solitary fibrous tumor of the dura
Solitary fibrous tumors of the dura are rare dural masses, histologically identical to solitary fibrous tumors found elsewhere and now believed to be closely related to hemangiopericytomas, a fact reflected in the current (2016) WHO classification of CNS tumors. They are composed of spindle cells thought to be of mesenchymal origin. They usually occur in middle-aged individuals and symptoms depend on the size of the mass and its location.
On imaging, these tumors are similar to meningiomas, appearing as extra-axial well circumscribed solid masses with vivid contrast enhancement.
Solitary fibrous tumors occur in older middle-aged individuals (mean 47-56 years of age), without a convincing gender predilection, although some reports have found increased incidence in females .
Clinical presentation is nonspecific and identical to other dural masses (e.g. meningioma). Symptoms depend on the size of the mass and its location. Headache is the most common presenting complaint .
Rarely, solitary fibrous tumors result in non-islet cell tumor hypoglycemia (NICTH) as a result of secretion of insulin-like growth factor-2 (IGF-2) . Rarer still is acromegaly .
In the latest (2016) update to the WHO classification of CNS tumors, hemangiopericytomas and solitary fibrous tumors of the dura are considered to be one entity with variable morphology, although both terms are retained to reflect the very different imaging and clinical features . This reflects the current understanding of this tumor; in fact, the term hemangiopericytoma has fallen out of favor outside of the CNS.
Although this is an area of some debate with alternative grading systems having been proposed, usually, solitary fibrous tumors of the dura are considered WHO grade I (whereas hemangiopericytomas are considered WHO grade II or III) .
Having said that, a proportion (5.8%) of solitary fibrous tumors appear malignant, with high mitotic rate, necrosis and hemorrhage .
These are well-circumscribed and firm .
Typically, solitary fibrous tumors demonstrate areas thick bands of collagen separating areas of hypercellularity and hypocellularity. Staghorn vessels, classically seen in hemangiopericytomas, are also commonly seen .
The immunohistochemical appearance of solitary fibrous tumors is not surprisingly the same as that of hemangiopericytomas, characterized by :
- CD34: positive
- vimentin: positive
- STAT6 protein: relocation of this protein to the nucleus
- EMA: usually negative
- PS100: negative
The radiographic appearances are those of a well-circumscribed mass arising from the dura (supratentorial convexity, falx and tentorium and posterior fossa). Unlike solitary fibrous tumors of the spinal cord, which most frequently arise from the cord itself and not the theca , parenchymal tumors of the brain are very rare.
Solitary fibrous tumors are well circumscribed and isodense to hyperdense compared to adjacent brain. They may demonstrate calcifications, and may erode bone if abutting it .
These tumors are similar to meningiomas, appearing as extra-axial well-circumscribed masses, with the following signal characteristics :
- typically, intermediate signal similar to brain
- iso- to hypointensity (the best clue to the diagnosis)
- heterogeneous signal: "yin-yang" appearance of separate areas with low signal and high signal intensity may be characteristic
- flow voids commonly seen
- T1 C+
- vivid diffuse heterogeneous contrast enhancement of low T2 signal components
- dural tail may be seen, but is much less common than in meningiomas
- DWI / ADC: regions of restricted diffusion are frequently seen
- lipid and lactate elevation
- myo-inositol elevation
- MR perfusion: elevated rCBV
Prominent tumor blush, with feeding vessels varying, depending on the location of the tumor. Importantly, in addition to meningeal supply, these tumors can receive blood supply from pial vessels .
Treatment and prognosis
Surgical resection, if complete, is usually curative. Recurrence rates are variably reported as low to relatively high; up to 50% in one series . If incomplete or recurrence occurs, then radiotherapy may be employed .
The differential of solitary fibrous tumors of the dura is essentially that of other dural masses, and common entities to be considered include :
- T2 signal iso- to hyperintense
- MRS: alanine and glutamate-glutamic acid peaks may be seen
- characteristic locations
- T2 signal usually iso- to hyperintense
- primary dural lymphoma
- dural metastases
The most useful feature that helps in making a preoperative diagnosis is the propensity for solitary fibrous tumor to be low signal on T2. Importantly, lymphoma, Erdheim-Chester disease and melanocytic lesions may also have low T2 signal. Meningiomas may also be low on T2, usually in the setting of mineralization/calcification; thus reviewing other sequences (T2*) or CT is useful.