves.  En plaque meningiomas should be distinguished from meningioangiomatosis, a task that is not always easy on MRIs.
WHO Histologic Grading of Meningiomas
The current (2007) WHO grading scheme for meningiomas  has largely adopted the criteria based on the clinicopathologic data from 2 large series [22, 23] ; the validity of these criteria has been subsequently confirmed by 2 other groups. [50, 51]
In contrast to many other tumors, one of the unique features in meningiomas is that invasion of bone, vascular structures, dura, dural sinus, and paranasal sinuses are not considered evidence of malignancy or aggressiveness. However, invasion of brain parenchyma is considered an indicator of aggressive behavior.
In the WHO grading system, meningiomas fall into WHO grades I-III. WHO grade I tumors comprise about 80% of all meningiomas. In comparison to WHO grade I tumors, atypical (WHO grade II) meningiomas have histologic features that indicate aggressive behavior, which include an increase in mitotic activity, brain invasion, and spontaneous necrosis. Atypical meningiomas comprise about 15%-20% of all meningiomas and carry a marked increase in local recurrence with reduced long-term survival. Cases with increased mitotic activity may not show impressive nuclear atypia. Careful search for mitotic activity and the mitotic count are important. An elevated mitotic count is defined as 4 or more mitoses per 10 consecutive high-powered fields (HPFs) (0.16 mm2),  regardless of whether the increase is focal or diffuse.
Although most histologic patterns do not reflect the tumor’s biologic potential, tumors with clear cell and chordoid histology are considered WHO grade II. Tumors with papillary and rhabdoid features are considered WHO grade III. In some cases, these features may be focal. There is a tendency for these aggressive components to become the dominant component in recurrent tumors.
Benign meningioma (WHO grade I)
WHO grade I meningiomas are defined by the following:
Histologic variant other than clear cell, chordoid, papillary, and rhabdoid
Lacks criteria of atypical and anaplastic meningioma
Atypical meningioma (WHO grade II)
Any of the following 3 criteria are used for WHO grade II meningiomas:
Mitotic index of ≥4/10 HPFs *
At least 3 of the 5 following parameters: (1) sheeting architecture (loss of whorling and/or fascicles), (2) small cell formation (high nucleus-to-cytoplasmic [N/C] ratio), (3) macronucleoli, (4) hypercellularity, or (5) spontaneous necrosis (ie, not induced by embolization or radiation)
Brain invasion †
* HPF is defined at 40X magnification of 0.16 mn2 or 0.452 mm in field diameter (as adapted from Meningiomas in WHO Classification of Tumours of the Central Nervous System  ).
† Brain invasion refers to breaching of the pial barrier but not mere extension or growth of tumor along the Virchow-Robin space. Fingerlike extension of the tumor into the parenchyma is the classic observation. In areas with confluent invasion, immunohistochemistry for glial fibrillary acidic protein (GFAP) may be needed to demonstrate entrapped gliotic parenchymal tissue. In the past, brain invasion was recognized as a definitive sign of malignancy for meningiomas. In the current WHO criteria, however, meningiomas that appear benign but with brain invasion are considered WHO grade II (atypical) rather than WHO grade III.
Genuine extracranial metastasis or subarachnoid spread is exceptional and most likely originates from anaplastic meningiomas. So-called "benign metastasizing meningioma" represents a stable or slowly growing metastasis from a meningioma exhibiting a typical low-grade histology. In such cases, the metastases are not regarded as evidence of malignancy. They are most often found in the lung and should be distinguished from the equally rare primary pulmonary meningiomas. [52, 53]
Anaplastic (malignant) meningioma (WHO grade III)
Either of the following 2 criteria is used for WHO grade III meningiomas:
Mitotic index ≥20/HPF *
Frank anaplasia (sarcoma, carcinoma, or melanomalike histology)
* HPF is defined at 40X magnification of 0.16 mn2 or 0.452 mm in field diameter (as adapted from Meningiomas in WHO Classification of Tumours of the Central Nervous System  ).
Meningioma is perhaps the primary neuroepithelial tumor with the widest diversification in histologic pattern. Some of these patterns may suggest a diagnosis other than meningioma, particularly when the biopsy sample is small and during intraoperative consultation. For example, secretory meningiomas can mimic metastatic adenocarcinoma, and lymphoplasmacyte-rich meningiomas may mimic a lymphoproliferative disorder. Some of these patterns carry significant histologic predictive values, whereas the others do not.
It is not common for a meningioma to have a homogeneous histology in all parts of the tumor. This feature is particularly important in the diagnosis of meningiomas with these troublesome patterns and in anaplastic meningiomas, as small areas with typical features of meningioma may be present somewhere on the slides, waiting to be discovered.
Components indicative of aggressive behavior, such as chordoid changes, may be focally present initially, but these components may become the dominant component on recurrent tumors and should be identified on the initial specimen. Identification of these focal patterns is important.
Histologic patterns recognized by WHO (as adapted from Meningiomas in WHO Classification of Tumours of the Central Nervous System  ) are reviewed in this section.
WHO grade I includes the following histologic patterns:
Meningothelial (syncytial) meningioma
Transitional (mixed) meningioma
Fibroblastic (fibrous) meningioma
Angiomatous (vascular) meningioma
WHO grade II includes the following histologic patterns:
Clear cell meningioma (intracranial)
WHO grade III includes the following histologic patterns:
Anaplastic (malignant) meningioma
Highlights of different histopathologic pattern of meningiomas
The highlights of different patterns are summarized below. Other than atypical and anaplastic meningiomas, these patterns roughly fall into 2 major categories. In the first category, an additional histologic pattern such as a substantial amount of lymphoplasmacytic cells (lymphoplasmacyte-rich meningioma) or specific mesenchymal component (metaplastic meningioma) is present in addition to the classic meningioma component. These patterns (except chordoid meningiomas) are usually not associated with an unfavorable prognosis. In the second category, specific cytologic features of the tumor cells, such as clear cell changes (clear cell meningioma) or rhabdoid changes (rhabdoid meningioma), deviate from classic meningioma. Many of these tumors are associated with an unfavorable prognosis.
WHO grade I
Meningothelial (syncytial) meningioma histologic highlights are as follows:
Epithelioid, round to polygonal, with a moderate amount of amphophilic to eosinophilic cytoplasm
Cells arranged in lobules of cells or whorls
Syncytial appearance due to fuzzy intercellular border
Intranuclear clear vacuoles and intranuclear pseudoinclusions are common
Transitional (mixed) meningioma histologic highlights include the following:
Intermediate features between fibroblastic type or a mixture of both meningothelial and fibroblastic components
Meningothelial whorls are well developed
Psammoma bodies are common
Fibroblastic (fibrous) meningioma histologic highlights include the following:
Spindle cells with bland nuclei arranged in fascicles or storiform pattern
May be very fibrous
Diagnostic features that help in the diagnosis include classic nuclear features, meningothelial whorls, and psammoma bodies. Immunohistochemistry and electron microscopy are also helpful. The differential diagnoses include other low-grade fibrous tumors, such as solitary fibrous tumors.
Psammomatous meningiomas are particularly common in the spinal region of older women and in tumors that are being monitored for a long time before resection. Histologic highlights of these tumors are as follows:
Numerous meningothelial whorls with psammoma bodies at their centers
Psammoma bodies should comprise about half of the tumor for this diagnosis to be made 
Angiomatous (vascular) meningiomas may have disproportionally large peritumoral edema relative to the size of the tumor. Histologic highlights include the following:
The histologic picture is dominated by blood vessels with small- to medium-sized vascular channels; most of these vessels are small and with hyalinized walls (this hypervascular pattern would suggest a hemangioma)
Two histologic patterns, the microvascular and macrovascular types, have been identified 
This angiomatous pattern occurs commonly in combination with microcystic meningiomas
Degenerative nuclear atypia is common and can be striking, thus raising the concern for malignancy or aggressiveness; however, these tumors are benign
The differential diagnoses include vascular tumors, hemangioblastomas, and hemangiopericytomas.
Microcystic meningiomas may also have disproportionally large peritumoral edema relative to the size of the tumor. Histologic highlights are as follows:
The extracellular microcysts contain pale, eosinophilic mucinous fluid (see image below)Image 18: In this microcystic meningioima, the tumor cells have thin, spidery cytoplasmic processes and numerous microcysts containing mucoid material. A rich vascular network (arrow) is present.
The tumor cells have thin, spidery cytoplasmic processes, which may suggest glioma when only small tissues are available or during the interpretation of frozen section diagnosis
Pleomorphic cells may be present and numerous, but they are not evidence for aggressive behavior
The differential diagnoses include hemangioblastomas, gliomas, and, less likely, low-grade metastatic carcinomas.
Secretory meningiomas may have disproportionally large peritumoral edema relative to the size of the tumor. In addition, these tumors may be associated with elevated carcinoembryonic antigen (CEA) levels in blood. Resection of the tumor would lead to a return of the CEA level to normal. Histologic highlights include the following:
Tumor cells with intracellular lumina containing eosinophilic (see the image below), periodic acid-Schiff (PAS)–positive secretionsImage 19: In this secretory meningioma, bright intracellular secretions (black arrows) are present. Note that the size of these vacuoles are several times the size of the nuclei. Intranuclear pseudoinclusions are also present (white arrows).
Although these secretory vacuoles are intracellular, they can be several times the size of the nuclei
These tumors have focal epithelial differentiation, particularly with cells containing the secretory vacuoles; these cells are positive for cytokeratin and CEA (see image below)Image 20: Expression of CEA is present in the tumor cells with secretory vacuoles (white arrow) in this case of secretory meningioma. Note that many intranuclear clear vacuoles are present in this case (black arrows).
There may be a high number of mast cells
The differential diagnoses include metastatic, mucin-secreting adenocarcinomas.
Lymphoplasmacyte-rich meningiomas may rarely have systemic findings, such as polyclonal gammopathy and anemia.  Histologic highlights are as follows:
This is the least common histologic pattern for meningiomas
There is extensive, chronic inflammatory cell infiltration, which obscures the meningothelial component; these inflammatory cells may represent a brisk inflammatory response to the tumor
Plasma cells with Russell bodies may be present
The differential diagnoses include Rosai-Dorfman disease (extranodal sinus histiocytosis), Castleman disease, inflammatory pseudotumor, and rheumatoid nodules; low-grade lymphomas; and meningitis and meningeal inflammatory changes secondary to lesions in the skull or adjacent structures.
Metaplastic meningioma histologic highlights include the following:
Metaplastic components, singly or in combination, including bone, cartilage, myxoid area, and xanthomatous and lipomatous components are present; bone and cartilage are far more commonly found than other components; some of these are reactive changes or mere accumulation of lipids but not genuine metaplasia
When an osseous component is present, it should be distinguished from bone invasion
The metaplastic components can be focal or widespread, but classic meningothelial components are often found, and the diagnosis is usually not a challenge
In extreme cases with widespread metaplastic changes, the concern of a mesenchymal tumor may be raised as part of the differential diagnoses.
WHO grade II
Chordoid meningioma's association with the systemic findings of Castleman syndrome, including polyclonal gammopathy and microcystic anemia, has been reported in children. [57, 58] These are usually large, dural-based, supratentorial tumors with a very high rate of recurrence following subtotal resection.  Histologic highlights are as follows:
The overall morphology mimics chordoma, featured by epithelioid cells with bubbly cytoplasm (see first image below) and an Alcian blue–positive myxoid stroma (see second image below)Image 20: Expression of CEA is present in the tumor cells with secretory vacuoles (white arrow) in this case of secretory meningioma. Note that many intranuclear clear vacuoles are present in this case (black arrows).Image 21: In this chordoid meningioma, the background substance resembles those in chordoma and the meningioma cells have fine bubbly cytoplasm (arrow). The nucleoli are also slightly prominent in this case.
This tumor is associated with lymphoplasmacytic infiltration that ranges in intensity from patchy to prominent
The proportion of chordoid component may increase in proportion in subsequent recurrences
Pure chordoid meningiomas are rare, and most of them are mixed with a variable amount of meningothelial component
The differential diagnoses include chordoma and chordoid glioma of the third ventricle. With chordomas, the location is helpful. Chordoid meningiomas are dural-based tumors that occur in a location where chordomas would not be expected. In addition, chordomas are positive for brachyury. [59, 60] Chordoid gliomas of the third ventricle are positive for GFAP.
Clear cell meningiomas have a predilection for the spinal cord and posterior fossa. Affected patients have a younger age of onset and an aggressive clinical course despite the typically bland histology.  Histologic highlights include the following:
Glycogen-rich cytoplasmic clearing but not extracellular secretory vacuoles best demonstrated by PAS with and without diastase digestion; this is an important feature that distinguishes these tumors from microcystic meningioma
The tumor is unique in being a pure clear cell tumor with subtle features of classic meningiomas in most cases
Dense stromal and perivascular blocky collagen deposition is present
The differential diagnoses include metastatic clear cell carcinoma, particularly renal cell carcinoma (clear cell meningiomas are negative for cytokeratin), and microcystic meningiomas.
The histologic highlights of atypical meningiomas are as follows:
When the tumor does not meet the diagnostic criteria of clear cell or chordoid meningioma, the histologic features should meet the criteria listed in WHO Histologic Grading of Meningiomas for the diagnosis of atypical meningioma, as listed above
Features of atypical changes can occur in any of the variants
WHO grade III
Papillary meningiomas are uncommon. The posterior fossa is a common site of predilection. These tumors have an increased frequency in young individuals and children,  and an aggressive clinical behavior and metastases to the lung is not uncommon. [62, 63] Histologic highlights include the following:
A perivascular arrangement of tumor cells is the dominant feature
The papillary or pseudopapillary growth pattern may dominate the histologic picture and results from dyscohesion of the tumor cells
These tumors often arise from more conventional meningiomas; the proportion of the papillary component may increase in subsequent recurrences
The differential diagnoses include ependymomas, astroblastomas, pituitary adenomas, hemangiopericytomas, and medulloblastoma/primitive neuroectodermal tumors.
Rhabdoid meningiomas are similar to many other rhabdoid tumors of adulthood in that this tumor is regarded as a pattern reflecting the malignant transformation of a meningioma. The demographics are similar to conventional meningiomas. The rhabdoid component may occur during recurrence. Histologic highlights are as follows:
The rhabdoid features can be very well recognized on cytologic preparations (see the image below)Image 8: The rhabdoid feature of this tumor is well demonstrated in this cytologic preparation. Note the eccentrically located nuclei and the large oval, amphophilic cytoplasm. In this particular case, the nuclear feature is rather bland for a rhabdoid tumor. The nuclei of rhabdoid tumors can be rather bizarre featuring enlarged, lobulated nuclei with huge, eosinophilic nucleoli.
Similar to other rhabdoid tumors, the rhabdoid cells have a discohesive growth pattern and an intracytoplasmic globular, eosinophilic inclusion that displaces the nuclei to the periphery; the nuclei have vesicular nucleoli and prominent nucleoli
Rhabdoid changes may be superimposed on a papillary meningioma background
Areas with conventional meningiomas are often present
The differential diagnoses include other primary or metastatic rhabdoid tumors, as well as atypical rhabdoid teratoid tumors (ATRTs). Rhabdoid meningiomas are rare in infants and children. ATRTs have a more diversified phenotypic immunohistochemical profile, including loss of expression of the HSNF5/SMARCB1/INI1 protein secondary to heterozygous or homozygous deletion of the INI1 gene. 
The histologic highlights of anaplastic meningioma include the following:
There are frank anaplastic changes, and the tumor may mimic metastatic carcinoma, melanoma, and sarcoma
Mitotic activity is ≥20 mitoses per HPF
Cytopathology of Meningiomas
Intraoperative cytologic preparations demonstrate helpful diagnostic features. Most meningiomas can be smeared out if an appropriate force is applied. For meningothelial and transitional meningiomas, the smears are usually composed of large clusters of epithelioid cells mixed with smaller clusters or individual cells. The degree of rubbery consistency depends heavily on the content of collagen. Fibroblastic meningiomas are a lot more difficult to generate a homogeneous smear than meningothelial or transitional meningiomas. Fibroblastic meningiomas usually yield cluster of spindle cells that tightly adhere to each other, but scattered single cells with epithelioid features are often present.
The scattered single cells typically give clues for the diagnosis of meningiomas. The nuclei are usually oval or slightly elongated and contain open chromatin (see image below).
For the grade I tumors, a prominent nucleoli should not be seen. A concern for grade II or III tumors, as well as nonmeningeal tumors such as metastatic carcinoma or melanoma, should be raised if prominent nucleoli are noted. Intranuclear pseudoinclusions (invagination of cytoplasm into the nucleus) and intranuclear clear vacuoles vary from scant to abundant. These nuclear features are typical but not unique to meningiomas.
The scattered single cells are usually epithelioid in appearance and in the shape of an elongated triangle with moderate amount of cytoplasm. To the eye of the author, these cells look like a veil being blown in the wind. Identification of definite meningothelial whorl formation (see first image below) is a clear help. Psammoma bodies are round, concentrically laminated, calcifications (see second image below) that are commonly seen.
Secretory meningiomas are featured by large, secretory vacuoles (see first image below), and rhabdoid meningiomas are featured by rhabdoid cells (see second image below). These cytologic features are well demonstrated in the cytologic preparations.
Meningothelial cells have features of epithelial and mesenchymal cells, and this mixed feature is well reflected in the histopathology of meningothelial (syncytial), transitional (mixed), and fibroblastic (fibrous) meningiomas. The meningothelial (syncytial) meningioma is perhaps the prototype of meningiomas and the most commonly encountered histologic pattern. In meningiomas with other uncommon histologic patterns, classic meningothelial areas may be focally present, and identification of these areas helps to confirm the diagnosis.
Meningothelial meningiomas are composed of lobules of meningothelial cells sometimes separated by thin fibrous septa. Sporadic psammoma bodies can be seen (see first image below). Meningothelial whorls are present but typically not as well formed as in transitional meningiomas. The intercellular border is typically fuzzy and poorly defined (see second image below). This feature reflects the interdigitating cytoplasmic border seen at the ultrastructural level. In fact, a well-defined cytoplasmic border in an otherwise typical meningioma raises the suspicion for a more aggressive behavior.
The nuclei are oval to elongated and typically without prominent nucleoli in low-grade cases. Clear intranuclear vacuoles as well as pseudonuclear inclusions are characteristic but not diagnostic nuclear features. Its abundance is rather variable (see image below).
At the other end of the spectrum, fibroblastic meningiomas are composed of fascicles of spindle cells mixed with a variable amount of collagen fibers. Overall features resemble a benign spindle cell mesenchymal neoplasm (see image below). Some of these tumors can be densely fibrotic. The nuclear features are helpful hints for the diagnosis, because the cell morphology is not particularly helpful. Pure fibroblastic meningiomas are rather uncommon, and areas with transitional and/or meningothelial components are often present and would help in confirming the diagnosis.
Transitional meningiomas are the intermediate version and are composed of areas with intermediate features between meningothelial and fibroblastic components (see first image below) as well as intermingled areas with distinct meningothelial and fibroblastic features. Meningothelial whorls are often well formed (see second image below).
Atypical meningiomas often do not possess impressive atypical nuclear changes. Increased mitotic activity can be seen independent of a significant increase in nuclear atypia (see first image below). In some atypical meningiomas with a high level of atypia, identification of intranuclear clear vacuoles and pseudoinclusions (see second image below) can help to confirm the diagnosis. Brain invasion can also be found independent of high-grade atypia (see third image below). Other features indicative of aggressive biologic potential for atypical meningiomas include loss of whorling and/or fascicles (sheeting of cells), small cell component, macronucleoli, hypercellularity, and spontaneous necrosis (see WHO Histologic Grading of Meningiomas).
Anaplastic meningiomas have high-grade anaplasia and high-mitotic activity (see WHO Histologic Grading of Meningiomas). These tumors may arise as de novo tumors or result from malignant progression of previously existing meningiomas. Because meningothelial cells have features of both epithelial and mesenchymal cells, it is not surprising to find that their high-grade tumors would span the histologic spectrum, resembling carcinoma at one end to sarcoma at the other. On first look, these tumors may not really suggest meningiomas but raise a consideration of a possible carcinoma, melanoma, or sarcoma. On closer scrutiny, histologic features of meningiomas may be found in some focal areas. Immunohistochemistry, electron microscopy, and genetic studies may be needed to confirm the diagnosis.
Proliferative markers determined by immunostaining for Ki-67 (MIB-1) or anti-phosphohistone-H3 (PHH3), as well as apoptotic index, have been shown to be correlated with the biologic behavior in meningiomas. In general, there is a high correlation between a high Ki-67 labeling index and histologic grade, but, owing to the differences in techniques used in different laboratories, the cutoff value for meningiomas, atypical meningiomas, and anaplastic meningiomas is not always reproducible. [23, 65, 66, 67, 68] Increased apoptotic index has also been correlated with increased biologic aggressiveness in meningiomas. 
Immunohistochemistry and electron microscopy
Vimentin is typically diffusely and strongly positive in meningiomas. The most reliable marker is perhaps epithelial membrane antigen (EMA), in which about 50%-100% of the cases are positive. [15, 60, 72, 73, 74] The staining pattern, however, is often weak and patchy rather than a diffuse positivity (see image below).
Meningiomas, with the exception of secretory meningiomas, are usually negative for cytokeratins. Focally positive immunoreactivity for S100 can also be seen.  Other markers with certain success, but not of wide clinical application, include prostaglandin D synthase,  E-cadherin,  claudin-1,  desmosomal plaque components such as desmoplakin,  and connexins. 
Meningiomas are often positive for androgen, progesterone, and estrogen receptors.  Expression is equally common in males and females, and there is no difference among different histologic subtypes. There is a significant correlation between negative or low expression of progesterone-receptor status and high tumor vascularity with high Ki-67 labeling index. [65, 81]
The ultrastructural features for meningiomas are highly diagnostic. Regardless of the histologic variants or patterns, the tumor cells have interdigitating cytoplasmic membrane. Intercellular junctions are present (see image below). 
Monosomy 22 is a unique cytogenetic alteration.  Mutation and/or deletion of the NF2 gene on chromosome 22q12 is present in most NF2-associated meningiomas and about 50%-80% of sporadic meningiomas. The NF2 gene codes for merlin (schwannomin), and its normal function is poorly understood. This gene is critical in constituting the early tumorigenic event in the 2-hit model of tumor-suppressor inactivation proposed by Knudson. 
In general, solitary meningiomas are clonal tumors as per X-chromosome inactivation studies.  In one study, most meningiomas shared the same single mutation, and recurrent tumors had the same clonality as the primary tumor.  In multiple meningiomas, it has been shown that most of these tumors share the same NF2 gene mutation  and inactivation of the same copy of X-chromosome.  Dural spread and peritumoral implants may well be the mechanism in cases with multiple meningiomas. 
Genetic events in the tumorigenesis and malignant progression of meningiomas
Various genetic alterations have been identified in the malignant progression of meningiomas and are summarized below (as adapted from Meningiomas in WHO Classification of Tumours of the Central Nervous System  ).
Benign meningioma (WHO grade I)
NF2 mutation (30%-60%)
Loss of 4.1B expression (20%-50%)
Loss of TSLC1 expression (30%-50%)
EGFR, PDGFRB activation
Atypical meningioma (WHO grade II)
+1q, 9q, 12q, 15q, 17q, 20q (30%-50% each)
Loss of TSLC1 expression (70%)
Loss of PR expression (60%-80%)
Telomerase/hTERT activation (60%-90%)
Notch, WNT, IGF, VEGF activation
Anaplastic meningioma (WHO grade III)
NDRG2 hypermethylation (70%)
Loss of TSLC1 expression (70%)
Loss of PR receptor (80-90%)
17q23 amplification (40%)
The diagnosis of classic, WHO grade I meningothelial, transitional, and fibroblastic meningiomas is usually straightforward. The higher-grade variants and meningiomas of the unusual patterns often pose a diagnostic challenge. Some of the more commonly encountered entities are discussed below.
Meningothelial hyperplasia refers to meningothelial proliferation over 10 cells thick and associated with an inciting event [92, 93] and is believed to be caused by reactive changes of meningothelial cells. Such hyperplasia is often prominent in optic nerve gliomas (see images below). In small biopsy specimens obtained during intraoperative consultation, these clusters may suggest meningiomas.
Meningioangiomatosis raises a strong suggestion of NF2, but sporadic cases can present. The initial presentation is usually seizures in a child or young adult. On imaging studies, these lesions may simulate an en plaque meningioma. Classically, this is a plaquelike leptomeningeal thickening that is firmly adhered to the cortex and is believed to be hamartomatous in nature.
Histologically, there is perivascular growth of spindle cells that often demonstrate features of meningothelial cells, including expression of EMA, psammoma bodies, and classic nuclear features of meningothelial cells. Coexisting meningiomas, with some of them presumably arising from the meningioangiomatosis, have been well described.
Schwannomalike cases may also present (meningoangio-neurilemmomatosis). In contrast to benign meningiomas that have a clear cleavage plane, the interface of meningioangiomatosis closely intermingles with the underlying cortex in which impressive gliosis is present. Entrapped neurons are often present.
With all these features, the diagnosis is often an enigma, particularly when the specimen is small. Meningioangiomatosis may be misinterpreted as a meningioma if meningothelial cells are prominent; as a glioma because of the overall hypercellularity and reactive gliosis; or a ganglion cell neoplasm, due to entrapment of preexisting neurons. A correct diagnosis depends largely on the recognition of the entity as an intracortical perivascular proliferation of meningothelial cells and fibroblasts. [90, 91] Correlation of clinical and imaging features are important.
Dural-based metastatic clear cell tumors
Dural-based metastatic clear cell tumors with low-grade morphology such as metastatic renal cell carcinoma and prostatic carcinoma may closely mimic clear cell meningioma. Diffuse cytokeratin positivity is present in these tumors, and this contrasts with a mostly negative pattern in classic meningiomas. Specific markers such as CD10 for renal cell carcinoma, prostatic acidic protein, and prostate-specific antigen (PSA) are helpful for confirming the diagnosis of metastatic renal cell carcinoma and prostatic carcinoma, respectively. Neurocytoma may also mimic clear cell meningioma. Neurocytomas occur most commonly within the ventricle and are synaptophysin-positive.
Hemangioblastomas may mimic microcystic and clear cell meningiomas. Primary dural-based hemangioblastoma is rare, and these tumors are positive for inhibin. Renal cell carcinoma is also one of the few carcinomas that are positive both for vimentin and cytokeratin.
Solitary fibrous tumors
Solitary fibrous tumor may closely mimic the fibroblastic type of meningiomas. In the past, some of these tumors were termed "sclerotic meningiomas." Solitary fibrous tumors are slow-growing spindle cell tumors that affect adults. Dural-based examples are rather uncommon.
Histologically, there is a great variation of the amount of collagen tissue. The tumor cells usually have bland or low-grade, elongated, cigar-shaped nuclei, and they often insinuate among collagen fibers. These features would suggest a fibroblastic meningioma.
Immunohistochemically, solitary fibrous tumors are positive for CD34 and Bcl-2 and negative for EMA (see images below). Faint positive CD34 immunoreactivity may be seen in meningiomas.
Schwannomas are common in the cerebellopontine angle and in the spinal nerve roots. On rare occasions, meningiomas may have a focal palisading arrangement that would suggest meningioma in these locations. Although uncommon, whorls can be seen in schwannomas.
Several features distinguish these 2 entities. The classic nuclear features of meningiomas are best recognized on cytologic preparations and are the first clues for the correct diagnosis. Schwannomas do not routinely contain collagen, and these tumors are strongly positive for S100; reticulin staining demonstrates deposition of reticulin fibers around the individual cells. In contrast, meningiomas are not strongly and diffusely positive for S100, and reticulin staining demonstrates nests of cells rather than pericellular reticulin fibers.
Although high-grade meningiomas may invade the brain and mimic a glioma, high-grade gliomas may also invade the leptomeninges and mimic meningiomas. Immunohistochemistry would demonstrate positivity for GFAP and S100 protein in the glioma component.
In the past, hemangiopericytomas were called angiomatous meningiomas. Hemangiopericytomas behave far more aggressively than low-grade meningiomas. In addition, hemangiopericytomas are hypercellular tumors with characteristic thin-walled branching, so-called "staghorn" blood vessels (see first image below). The tumor cells are rather monotonous in size, with large nuclei, and they often share a comparable level of atypia among different cells (see second image below). These cells lack the characteristic nuclear features of meningothelial cells, nor are meningothelial whorls found. Hemangiopericytomas are negative for EMA and may be positive for CD34. A reticulin stain would demonstrate deposition of reticulin among individual tumor cells.
Although anaplastic meningioma may mimic metastatic carcinomas, secretory meningiomas often suggest mucin-containing adenocarcinomas at intraoperative consultation. The exuberant edema associated with secretory meningiomas often suggests metastatic neoplasm on imaging studies. The large vacuoles would fuel this suspicion. Cytologically and histologically, the secretory vacuoles of secretory meningiomas are huge and are several times the size of the nuclei or even the cell (see the first 2 images below). In contrast, mucin-secreting adenocarcinomas rarely have their vacuoles 2-3 times the size of the nuclei (see the third image below). In addition, they contain bluish mucoid substance rather than the bright eosinophilic substance that are seen in secretory meningiomas.