Origin and macroscopic appearance
The tumor grows from meningocytes or residual arachnoid cells. Visually, it can present as a well-circumscribed spherical nodule or have a flat, spreading shape that infiltrates the meninges (the so-called en plaque form).
Most frequently (in 90% of cases), meningiomas are located supratentorial: parasagittal, on the convexity of the brain, in the sphenoid wing region, or the olfactory groove. When growing infratentorial, they usually occupy the posterior surface of the petrous temporal bone or the clivus.
Histology and WHO classification
There are four main histological types of meningiomas: meningothelial, fibrous (fibroblastic), transitional, and psammomatous.
According to the WHO classification, meningiomas are divided into three grades of malignancy:
- Grade I (typical): Account for the vast majority. They have a favorable prognosis and a low risk of recurrence after complete surgical removal.
- Grade II (atypical): Characterized by a high mitotic index and areas of disordered cellular growth. If the tumor invades the brain parenchyma, it is prognostically classified as Grade II.
- Grade III (anaplastic): Exhibit marked mitotic activity. Morphologically, they may resemble sarcoma, carcinoma, or melanoma.
Diagnostic imaging (CT and MRI)
Meningiomas are characterized by classic signs of extra-axial growth: a broad base adjacent to the dura mater, buckling and medial displacement of the gray-white matter junction, and the presence of a CSF cleft separating the mass from the brain.
Computed tomography (CT):
- Excellently visualizes bone changes: hyperostosis in benign variants and destruction in atypical ones.
- About 60% of tumors are initially hyperdense.
- Approximately 20% contain calcifications.
- Following contrast administration, intense and homogeneous enhancement is observed.
Magnetic resonance imaging (MRI):
- On T1- and T2-weighted images, the tumor is often isointense to the brain cortex (without contrast, it is easy to miss).
- “Capping cysts” with CSF signal intensity may be present.
- Vasogenic edema is common, and its severity does not depend on the size of the lesion (intra-axial edema increases the risk of recurrence).
- After gadolinium administration, the tumor enhances brightly and uniformly.
- Dural tail sign — linear contrast enhancement along the dura mater. A common sign, but not strictly pathognomonic.
Advanced imaging techniques
- Perfusion-weighted MRI (PWI): Meningiomas are highly vascular tumors, and therefore characterized by a significant increase in relative cerebral blood volume (rCBV). This helps differentiate them from dural metastases.
- MR spectroscopy (MRS): An alanine peak is detected in less than 50% of cases.
- Positron emission tomography (PET): Meningioma tissue has a high density of somatostatin receptors, which ensures intensive uptake of the radiopharmaceutical 68Ga-DOTATOC. This is critical for radiation therapy planning and differentiating recurrence from postoperative changes.
- Angiography: Used for preoperative embolization. The typical picture is blood supply from meningeal arteries and dense, persistent contrast enhancement (blush).
Briefly: answers to topical questions
By what morphological criteria is a meningioma classified as Grade II (atypical) according to the WHO classification?
A meningioma is classified as Grade II (atypical) in the presence of a high mitotic index, areas of disordered growth, or invasion into the brain parenchyma. Brain invasion prognostically corresponds to Grade II, although it is not formally a pathomorphological criterion.
With which pathologies should the MRI "dural tail sign" be differentiated?
The MRI "dural tail sign" should be differentiated with schwannomas and metastases. It represents linear contrast enhancement along the dura mater originating from the tumor. Previously considered pathognomonic for meningioma, it can also occur in schwannomas and metastases.
What is the typical angiographic appearance of a meningioma?
The typical angiographic appearance of a meningioma is characterized by blood supply from meningeal vessels and persistent contrast enhancement. Cardinal angiographic signs include:
- Blood supply from meningeal vessels — serves as the main nutritional source for the neoplasm.
- Dense contrast enhancement (blush) — homogeneous and persistent staining of the tumor nodule.
- Vascular parasitization — recruitment of cortical vessels or branches of the ophthalmic artery into the blood supply.
Currently, angiography is used primarily for preoperative embolization to minimize intraoperative blood loss, after which perfusion in the devascularized segment decreases.
To memorize the signs of extra-axial lesions, use the three "C"s: Cortical buckling, Cerebrospinal fluid cleft (CSF cleft), and Connection to the dura (broad dural base).
Class and exam questions
How to distinguish a meningioma from a vestibular schwannoma in the cerebellopontine angle on CT?
Meningioma usually causes sclerosis (hyperostosis) of the adjacent bone, whereas a schwannoma leads to bone thinning and smoothly expands the internal acoustic meatus.
Does the MRI "dural tail sign" always indicate a meningioma?
No. Although this sign is very characteristic of meningiomas, it is not pathognomonic and can be observed in schwannomas or dural metastases.
Does the size of vasogenic edema correlate with the size of the meningioma itself?
It does not correlate. Even small meningiomas can cause extensive vasogenic edema of the surrounding brain tissue.
What else to study on this topic
- Differential diagnosis with hemangiopericytoma on MRI.
- Evaluation of the tumor's relationship to venous sinuses using CTA and MRA.
- Preoperative angiography and embolization of meningiomas.
- The role of 68Ga-DOTATOC PET in radiation therapy planning.
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