Atypical meningioma

Last revised by Arlene Campos on 7 May 2024

Atypical meningioma refers to a more aggressive form of meningioma and denotes a WHO grade 2 tumor (along with two histological variants, clear cell meningioma and chordoid meningioma). Atypical meningiomas account for 20-30% of all meningiomas 1,2

It should be noted that epidemiology, clinical presentation, and radiographic features do not reliably distinguish grade 1 (benign) from grade 2 (atypical) meningiomas, and thus they are not unnecessarily repeated here. Generally, these tumors 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). This is perhaps unfortunate as it can cause confusion among clinicians who are used to refer to something being 'atypical' when it doesn't look like the 'usual' garden-variety tumor/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 1 tumors (see meningioma article for further discussion of histological variants). 

Atypical (WHO grade 2) meningiomas are characterized histologically by 1,2

  • 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 of brain parenchyma

It is important to note when reading older literature, that brain invasion of an otherwise "benign" grade 1 tumor has only been sufficient to designate it a grade 2 tumor in WHO classifications 2. Furthermore, this feature remains somewhat controversial due to the lack of consistent sampling of the adjacent brain when samples are sent for histological assessment. In the 5th edition of the WHO classification (2021) this criteria explicitly requires tumor breaching the pia mater and not merely indenting the brain or extending along perivascular spaces 7.

Generally, it is impossible to distinguish between benign (WHO grade 1) from atypical (WHO grade 2) and anaplastic (WHO grade 3) meningiomas with any confidence based on general morphology. The most reliable feature is the presence of lower apparent diffusion coefficient values (reflecting higher cellularity) 3,4.

Importantly, the presence of vasogenic edema 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 tumor, is also surprisingly difficult to predict on MRI. 

First-line therapy is surgical resection, with radiotherapy (external beam or brachytherapy) often added both to 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

The five-year recurrence rate is significantly higher (41%) than that seen in grade 1 (benign) meningiomas (12%) 3

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