Meningeal hemangiopericytoma (historical)

Meningeal haemangiopericytomas are rare tumours of the meninges, relatednow considered to be aggressive versions of solitary fibrous tumours of the dura, often presenting as a large and locally aggressive dural mass, frequently extending through the skull vault. They are difficult to distinguish on imaging from the far more common meningioma, but but are treated similarly with surgical resection with or without radiotherapy to reduce the risk of recurrence, which is high. 

For a general discusiondiscussion of non-meningeal haemangiopericytomas please refer to the general article on haemangiopericytoma.

Epidemiology

Haemangiopericytomas account for less than 1% of all intracranial tumours ¹. They are typically encountered in younger adults (30s-40s) with up to 10% being diagnosed in children ³. Slight male predilection (M:F 1.4:1) ^3,6. 

Clinical presentation

As these tumours are typically large, usually supratentorial,Clinical presentation is usually due to mass effect and will vary depending on location. Headache, seizures, focal neurological dysfunction may all be presenting features ³. Additionally, in up to 20% of cases these tumours (particularly anaplastic haemangiopericytomas) cancan metastasize systemically, typically to liver, lung and bone ^1,3,6.

Pathology

Haemangiopericytomas were previously classified as angioblastic sub-type meningiomas, then considered to arise from smooth muscle perivascular pericytes of dural capillaries (pericytes of Zimmerman) ^3, but the most recent studies suggesting that these lesionlesions are actually arising from fibroblast and considered in the spectrum of the solitary fibrous tumours of the dura ⁴. This is further supported by the fact that both entities share a similar genetic alteration: genomic inversion of 12q13 locus resulting in fusion of NAB2 and STAT6 genes, the latter expressed and able to be assessed using immunohistochemistry techniques ⁶. In fact, in the latest (2016)2016 update to the WHO classification of CNS tumours the two entities have been combined ⁵. 

They are more aggressive than meningiomas, have a higher frequencySolitary fibrous tumours of recurrence, and are considered a grade II tumour in the WHO Classification (in contrast to meningiomas whichdura are WHO I grade I). Anaplasticone lesions, whereas haemangiopericytomas are WHO grade II or III (anatplastic) tumours,⁶. 

Microscopic appearance

Haemangiopericytomas are highly cellular tumours with frequent mitoses (grade II <5 per 10 HPF; grade III ≥5 per 10 HPF) and often with areas of necrosis ⁶.  The cells are separated by a limited amount of delicate reticulin fibers and have numerous 'staghorn' thin walled branching vessels, the latter a propensity to metastasise systemicallyfeature shared by solitary fibrous tumours of the dura ³⁶. 

Immunophenotype

Ideally, the diagnosis is confirmed by assessing for STAT6 expression by immunohistochemistry or identifying NAB2-STAT6 fusion ⁶. Haemangiopericytomas have a number of useful immunohistochemical markers ⁶: 

• STAT6: positive
• CD34: positive
• vimentin: positive

Ki-67 proliferation index is typically around 10% ⁶. 

Radiographic features

Haemangiopericytomas are almost always solitary, usually supratentorial masses, often lobulated in contour. They are highly vascular and have a tendency to erode adjacent bone ³.

Other common location is posterior fossa in posterior occipital region.

CT

• vivid enhancement
• erosion of adjacent bone
• no hyperostosis
• no calcification

MRI

Features on various sequences include

• T1: isointense to grey matter
• T1 C+ (Gd)
  • vivid enhancement
  • heterogeneous
  • may have a narrow base of dural attachment
  • dural tail sign is seen, more commonly in grade II tumours
• ​T2
  • isointense to grey matter
  • multiple flow voids on MRI (need to distinguish from spoke-wheel appearance of meningioma)
  • adjacent brain oedema frequently present
• MR spcetroscopy
  • high myo-inositolmyoinositol ³
  • absent alanaine peak (present in meningiomas) ³
• DWI
  • intermediate restricted diffusion (less than meningioma) 
  • minimum ADC ~ 1100 (+/- 130) x 10^-6 mm²/s 

Angiography

• ECA, ICA and vertebral supply common
• highly vascular
• corkscrew arteries
• fluffy tumour stain
• lack of early draining veins ³
• useful for pre-operative embolisation
• assessment of dural venous sinus involvement 

Treatment and prognosis

Total surgical excision is recommended, with pre-operative catheter embolisation helpful in limiting blood loss ³. Adjuvant radiotherapy to reduce the incidence of recurrence has also been advocated ^1,3. 

Differential diagnosis

The main differential diagnosis is that of menigioma although all other dural masses should be considered. Distinguishing a haemangiopericytoma from a meningioma can be difficult as they have similar appearances on both CT and MRI. 

• meningioma
  • older patients (>50 years of age)
  • smoother
  • central vascular spoke-wheel vascular supply
  • less likely to erode adjacent bone
  • more likely to cause hyperostosis
  • more likely to be multiple
  • do not metastasisevery unlikely to metastasize
  • almost alwaysusually have a broad dural attachment and dural tail, both of which haemangiopericytomas sometimes lack
  • MRS: alanine peak, absent myo-inositolmyoinositol peak
  • Immunohistochemistry: EMA positive, CD34 and STAT6 negative
• solitary fibrous tumour