Meningeal hemangiopericytoma (historical)

Hemangiopericytomas of the meninges are rare tumors of the meninges, now considered to be an aggressive form of solitary fibrous tumors of the dura. They often present as large and locally aggressive dural masses, frequently extending through the skull vault. They are difficult to distinguish on imaging from the far more common meningioma. Still, they are treated similarly with surgical resection with or without radiotherapy to reduce the risk of recurrence, which is high. 

The remainder of the article presents a historical perspective of this entity. For an updated discussion, please refer to solitary fibrous tumors of the dura.

Terminology

Hemangiopericytomas have been enigmatic tumors with a long and checkered history of changing name and classification. 

They were previously classified as angioblastic subtype meningiomas, then considered to arise from smooth muscle perivascular pericytes of dural capillaries (pericytes of Zimmerman) ³.

It has since been shown that these lesions arise from fibroblasts and are in the spectrum of the solitary fibrous tumors 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 the fusion of NAB2 and STAT6 genes, the latter expressed and able to be assessed using immunohistochemistry techniques ⁶. 

This resulted in the term being abandoned throughout the body, but for some time it tenaciously persisted in the CNS classification on account of what was felt to be distinctive imaging features. 

In the revised 4^th edition (2016) of the WHO classification of CNS tumors, it ceased to be a distinct entity but the term remained in use under the compound diagnosis "solitary fibrous tumor/hemangiopericytoma" ⁶. In the 5^th edition (2021) the term was finally officially retired ⁷. Solitary fibrous tumors grade 2 or 3 are equivalent to hemangiopericytomas ⁷.  

Epidemiology

Hemangiopericytomas accounted for <1% of all intracranial tumors ¹. They were typically encountered in younger adults (30-50 years) with up to 10% being diagnosed in children ³. There is a slight male predilection (M:F = 1.4:1) ^3,6. 

Clinical presentation

Clinical presentation was 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 tumors can metastasize systemically, typically to liver, lung, and bone ^1,3,6.

Pathology

Solitary fibrous tumors of the dura can be graded from WHO grade 1 to 3 with what traditionally has been termed hemangiopericytomas being grade 2 or 3 ⁶.

Microscopic appearance

Hemangiopericytomas were highly cellular tumors 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" vessels, the latter a feature shared by solitary fibrous tumors of the dura ⁶. 

Immunophenotype

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

• STAT6: positive

• CD34: positive

• vimentin: positive

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

Radiographic features

Hemangiopericytomas were almost always solitary, usually supratentorial masses, often lobulated in contour. They were highly vascular and had a tendency to erode adjacent bone ³.

Another common location was the posterior fossa in the posterior occipital region.

CT

• vivid enhancement

• erosion of adjacent bone

• no hyperostosis

• no calcification

MRI

Features on various sequences included:

• 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 tumors

• ​T2

  • isointense to grey matter

  • multiple flow voids on MRI (need to distinguish from the spoke-wheel appearance of meningioma)

  • adjacent brain edema frequently present

• MR spectroscopy

  • high myoinositol ³

  • absent alanine peak (present in meningiomas) ³

• DWI/ADC

  • intermediate restricted diffusion (less than meningioma) 

  • minimum ADC ~1100 (+/- 130) x 10^-6 mm²/s 

Angiography (DSA)

• external carotid, internal carotid, and vertebral artery supply common

• highly vascular

• corkscrew arteries

• fluffy tumor stain

• lack of early draining veins ³

• useful for pre-operative embolization

• assessment of dural venous sinus involvement 

Treatment and prognosis

Total surgical excision was recommended, with pre-operative catheter embolization 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 was that of meningioma although all other dural masses should be considered. Distinguishing a hemangiopericytoma from a meningioma was 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

  • very unlikely to metastasize

  • usually, have a broad dural attachment and dural tail

  • MRS: alanine peak, absent myoinositol peak

  • immunohistochemistry: EMA positive, CD34 and STAT6 negative