Molecular Alterations, Histopathology and Squash Cytology of Meningioma

Abstract

Meningioma originates from the arachnoid cap cells of duramater. It is a slow growing tumour of Central Nervous System. It is seen more commonly in females at around 66 years of age. The most common genetic abnormality is monosomy of chromosome 22. Since the inception of World Health Organisation (WHO), meningioma was graded based on histo-morphological appearance. At present, they are categorised into 3 grades. This chapter describes the molecular alterations, histopathological grading, histopathological subtypes and squash cytology of meningioma.

1. Introduction

Meningioma is a slow growing tumour of Central Nervous System (CNS) arising from the meninges of brain and spinal cord. They comprise majority of the primary CNS tumours. The tumour is more commonly seen in females, compared to males [1]. Apart from gender, the increased incidence is also noted in first degree relatives and the tumour has association with various genetic syndromes such as neurofibromatosis type 2 (NF2), Von Hippel Lindau (VHL) disease, multiple endocrine neoplasia type 1 (MEN1), Li-Fraumeni syndrome, Cowden syndrome and Gorlin syndrome [2].

The World Health Organisation (WHO) has graded meningioma into three grades and 15 subtypes, with each subtype having its own histopathological characteristics and molecular association [1]. Meningothelial subtype is the most common (63%) followed by transitional meningioma (19%) [3]. The clinical presentation depends upon the size and site of tumour. Smaller tumours are usually asymptomatic and are diagnosed incidentally. However, larger tumours cause symptoms and neurological deficits. Majority of the meningiomas are grade 1 (benign) and have favourable prognosis. Grade 2 meningioma (atypical) have a higher chance of recurrence whereas grade 3 meningioma (anaplastic/malignant) have worst prognosis [1].

2. History

• The earliest written record of meningioma dates back to 1614, when Felix Plater from the University of Basel performed an autopsy on Sir Caspar Bonecurtius [4].

• In the sixteenth century, first surgery was performed for removal of meningioma. However, the first successful surgery was done in the year 1770 by Anoine Luis and removed meningioma from convexity (parasagittal) plane [5].

• In 1835, the first skull-based meningioma was removed successfully by Zanobi Pecchioli, Professor of Surgery at the University of Siena [6].

• William Macewen, a Scottish surgeon, was also known for performing successful removal of dural based meningioma [7].

• In 1922, Harvey Cushing used the term “meningioma” to describe group of tumours that can develop anywhere along the neuraxis (brain and spinal cord) of the body, but share many of the features in common [4].

• Based on cell structure, Charles Oberling segregated meningioma into several subtypes, and over the years, other different types of meningiomas were identified by many scientists. Later on, in 1979, the first WHO “blue book” Histological Typing of Tumours of the Central Nervous System was released, which classified meningioma into seven subtypes, which got upgraded in its further editions 1993, 2000, 2007 and 2016. The latest fifth edition of “WHO Classification of Tumours of the Central Nervous System (CNS)” blue book, was published in 2021 and has graded meningioma into 3 different grades and described 15 histopathological subtypes of meningioma [8]. Each WHO “blue book” included revisions that reflected changes in concepts, knowledge, updates and represented an improvement over its previous editions.

3. Epidemiology

Meningiomas comprise 37.6% of primary CNS tumours. Meningiomas are more common with increasing age, 66 years being the median age of diagnosis. It is more commonly seen in females, compared to males, with female: male ratio of 2.3:1 [1].

Incidence rate ranges from 0.16/100,000 for patients aged 0 to 19 to 18.69/100,000 for those over 40 years. It has been observed that children present more frequently with high grade meningioma and have poor prognosis [9]. Typically, 2–3% of individuals have meningioma identified incidentally during autopsy [10].

According to WHO grading system of meningioma, 80–82% are grade 1, 17–18% comprise grade 2 and 1.7% forms grade 3 [1]. The recurrence rate of meningioma increases 2% every year, that is, at 10 years, the chances of recurrence increase by 20% [11]. According to the literature, higher-grade meningiomas have high recurrence rates. The recurrence rate in grade 3 meningiomas ranges approximately from 50 to 94%. In contrast, the recurrence rate in grades 1 and 2 meningioma is 7 to 25% and 29 to 52%, respectively [12].

4. Sites

4.1 CNS sites

Meningiomas most commonly arise in the cerebral convexities (parasagittal/falx cerebri/venous sinuses). Other sites include olfactory grooves, sphenoid ridges, parasellar/suprasellar regions, optic nerve sheath, petrous ridges, tentorium and posterior fossa. Very rarely, they are found in intraventricular and epidural sites [13].

Few subtypes of meningioma have predilection for specific locations: [12, 14, 15].

• Meningothelial meningioma–midline or paramedian skull base

• Psammomatous meningioma–thoracic spine

• Chordoid meningioma–supratentorial

• Clear cell meningioma–cerebellopontine angle and spine especially cauda equina

• Papillary meningioma–supratentorial

• Rhabdoid meningioma–supratentorial, infratentorial or spinal

• Anaplastic meningioma–Cerebral or spinal meninges, cerebral ventricle

4.2 Ectopic sites

The most common ectopic site is head and neck region which includes scalp skin, intraosseous, orbit, paranasal sinuses, temporal bone and ear [16]. Very rarely, few cases have also been reported in lungs, salivary glands and mediastinum [17, 18, 19].

5. Pathophysiology

5.1 Pathogenesis

The origin of meningioma is from arachnoid cap cells of the duramater or choroid plexus. The most common genetic abnormality seen in majority of the meningioma is monosomy of chromosome 22 with more than 50% of tumours show allelic loss in 22q12.2, the region encoding the neurofibromatosis 2 (NF2) gene, also known as merlin gene [1]. On the contrary, paediatric meningiomas do not demonstrate NF2 mutations, instead they predominantly have YAP1 alterations, further leading to activation of the Hippo pathway [20].

A study of genomic sequencing was carried on sporadic meningioma and it revealed two subsets of mutations. The first subset had NF2 mutations and loss of chromosome 22. The second subset lacked NF2 mutation, but however manifested recurrent oncogenic mutations in AKT1 and PIK3CA. In addition, alterations in TRAF7, KLF4 and SMO were also observed [13, 21].

5.2 Diagnostic molecular pathology

The subtypes of meningioma are closely correlated with genetic changes (e.g. in AKT1, SMO, and PIK3CA), although these changes do not define them. DNA sequencing can be used to determine the status of the majority of genetic modifications that are directly relevant to subtyping and grading, including the TERT promoter, SMARCE1, KLF4 and TRAF7, and other abnormalities. The most malignant-appearing and proliferative sites should be the focus of tissue selection for DNA extraction because TERT mutations can develop throughout progression of disease. In situ hybridization can be used to detect homozygous deletion of CDKN2A and/or CDKN2B, or it can be computed using a variety of high-throughput sequencing or hybridization assays. However, it carries a limitation, as FISH probes are large, this method occasionally misses minor deletions. High-resolution copy-number plots may occasionally be used to infer rare events such as TERT activation by gene fusion or gene fusions involving YAP1, although RNA sequencing or in situ hybridization are often required for their confirmation. Because BAP1 and PBRM1 are susceptible to both mutation and deletion, DNA sequencing and independent copy-number analysis are required to analyse such mutations. Apart from DNA based methods, various immunohistochemical (IHC) stains can be used to detect genetic alterations such as SMARCE1 loss in clear cell meningioma and BAP1 loss in rhabdoid meningioma [14, 22]. Various diagnostic modalities used to assess genetic mutations in meningioma are summarised in Table 1.

Diagnostic methods	Genetic mutations
Immunohistochemistry	SMARCE1 loss in clear cell meningioma and BAP1 loss in rhabdoid meningioma
DNA sequencing	TERT promoter, SMARCE1, KLF4 and TRAF7
In situ hybridization	homozygous deletion of CDKN2A and/or CDKN2B
Copy-number plots	TERT activation, YAP1 gene fusions
DNA sequencing and copy-number analysis	BAP1 and PBRM1

Table 1.

List of methods for diagnosing genetic mutations in meningioma [14, 22].

5.3 Subtypes of meningioma and genetic alterations

It has been observed that certain types of meningioma have specific genetic alterations (summarised in Table 2).

Subtypes of meningioma	Genetic alterations
Meningothelial meningioma	AKT1, TRAF7, SMO, and/or PIK3CA mutations [13, 23]
Fibrous, transitional and psammomatous meningioma	22q deletion and NF2 allele mutation [14, 24]
Secretory meningioma	KLF4/TRAF7 mutation [22]
Angiomatous, microcystic, and metaplastic meningioma	Gain of chromosome 5 [25]
Chordoid meningioma	Deletion of chromosome 2p [26]
Clear cell meningioma	Both germline and somatic SMARCE1 mutations [27]
Papillary and rhabdoid meningioma	PBRM1 and BAP1 mutation or deletion [14]
Anaplastic meningioma	Mainly NF2 inactivation by mutation or chromosome 22q loss [14]; other alterations includes TERT promoter mutation, homozygous deletion of CDKN2A and/or CDKN2B and loss of H3 p.K28me3 (K27me3) [28, 29, 30]

Table 2.

Genetic alterations observed in meningioma subtypes.

5.4 Tumour location and mutation spectrum

Certain mutations have strong correlation with tumour site. While skull base meningiomas have mutations in AKT1, TRAF7, SMO, and/or PIK3CA, the convexity meningiomas and the majority of spinal meningiomas frequently have a 22q deletion and/or NF2 mutations [13]. Meningiomas harbouring 22q alterations (e.g. NF2, SMARCB1) originate from neural crest derived meninges and includes convexity, falx, tentorium and spinal cord. Whereas, the meningiomas driven by hedgehog signalling pathway, PI3K signalling, TRAF7, KLF4 and POLR2A mutations, arise from the mesoderm derived meninges of midline/paramedian anterior, central and ventral skull base [24].

6. Aetiology

Certain risk factors are attributed for increasing and decreasing the size of tumour.

6.1 Risk factors leading to increase in size of tumour

1. Ionising radiation, either low dose or high dose, especially for chordoid and anaplastic meningiomas [31, 32]

2. Hormone replacement therapy or oral contraceptives [33]

3. Excess body fat (obesity) and alcohol [34]

4. History of prior surgery, male gender, especially in non-skull based atypical meningioma [35]

5. Long term progestin therapy responsible for enrichment of PIK3CA mutations [36]

6. Mutations in SMARCB1 and SMARCE1 predispose to multiple meningiomas [37]

7. Breast cancer [2]

6.2 Factors leading to decrease in size of tumour

1. Breastfeeding for ≥6 months [2, 38]

2. Allergic diseases such as asthma and eczema [39]

7. Clinical presentation

Majority of the meningiomas are diagnosed in females at around 60–65 years, however, there are some exceptions [1]. Chordoid meningioma can be seen in adults, with average age at diagnosis being 45 years [14]. Clear cell meningioma is mostly seen in children and younger population, median age at diagnosis being 24 years [15]. Papillary meningioma is seen in both children and adults [14].

Meningiomas are slow growing tumours and can cause a variety of neurological impairments, depending upon the tumour’s location. Small tumours are usually asymptomatic and diagnosed incidentally during autopsy. Larger tumours cause compression of nearby structures and produce clinical signs and symptoms. Typical symptoms of meningiomas include headaches, weakness, and convulsions [40].

However, there are few specific symptoms seen in meningioma attributed to their certain locations. Meningioma that cover the cerebrum, sphenoid bone, supra orbital bone or cavernous sinus may produce seizures. Tumours extending into the parasagittal region cause progressive spastic weakness/paralysis in the contralateral limbs and urinary incontinence [41]. Tumours of the Sylvian aqueduct or ventricles can induce a wide range of motor, sensory, aphasic and seizure symptoms. Further, rise in intracranial pressure and hydrocephalus can also occur [42]. Spinal meningiomas cause pain, radiculopathy and local site tenderness. Symptoms include decreased muscle tone, weakness, muscle fasciculations and hyporeflexia [43].

7.1 Metastasis

Anaplastic (malignant) meningiomas are aggressive in nature and have a higher chance to metastasize, although the incidence of meningioma metastasis is very rare (around 0.18%). Ideally, brain tumours do not metastasize to distant sites because of the protective blood brain barrier (BBB), but exceptionally anaplastic meningiomas can. The reason being, these tumours are located towards the blood side of BBB and they are directly connected to the blood vessels. Hence, the malignant cells escape into the blood stream and frequently metastasize into lungs [44]. Similarly, papillary meningioma display papillary growth architecture having propensity for brain invasion and producing dissemination and metastasis, primarily to the lung [14]. Apart from lungs, very rarely other metastatic sites are pleura, bones, liver, lymph node and kidney [10].

8. Diagnosis of meningioma

WHO CNS fifth edition has laid down certain “essential and desirable diagnostic criteria” for the diagnosis of meningioma: [14, 22].

8.1 Essential criteria

It includes classic histopathological features matching at least one of the meningioma subtypes OR suggestive histopathological features combined with biallelic inactivation of NF2 or other classic drivers of conventional meningioma (TRAF7, AKT1, KLF4, SMO, PIK3CA), clear cell meningioma (SMARCE1), or rhabdoid meningioma (BAP1) OR suggestive of histopathological features combined with one of the defined DNA methylation classes of meningioma.

8.2 Desirable criteria

It includes tumour localised to meninges; EMA and SSTR2A positivity on immunohistochemistry; classic copy-number alterations of NF2-mutant meningioma, such as monosomy 22/22q in lower-grade meningiomas, with additional losses of 1p, 6, 10q, 14q, and/or 18 in higher-grade meningiomas.

9. ICD coding for meningioma

• ICD-O coding: 9530/0 Meningioma [14]

• ICD-11 coding: 2A01.0Z Meningiomas, unspecified

10. Evolution of grading system in meningioma

10.1 WHO CNS first edition (1979)

This edition included following categories of meningioma: meningotheliomatous (syncytial), fibrous, transitional, psammomatous, angiomatous, hemangioblastic, hemangiopericytoma, papillary and anaplastic meningioma [45, 46]. Strict criteria for WHO grade I, II and III were however not specified, but it suggested that tumours should be designated as grade II when following features are present: increased cellularity, frequent mitotic activity, high nucleus: cytoplasmic ratio, prominent nucleoli, patternless or sheet like growth and foci of necrosis (either spontaneous or geographic). Grade III meningiomas are anaplastic (malignant) in nature and should have features of malignancy in excess to that observed in grade II tumours [47].

10.2 WHO CNS second edition (1993)

In this edition, more new variants were added: microcystic, secretory, clear cell, chordoid, lymphoplasmacytic and metaplastic meningioma. Atypical meningioma was introduced as a category, but was not clearly defined. Further, Hemangioblastic category got deleted. Hemangiopericytoma was moved to “Mesenchymal, Non-Meningothelial Tumours” category [46]. Meningiomas were graded into three grades: I, II and III. Meningiomas categorised under grade I had low proliferative activity, distinct in nature and can be cured by surgical resection without any chance of recurrence. Grade II meningiomas had features similar as described in previous edition. Grade III meningiomas constitute anaplastic variant. These tumours are either anaplastic from the beginning or develop as a result of transition from grade I or II meningiomas and carry the highest tendency to metastasize [45].

10.3 WHO CNS third edition (2000)

A new variant rhabdoid meningioma was added and was categorised into grade III [48]. Further, the boundaries between benign and atypical meningioma and between atypical and malignant meningioma were not well delineated in the 1993 WHO classification. To address this issue, the 2000 WHO classification, recommended more objective criteria, which was largely based on a series of research from Mayo Clinic [49, 50].

Meningiomas having a low probability of recurrence and/or aggressive growth were included in WHO grade I. It included meningothelial meningioma, fibrous/fibroblastic meningioma, transitional (mixed), psammomatous meningioma, angiomatous meningioma, microcystic meningioma, secretory meningioma, lymphoplasmacyte-rich meningioma, and metaplastic meningioma. Meningiomas with a higher risk of recurrence and/or aggressive growth were classified by the WHO as grades II and III. Atypical meningioma, intracranial clear cell meningioma, and chordoid meningioma were all classified as WHO grade II. Rhabdoid meningioma, papillary meningioma, and anaplastic (malignant) meningioma were classified as WHO grade III. Further, it was recommended, that any subtype or grade of meningioma having high proliferative index and/or brain invasion should be additionally reported [48].

Indicators of tumour proliferation, like MIB-1 labelling index, are helpful in determining the likelihood of meningioma recurrence. Cut-off levels of MIB-1 labelling index distinguishing benign from atypical meningiomas and atypical from malignant meningiomas were not recommended, nonetheless, due to inter-institutional and inter-observer heterogeneity. However, high proliferation indices do offer independent predictive data of tumour recurrence. Therefore, it was advised that if the labelling indices were significantly higher than predicted, a term “with high proliferative activity” should be included to the diagnoses of benign or atypical meningioma [48].

Similarly, it has been demonstrated that grade 1 meningioma have an increased recurrence risk similar to an atypical meningioma, if there is presence of brain invasion [49, 50]. Additionally, the development of a malignant course cannot be necessarily predicted by the presence of brain invasion in an atypical meningioma. Hence, in the third edition, it was suggested that if brain invasion was present, a word “with brain invasion” should be included to the diagnosis of benign or atypical meningiomas. This will alert the clinician to a higher likelihood of recurrence, particularly in the case of a histologically benign (grade 1) meningioma [48].

10.4 WHO CNS fourth edition (2007)

There were no major changes. The only notable change to the WHO grading system in 2007 was the addition of brain invasion as a criteria for atypical meningioma, whereas in WHO third edition (2000) it was suggested as an additional finding to be included in reporting, if present [46, 51].

10.5 Updated WHO CNS fourth edition (2016)

The 2007 edition was further updated in 2016, however no substantial changes were observed in meningioma [46].

10.6 WHO CNS fifth edition (2021)

This edition introduces the major changes incorporating the molecular diagnostics in CNS tumour classification. Roman numerals (I, II, and III) were used to indicate the CNS WHO tumour grades in the past. But the latest fifth edition has recommended the use of Arabic numerals 1, 2, and 3 in the grading system. This change was to bring CNS tumour grades in line with other organ systems [22].

In WHO CNS fifth edition, meningioma is recognised as a single type, having further 15 subtypes reflecting its wide morphological variety. It is currently stressed that the criteria defining atypical or anaplastic (grade 2 or 3) meningioma should be implemented regardless of the underlying subtype. Whereas, chordoid and clear cell meningiomas have been solely placed in CNS WHO grade 2 as they are more likely to recur. Similarly, rhabdoid and papillary morphology are categorised into CNS WHO grade 3 [22].

Following is the WHO CNS 5 grading system for meningioma: [12, 14, 22].

1. CNS WHO grade 1: Tumours have low mitotic activity (< 4/10 high power field); brain invasion absent; < 3/5 atypical features (tumour necrosis, increased cellularity, small tumour cells with high nucleus: cytoplasmic ratio, sheet like growth and prominent large nucleoli).

2. CNS WHO grade 2: This includes intermediate mitotic count (4–19/10 high power field); presence of brain invasion; chordoid or clear cell histopathological subtype; at least presence of 3/5 atypical features (increased tumour cellularity, small tumour cells with high nucleus: cytoplasmic ratio, nucleoli prominence, sheet like or uninterrupted growth pattern, presence of spontaneous necrosis).

3. CNS WHO grade 3: Includes high mitotic activity (≥20/10 high power field); anaplastic histopathological features (sarcoma/carcinoma/melanoma–like morphology), TERT promoter mutation; homozygous deletion of CDKN2A and/or CDKN2B genes.

11. Errors in grading due to treatment related effects

Few hospitals perform preoperative embolization of meningioma to reduce intraoperative bleeding and the pathologist might not be informed regarding embolization procedure. The histological alterations caused by embolization, such as macro nucleoli, necrosis and compensatory proliferation with more mitotic figures, may result in the tumour being over graded. Over grading can be prevented through communication between the neurosurgeon and neuropathologist regarding the use of embolization. Similarly, acknowledging the patient’s prior radiation therapy would help to allay features of necrosis or cellular atypia [52].

12. Macroscopic appearance

On gross appearance, majority of the meningiomas are well circumscribed, globular, solid masses attached to the underlying duramater. These tumours are firm to rubbery in consistency. Some meningiomas are lobulated or have bilobed appearance and grow in a flat en plaque like pattern along the dura of sphenoid bone. Few subtypes of meningioma have characteristic morphology which provides a clue in their diagnosis. Psammomatous meningiomas of the spinal region have gritty texture because of presence of psammoma bodies (calcified spherules). Fibrous meningioma has a smooth surface [14]. Chordoid meningioma have a soft, gelatinous, cystic consistency with a smooth contour and has translucent areas on cut surface. Atypical meningiomas are seen invading the adjacent brain parenchyma [35].

Majority of grade 1 meningiomas compress and displace the adjacent brain, but are neither adherent nor invasive, and thus can be easily removed. However, higher-grade meningiomas are widely adherent and invasive to the adjacent brain tissue and show necrotic changes. Sometimes, these tumours are able to infiltrate the dural sinuses as well. Parasagittal meningiomas have the ability to partially or totally block the superior sagittal sinus. Rarely, meningiomas can penetrate the skull bone and produce reactive hyperostosis of the skull vault. Cerebral arteries and/or cranial nerves may be attached to or encased by meningiomas, but are usually not infiltrated [12, 14].

13. Histopathological subtypes of meningioma

13.1 Meningothelial meningioma

This is the most common subtype of meningioma. The tumour cells resemble the morphology of arachnoid cap cells. The cells are arranged in syncytia like lobules separated by thin collagenous septae (Figure 1a). The cells are monomorphic, with few cells having nuclear haloes, pseudo inclusions and eosinophilic cytoplasm. Very rarely, psammoma bodies and meningothelial whorls are noted [12, 14].

Figure 1.

Photomicrographs of (a) meningothelial meningioma having cells arranged in syncytia like lobules separated by thin collagenous septae (microscopic filed with 100X in haematoxylin and eosin stain); (b) fibrous meningioma characterised by spindle cells arranged in parallel and interlacing bundles (microscopic filed with 100X in haematoxylin and eosin stain); (c) Psammomatous meningioma displaying calcified psammoma bodies (black arrow, microscopic filed with 40X in haematoxylin and eosin stain); and (d) angiomatous meningioma having hyalinised thick walled blood vessels (orange arrow, microscopic filed with 40X in haematoxylin and eosin stain).

13.2 Fibrous meningioma

This subtype is characterised by spindle cells arranged in parallel, storiform, or interlacing bundles in a collagen-rich matrix (Figure 1b). The fascicles formed by tumour cells contain variable amounts of intercellular collagen [14, 24].

13.3 Transitional meningioma

This subtype has features of both meningothelial and fibrous patterns, as well as some transitional characteristics. There are places that have both lobular and fascicular architecture, with some of these areas being in between the two patterns (thus the term “transitional”). This subtype frequently exhibits whorls and psammoma bodies [12, 14].

13.4 Psammomatous meningioma

Psammoma bodies predominate over the tumour cells in this subtype. (Figure 1c). Sometimes, these psammoma bodies overlap each other and combine to form enormous, calcified masses. Although actual meningioma cells can be scarce and difficult to identify, immunohistochemistry markers for EMA or SSTR2A can highlight them. The non-calcified foci resemble the morphology of fibrous or transitional subtype [12, 14].

13.5 Angiomatous meningioma

This subtype is characterised by predominance of small blood vessels, which can be thick or thin walled and are variably hyalinised (Figure 1d). The actual meningioma tumour cells may be difficult to locate between these vessels. Additionally, microcytic or metaplastic areas can also be present, however the tumour cells of these regions show degenerative changes and nuclear atypia. Adjacent brain tissue shows oedematous changes [12, 14].

13.6 Microcystic meningioma

On histology, the microcystic subtype of meningioma comprises microcysts made of cells with thin, elongated processes that give the tissue a cobweb-like appearance (Figure 2a). The cysts may coalesce together and form large macrocyst. Degenerative nuclear atypia can also be observed in microcystic meningioma, just like in angiomatous meningioma, which raises the suspicion of a higher-grade tumour, however, this variant is benign in nature. Associated cerebral edema can be noticed [12, 14]. On immunohistochemistry, these microcystic areas are diffusely and weakly positive for carbonic anhydrase IX (hypoxic marker) [53].

Figure 2.

Photomicrographs of (a) microcystic meningioma having microcysts with cobweb like appearance (microscopic filed with 40X in haematoxylin and eosin stain); (b) secretory meningioma characterised by gland like epithelial pattern with lumen containing eosinophilic secretions (microscopic filed with 100X in haematoxylin and eosin stain); (c) Lymphoplasmacyte-rich meningioma displaying dense collection of lymphocytes (microscopic filed with 100X in haematoxylin and eosin stain); and (d) clear cell meningioma having large polygonal clear cells with clear cytoplasm and presence of interstitial collagen (microscopic filed with 100X in haematoxylin and eosin stain).

13.7 Secretory meningioma

This subtype is characterised by gland like epithelial pattern, the lumen of which contains PAS positive eosinophilic secretions which resemble psammoma bodies, hence termed as pseudo psammoma bodies (Figure 2b). These secretions are positive for carcino embryonic antigen (CEA). Therefore, elevated CEA levels is observed in these patients, the levels fall down by resection and again rises up by recurrence of tumour. Peritumoral oedema is present. KLF4 and TRAF7 mutations are seen. The surrounding tumour cells are also positive for CEA and keratin [54].

13.8 Lymphoplasmacyte-rich meningioma

This subtype is very rare, seen in less than 1% cases. On histopathology, the tumour is characterised by presence of extensive chronic inflammatory cells, predominantly macrophages and very few plasma cells (Figure 2c). Meningothelial component is generally scant [55].

13.9 Metaplastic meningioma

This variant has presence of mesenchymal components which can be present singly or in combinations and it includes bone, cartilage, fat, myxoid or xanthomatous changes. However, this does not constitute true metaplasia and do not carry any clinical significance [12, 14].

13.10 Atypical meningioma

Atypical meningioma is categorised into CNS WHO grade 2. It is an intermediate grade meningioma. This subtype is characterised by brain invasion, which is described as irregular, tongue shaped protrusion of tumour cells into the brain parenchyma without intervening leptomeninges. Infiltration of tumour cells into the perivascular Virchow-Robin spaces does not come under brain invasion as piamater is not breached. Sometimes, it is difficult to locate brain parenchyma in between the tumour cells, in such cases immunohistochemistry marker GFAP is useful for highlighting intervening glial tissue [12, 14]. Studies say that these tumours have higher chances of recurrence despite total resection and show bony involvement [56, 57]. Interestingly, it should be noted that mere presence of bone involvement is not considered as a criteria for atypical meningioma.

13.11 Chordoid meningioma

This is a rare subtype accounting for 0.5–1% of intracranial meningiomas. The tumour has equal preponderance for both males and females [58]. The tumour is named “chordoid” because it resembles “chordoma” on histopathology, and is characterised by small epitheloid to spindled shaped tumour cells arranged in cords and trabeculae having foamy vacuolated cytoplasm embedded in a mucin rich matrix. Interspersed chronic inflammatory cells are also present [14, 59]. On immunohistochemistry, the tumour cells are positive for EMA, podoplanin and patchy cytokeratin whereas negative for S100, brachyury and GFAP [58]. Immunoreactivity with NHERF1 has also been reported [60]. Rarely these patients have associated comorbid haematological disorders like Castleman disease and anaemia [61].

Chordoid meningiomas have been designated as CNS WHO grade 2 because these tumours have high chance of recurrence. The increased recurrence risk is due to high proliferative activity and atypical histopathological features [58].

13.12 Clear cell meningioma

It is a rare variant and accounts for 0.2–0.8% of all meningiomas. This subtype has round to polygonal cells with clear, glycogen-rich cytoplasm and abundant perivascular and interstitial collagen (Figure 2d). The clear cells are arranged in pattern-less or sheet like architecture [14]. The cytoplasmic glycogen is PAS-positive and diastase-sensitive. Occasionally, perivascular and interstitial collagen combines to form large acellular eosinophilic collagen areas. Whorls and psammoma bodies are absent. Mitotic activity is not prominent [15]. On immunohistochemistry, the tumour cells are positive for EMA, PR and Vimentin. Characteristic loss of nuclear expression with SMARCE1 is noted [22]. Clear cell meningiomas are classified as CNS WHO grade 2 because of their association with more aggressive behaviour, recurrence and cerebrospinal fluid seeding [14]. Clear cell meningioma should be distinguished from metastatic clear cell renal cell carcinoma, the latter is strongly positive for keratin, EMA and negative for S100 protein [45].

13.13 Anaplastic (malignant) meningioma

Anaplastic meningioma is rare and comprises 1–3% of all meningiomas. It is designated under CNS WHO grade 3 category and has an aggressive malignant morphology. The tumours can invade brain parenchyma and show extensive necrosis (Figure 3). These tumours can be primary (arise de novo) or secondary (progress from grade 1 or 2 meningioma). Most of the time, due to higher grade morphology, meningothelial origin is difficult to identify, thus it can be confirmed by immunohistochemistry and/or genetic testing. On immunohistochemistry, the cells are positive for EMA, SSTR2A, focal CK AE1/AE3 and STAT6. Genetically, these tumours show TERT promoter mutation and homozygous deletion of CDKN2A and/or CDKN2B genes, which can by identified by molecular testing [14, 22].

Figure 3.

Photomicrographs from a case of anaplastic meningioma (a) tumour necrosis (microscopic filed with 100X in haematoxylin and eosin stain), (b) malignant cells with large prominent nucleoli and bizzare morphology (microscopic filed with 400X in haematoxylin and eosin stain), (c) atypical tripolar mitotic figure (blue arrow, microscopic filed with 400X in haematoxylin and eosin stain), and (d) tumour cells showing strong intense cytoplasmic positivity for EMA (microscopic filed with 400X IHC).

The differentials of anaplastic meningioma include metastatic carcinoma, melanoma of meninges, sarcoma and solitary fibrous tumour (SFT). Immunohistochemistry plays a major role in distinguishing them. Metastatic carcinoma is strongly and diffusely positive with CK AE1/AE3. Meningeal melanoma will show brown coloured melanin pigment, which can be confirmed with melanin bleach and Fontana-Masson special stain. Further melanoma will be positive for HMB45, Melan A and negative for EMA. Sarcomas will be Vimentin positive diffusely and SSTR2A negative. SFT will display staghorn vessels and positivity with CD34, STAT 6, BCL2 whereas negative for EMA and SSTR2A [12, 29, 40].

13.14 Papillary meningioma

They are categorised into CNS WHO grade 3. The tumour cells surround the thin walled blood vessels in a perivascular, pseudo rosette-like pattern. There is peritumoral oedema, bony destruction, hyperostosis reaction and rarely cyst formation [14]. A close genetic and molecular link is suggested between papillary and rhabdoid meningioma as few tumours display rhabdoid cytomorphology arranged in papillary architecture [62].

13.15 Rhabdoid meningioma

This subtype comes under CNS WHO grade 3. The tumour cells have typical rhabdoid morphology, which are large plump cells, with eccentrically placed round nuclei, open vesicular nuclear chromatin, prominent large nucleoli and hyaline paranuclear inclusions. Whorl formation is sometimes retained. Few tumour cells may have papillary features suggesting close association with papillary meningioma. These tumours have high mitotic activity and are aggressive in nature [14]. On immunohistochemistry, the tumour cells are EMA, PR, Vimentin positive. The inclusions are positive for cytokeratin. Characteristically, INI1 expression is retained [63].

14. Other rare variants of meningioma

14.1 Cystic meningioma

This variant accounts for 4–7% of meningiomas and is characterised by the presence of intratumoural or peritumoral cysts. The pathogenesis of cyst formation is tumour degeneration and necrosis which produces a macro–cavitation attributed to intracellular regressive process [64]. The content of the cyst may be CSF, xanthochromic fluid, clear fluid or haemorrhagic fluid [65].

There are four types of intracranial cystic meningiomas as per Nauta classification: [66].

1. Type 1–intratumoral cyst, present centrally, within the tumour

2. Type 2–intratumoral cyst, present peripherally, surrounded by tumour

3. Type 3–peritumoral cyst, present peripherally, adjacent to brain parenchyma

4. Type 4–peritumoral cyst, present between tumour and brain parenchyma, cyst wall formed by arachnoid layer

14.2 Radiation induced meningioma (RIM)

This tumour occurs due to exposure to ionising radiation, with a long latent period of 2 to 63 years. These are multiple and aggressive. High grade RIM have increased VEGH levels and mRNA overexpression [67]. These tumours have high MIB-1 labelling index of more than 10% due to increased proliferative activity and have a higher chance of recurrence within 1 year of resection [68].

14.3 Ossified meningioma

This variant comprises 1–5% and is located intraspinal or intracranial. It is a very slow growing tumour characterised by complete ossification/calcification of tumour foci. It is different from psammomatous meningioma, as viable meningothelial cells are present in the latter. It is considered as a subtype of grade 1 metaplastic meningioma as the pathogenesis postulated is the metaplastic change of arachnoid and interstitial cells [69, 70].

15. Squash cytology

Squash cytology is a simple, rapid and friendly technique for preliminary intraoperative diagnosis of CNS tumours/space occupying lesions. It helps to arrive at a reliable diagnosis on crush prep smears and gives guidance to the operating surgeon [71]. The cytological feature of meningioma seen on squash cytology include epithelioid cells with round to oval nuclei and delicate streaked cytoplasm. Sometimes, intranuclear pseudo inclusions are also noted. Meningothelial whorls/nests and psammoma bodies are highly characteristic. A major limitation observed is with tumours having abundant collagen and firm rubbery consistency, because adequate preparation of smears become difficult and challenging [12, 14].

Apart from the above described classical morphology, few subtypes of meningioma have additional characteristic specific features which provides a clue in their intra operative diagnosis:

• Chordoid meningioma–tumour cells are arranged as cords, have eosinophilic cytoplasm and embedded in an abundant myxoid rich background [72]

• Clear cell meningioma–spindled to polygonal cells having whorled and syncytial architecture, cells have bland nuclear chromatin and vacuolated cytoplasm [73]

• Rhabdoid meningioma–rhabdoid cells having eccentrically placed nuclei, vesicular chromatin, prominent nucleoli, dense eosinophilic cytoplasm and distinct cell borders. Cytoplasmic hyaline inclusions, pseudo nuclear inclusions, stout processes and increased mitotic activity may be identified [74]

• Papillary meningioma–papillary architecture seen on low power field [62]

• Anaplastic meningioma–increased atypical mitotic figures [40].

16. Conclusion

Meningioma is the most common tumour of CNS, seen most commonly in females at around 66 years of age, however rarely reported in children also. They can be asymptomatic or sometimes, symptomatic. Since the inception of WHO CNS blue books, with every edition, there has been an evolution in histopathological grading system of meningioma. The current WHO grading system has categorised meningioma into 3 grades. Grade 1 is the most common and has best prognosis. Grade 2 is intermediate and grade 3 is rare with worst prognosis. Various molecular and genetic alterations have been identified with novel diagnostic techniques. Immunohistochemistry helps to differentiate meningioma from other mimicking lesions and also helps to highlight entrapped glial tissue within the tumour foci. Squash cytology plays unique role in intra–operative diagnosis of the lesion with their characteristic morphology. Very rarely meningioma metastasize, however if present, lungs are the most common site.

17. Future aspects

Few more rare subtypes of meningiomas have been identified. These include oncocytic, mucinous, sclerosing, whorling–sclerosing, GFAP expressing, meningothelial rosettes and granulo-filamentous inclusion–bearing variants [75, 76, 77]. However, more studies and research is needed, to understand the same. Many treatment related (particularly immunotherapy) studies are being done, which is out of the purview of our chapter, as it focuses mainly on pathological aspects.

Acknowledgments

The authors wish to thank senior histopathology technical staff Mrs. Nazia and Mr. Rahul for excellent section cutting and staining.

Conflict of interest

The authors declare no conflict of interest.

Abbreviations

WHO	World Health Organisation
CNS	central nervous system
IHC	immunohistochemistry
NF2	neurofibromatosis type 2
VHL	Von Hippel Lindau
MEN1	multiple endocrine neoplasia type 1
YAP1	yes associated protein 1
AKT1	AKT serine/threonine kinase 1
PIK3CA	phosphatidyl inositol-4, 5-bisphosphate 3-kinase catalytic subunit alpha
TRAF7	TNF receptor associated factor 7
KLF4	kruppel like factor 4
SMO	protein smoothened
DNA	deoxyribo nucleic acid
TERT	telomerase reverse transcriptase
SMARCE1	SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily E member 1
CDKN2A/B	cyclin dependent kinase inhibitor 2A/B
FISH	fluorescence in situ hybridization
BAP1	BRCA1 associated protein 1
PBRM1	polybromo 1
RNA	ribo nucleic acid
H3 p.K28me3 (K27me3)	tri methylation of lysine 27 on histone H3
PI3K	phosphoinositide 3 kinases
POLR2A	RNA polymerase II subunit A
EMA	epithelial membrane antigen
SSTR2A	somatostatin receptor 2
ICD	international classification of diseases
MIB-1	E3 ubiquitin protein ligase
CEA	carcino embryonic antigen
GFAP	glial fibrillary acidic protein
NHERF1	Na+/H+ exchanger regulatory factor 1
PAS	periodic acid schiff
PR	progesterone receptor
CK AE1/AE3	cytokeratin AE1/AE3
STAT6	signal transducer and activator of transcription 6
SFT	solitary fibrous tumour
HMB45	human melanoma black 45
CD34	cluster of differentiation 34
BCL2	B cell lymphoma 2
INI1	integrase interactor 1
CSF	cerebro spinal fluid
RIM	radiation induced meningioma
VEGF	vascular endothelial growth factor
mRNA	messenger ribonucleic acid