Aetiology

Most meningiomas are idiopathic. Known risk factors include a history of cranial radiotherapy or exposure to x-rays (as low as 1-2 Gy). Meningioma is a common secondary neoplasm after treatment of childhood cancers with radiation, with an estimated incidence of roughly 5%, and a median time to development of 22 years.[8][9][10][11] Patients who received a radiation dose of 24 Gy or higher have a 30-fold increased risk of meningioma development.[9]

There is an association with a history of breast or thyroid cancer.[12][13][14] However, one systematic review and meta-analysis did not find a statistically significant odds ratio of meningioma in female patients with breast cancer.[15] An increased incidence has been reported in women exposed to fertility treatments or cyproterone, and in transgender people exposed to high-dose oestrogen/progesterone, although the mechanisms and causality remain to be determined.[16][17][18][19] Rare genetic syndromes are associated with meningiomas, most commonly neurofibromatosis 2.[20]

There is a possible link to previous head trauma; however, the evidence for this is weak.[21][22][23]

Pathophysiology

Meningiomas are thought to arise from the meningothelial cells of the arachnoid layer (arachnoid-cap cells).[24]​ They are extramedullary tumours that grow from the meninges surrounding the brain and spine, and can also occur in the ventricles of the brain, or anywhere along the dura in the intracranial vault and along the spinal canal. As meningiomas grow, they can cause compression of the surrounding brain and nerves, leading to symptoms of mass effect such as vision loss caused by optic nerve compression. Meningiomas may involve the bone, producing hyperostosis with visible bony growth in some cases, and leading to mass effect in affected regions (e.g., proptosis).

Classification

World Health Organization (WHO) classification of meningioma (5th ed)[2]​​

The WHO Classification of Tumours of the Central Nervous System was revised in 2021 for the 5th edition and was the first foray into integrating molecular features into meningioma grading.[3]​ The histopathological grading and classification of meningiomas remains largely unchanged, with brain invasion as a diagnostic criterion for atypical meningiomas (WHO grade 2). TERT promoter mutations and homozygous loss of CDKN2A/B have now been included as standalone criteria for WHO grade 3 meningiomas.

Grade 1:

  • Meningothelial meningioma

  • Fibrous (fibroblastic) meningioma

  • Transitional (mixed) meningioma

  • Psammomatous meningioma

  • Angiomatous meningioma

  • Microcystic meningioma

  • Secretory meningioma

  • Lymphoplasmacyte-rich meningioma

  • Metaplastic meningioma

Grade 2 (Atypical):

  • Chordoid meningioma

  • Clear cell meningioma

  • More than 4 to 19 mitoses per 10 high power fields

  • Brain invasion

  • Cellular atypia such as sheeting, necrosis, hypercellularity, high nuclear/cytoplasm ratio

Grade 3 (Malignant):

  • Papillary meningioma

  • Rhabdoid meningioma

  • 20 or more mitoses per 10 high power fields

  • CDKN2A/B homozygous loss, TERT promoter mutation, BAP1 mutation

The WHO classification has evolved to identify more high-risk features of meningiomas that confer a higher risk of recurrence. This has resulted in an increased prevalence of atypical meningiomas; traditionally, they accounted for about 5% of cases, but they now comprise 20% to 35% of all cases.[4]​

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