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 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 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). 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).
Atypical (WHO grade II) meningiomas are characterized histologically by 1:
- 4 to 19 mitoses per ten high-power fields
- 3 or more of the following 5 histologic features:
- 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% 1.
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) 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.
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 6.
No effective chemotherapeutic agents have been identified 5.
The five-year recurrence rate is significantly higher (41%) than that seen in grade I (benign) meningiomas (12%) 3.
- 1. Backer-Grøndahl T, Moen BH, Torp SH. The histopathological spectrum of human meningiomas. Int J Clin Exp Pathol. 2012;5 (3): 231-42. Free text at pubmed - Pubmed citation
- 2. Watts J, Box G, Galvin A et-al. Magnetic resonance imaging of meningiomas: a pictorial review. Insights Imaging. 2014;5 (1): 113-22. doi:10.1007/s13244-013-0302-4 - Free text at pubmed - Pubmed citation
- 3. Toh CH, Castillo M, Wong AM et-al. Differentiation between classic and atypical meningiomas with use of diffusion tensor imaging. AJNR Am J Neuroradiol. 2008;29 (9): 1630-5. doi:10.3174/ajnr.A1170 - Pubmed citation
- 4. Filippi CG, Edgar MA, Uluğ AM et-al. Appearance of meningiomas on diffusion-weighted images: correlating diffusion constants with histopathologic findings. AJNR Am J Neuroradiol. 2001;22 (1): 65-72. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 5. Modha A, Gutin PH. Diagnosis and treatment of atypical and anaplastic meningiomas: a review. Neurosurgery. 2006;57 (3): 538-50. Pubmed citation
- 6. Walcott BP, Nahed BV, Brastianos PK et-al. Radiation Treatment for WHO Grade II and III Meningiomas. Front Oncol. 2013;3: 227. doi:10.3389/fonc.2013.00227 - Free text at pubmed - Pubmed citation
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