Atypical meningioma

Atypical meningioma refers to a more aggressive form of meningioma and denotes a WHO grade II tumour (along with two histological variants clear cell meningioma and chordoid meningioma). Atypical meningiomas account for 20-30% of all meningiomas 1,3

It should be noted that epidemiology, clinical presentation and radiographic features do not reliably distinguish grade I (benign) from grade II (atypical) meningiomas, and thus they are not unnecessarily repeated here. Generally, these tumours grow faster, have more heterogeneous/aggressive imaging appearances, and have a tendency to recur early. 

The use of the word "atypical" to denote a higher grade, refers to the histological appearances (see below). However, it is perhaps unfavourable as it causes confusion for many other clinicians who are used to referring to something being 'atypical' to indicate that it doesn't look like the 'usual' garden-variety tumour/condition. One should therefore not confuse "atypical meningioma" with histological variants, many of which have unusual imaging and histological features but are nonetheless WHO grade I tumours (see meningioma article for further discussion of histological variants). 

Atypical (WHO grade II) meningiomas are characterised histologically by 1

  • 4 to 19 mitoses per ten high-power fields
  • 3 or more of the following 5 histologic features:
    • necrosis
    • sheet-like growth
    • small cell change
    • increased cellularity
    • prominent nucleoli
  • direct invasion into brain parenchyma

It is important to note when reading older literature, that it is only since the WHO 2007 classification, that infiltration into brain parenchyma of an otherwise "benign" grade I tumour was sufficient to designate it a grade II tumour. As such, the incidence of grade II tumours increased to ~30% 1.

Generally, It is not possible to confidently distinguish benign (WHO grade I) from atypical (WHO grade II) and anaplastic (WHO grade III) meningiomas on general morphology. The most reliable feature is the presence of lower ADC values (reflecting higher cellularity) 3,4.

Importantly presence of vasogenic oedema in adjacent brain parenchyma is not a predictor of atypical or anaplastic histology 3

Brain invasion, although by definition denoting at least a grade II tumour, is also surprisingly difficult to predict on MRI. 

First line therapy is surgical resection with radiotherapy (external beam or brachytherapy) often added both for complete and incomplete resections (see Simpson grade). Radiation has been shown to improve local control and prolongs overall survival 6

No effective chemotherapeutic agents have been identified 5

Five year recurrence rate is significantly higher (41%) than that seen in grade I (benign) meningiomas (12%) 3

 


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Article Information

rID: 42383
Section: Pathology
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