atypical meningioma

Dural masses:

meningiomas and their mimics. Atypical meningioma. a, b Pre- and post-contrast axial T1-weighted MR images showing a heterogeneously enhancing mass with a wide dural base, CSF cleft and bony involvement. The cortical interface is less distinct when compared with Fig. 1, and there is a moderate amount of associated vasogenic oedema. c Axial T2-weighted MR image of the same lesion. d Susceptibility-weighted images show internal calcification

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Dural masses:

meningiomas and their mimics. Atypical meningioma. a, b Pre- and post-contrast axial T1-weighted MR images showing a heterogeneously enhancing mass with a wide dural bass, dural tail and CSF cleft. The cortical interface is poorly outlined suggestive invasion of the underlying brain parenchyma as well as the adjacent skull. c Axial T2-weighted MR image of the same lesion with surrounding vasogenic oedema and midline shift. d A diffusion-weighted image shows heterogeneous areas of restricted diffusion extending into the adjacent cortex

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Radiopaedia • CC-by-nc-sa 3.0 • de

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

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 tumors grow faster, have more heterogeneous/aggressive imaging appearances, and have a tendency to recur early.

Terminology

The use of the word "atypical" to denote a higher grade, refers to the histological appearances (see below). However, it is perhaps unfavorable 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 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 I tumors (see meningioma article for further discussion of histological variants).

Pathology

Atypical (WHO grade II) meningiomas are characterized histologically by :

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 it is only since the WHO 2007 classification, that infiltration into brain parenchyma of an otherwise "benign" grade I tumor was sufficient to designate it a grade II tumor. As such, the incidence of grade II tumors increased to ~30% .

Radiographic features

Generally, it is impossible to confidently distinguish benign (WHO grade I) from atypical (WHO grade II) and anaplastic (WHO grade III) meningiomas based on general morphology. The most reliable feature is the presence of lower apparent diffusion coefficient values (reflecting higher cellularity) .

Importantly, the presence of vasogenic edema in adjacent brain parenchyma is not a predictor of atypical or anaplastic histology .

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

Treatment and prognosis

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 .

No effective chemotherapeutic agents have been identified .

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