Imaging of intracranial meningioma Dr. Hazem Abu Zeid Yousef Lecturer of Diagnostic Radiology

Background Meningiomas are the most common non-glial tumours of the CNS accounting for between 16 - 20 % of all intracranial tumours. They have been classified into three basic categories. The largest one includes tumors growing within the neuraxis called primary neuraxial meningiomas (PNM).

The second group includes tumors that grow outside the neuraxis referred to as primary extraneuraxial meningiomas (PEM) and the third group comprises tumors extending directly outside the neuraxis or metastasize and called secondary meningiomas.

Symptomatic meningiomas occur 2–3 times more often in female patients, especially during middle-age. Tumor location is the single most important feature regarding therapy since it practically defines the terms of surgical intervention.

The frequency of meningiomas at various intracranial sites varies from study to study. In general, the convexity and parasagittal meningiomas tend to occur for approximately 50% of all intracranial meningiomas, while sphenoid ridge meningiomas account for about 20% the anterior cranial fossa for 10% and those of parasellar regions for approximately 10%

According to the WHO 2007 classification system, the meningiomas are classified into 3 histological grades and 15 subtypes. Most meningiomas are benign, well-circumscribed, slow growing tumors corresponding to WHO grade I. Some meningiomas, including WHO grade II atypical (4.7% to 7.2%) and grade III anaplastic (1.0 to 2.8%) tumors, are clinically and histologically aggressive.

Much larger proportion, 20% of meningioma, demonstrates aggressive behavior. This suggests that a borderline group of grade I meningioma which behaves aggressively and may have recurrent or progressive disease. Therefore, histopathological grading alone might not correlate with the patient outcome. It is important to distinguish WHO-grade I meningiomas with aggressive behavior from their non-aggressive counterparts.

Typical CT features In 72%-85% of meningiomas, CT demonstrates typical diagnostic features, including a sharply circumscribed unilobular mass with a broad-based dural attachment. On NCCT scans the mass appears as an area of homogeneous hyperattenuation “40-50 HU”. After the IV administration of contrast material, the mass homogeneously enhances.

Calcification: Calcification is seen on CT in 20–27% of meningiomas. It is usually microscopic or punctuated, but may be large, conglomerate, peripheral, or central. The presence of calcification indicates slowly progressive benign nature. Malignant meningiomas are rarely calcified.

Hyperostosis Another typical imaging characteristic of meningioma is hyperostosis of the adjacent calvarium in 18–50% of cases. It occurs only in those masses that are immediately adjacent to the bone. Osteoma, fibrous displasia, Paget’s disease, and hyperostosis frontalis may imitate the hyperostosis typically seen in meningiomas.

Bone destruction by meningiomas is an uncommon feature, found in approximately 3% of cases. Benign as well as malignant meningiomas may invade the skull. These destructive meningiomas are typically associated with adjacent areas of hyperostosis. If a purely destructive skull lesion is identified, this is more likely due to metastasis, sarcoma, or myeloma.

Typical MRI features Typically, meningiomas are peripheral unilobular masses with broad-based dural attachments and smooth, well-defined borders. On T1W and T2W images, meningiomas are usually isointense to normal gray matter (in contrast with other intracranial tumors). Nearly all meningiomas enhance rapidly and intensely following IV contrast administration.

In general, T1WI offer little to no specificity whereas T2W images can give information about histological subtype, vascularity, and consistency. Meningiomas hyperintense on T2WI are usually soft, more vascular, and more frequently of syncytial or angiomatous subtype. Tumors hypointense or on T2WI tend to have harder consistency and are more of fibroblastic or transitional subtype.

White matter buckling A useful feature in confirming the extra-axial location of the suspected meningioma is the inward bowing of the gray–white junction of the adjacent brain parenchyma often called ‘white matter buckling’.

Pseudocapsule Another useful MR characteristic is the presence of signal void pseudocapsule which consists of linear signal void representing the dura itself, interposed between the tumor and the brain parenchyma, as well as of punctuate foci of signal void owing to the displaced vessels. There may also be a CSF cleft trapped between the cortex and the meningioma.

Dural tail sign A linear enhancement along the dura mater, on either side of meningioma on contrast-enhanced MRI. This sign is not specific to meningioma and also observed in several conditions including glioma, brain metastasis, acoustic neuroma and lymphoma.

The triple criteria for DTS are: (1) Presence of at least two consecutive sections through the tumor at the same site in more than one imaging plane; (2) Greatest thickness adjacent to the tumor and tapering away from it; and (3) Enhancement more intense than that of the tumor itself.

Atypical imaging features Several imaging features such as peritumoral edema, cystic changes, lipomatous transformation, intracranial hemorrhage, irregular contour, poorly defined margins, and ring enhancement are considered unusual or atypical.

Peritumoral edema (PTE). About 60% of meningiomas are associated with PTE. It is more common with large lesions but may be extensive with small ones. Reports have found that severe edema is associated with more aggressive syncytial and angioblastic cell types, and mild-to-moderate edema associated with fibroblastic and transitional cell types.

Cystic meningioma The term cystic meningioma has been used to describe two different morphologies: intratumoral cavities, and extratumoral or arachnoid cysts. The presence of neoplastic cyst should be suspected when ring enhancement of the wall is seen.

Lipoblastic meningioma Lipoblastic meningioma represents a variant in which there is a metaplastic change of meningothelial cells into adipocytes. Lipomatous meningiomas are markedly hypodense on CT (negative HU) and may have minimal to slight enhancement within the fatty regions.

Spontaneous intracranial hemorrhage Spontaneous intracranial hemorrhage associated with meningioma is an uncommon condition with incidence 1.3% of all meningiomas The most common type of bleeding is subarachnoid hemorrhage, followed by intracerebral and intratumoral hemorrhage.

Utility of advanced MRI techniques in the diagnosis of meningioma Advanced MR imaging techniques are usually of little value in making the diagnosis in patients with typical imaging findings of meningioma. Atypical and malignant meningiomas tend to be markedly hyperintense on diffusion-weighted MR images and exhibit marked decrease in ADC values when compared with normal brain parenchyma.

MR spectroscopy may provide additional information in differential diagnosis. The most common finding in meningiomas is a high Cho peak with low or absent NAA and Cr and variable amounts of lactate. Most importantly, y high ratio of Ala to Cr has been found in meningiomas (relatively specific finding). MR spectroscopy may differentiate histologically atypical meningiomas on the basis of lactate peak at 1.3 ppm.

Independent predictors of non-grade I meningioma Hyperintensity on DWI. Disruption of arachnoid at brain-tumor interface. PTE. Heterogenicitiy on contrast enhanced MRI. Irregular tumor shape.

En plaque meningioma En plaque meningiomas is characterised by diffuse and extensive dural involvement, usually with extracranial extension into calvarium, orbit, and soft tissues. Both CT and MR imaging are useful to evaluate the extent of extradural and calvarial involvement. 

CPA meningiomas CPA meningiomas represent the second most common mass lesions of the CPA, although less than 5% of all meningiomas occur in CPA. Approximately, 80% of masses in the CPA are acoustic schwannomas and half of the remainder 20% are meningiomas

Tuberculum sella meningioma Tuberculum sella meningioma represent 3% to 10% of all meningiomas. They may be associated with hyperostosis of the sphenoid ridge and may be very invasive, spreading to the dura of the frontal, temporal, orbital, and sphenoidal regions. Medially, this tumor may expand into the wall of the cavernous sinus, anteriorly into the orbit, and laterally into the temporal bone.

Orbital meningiomas Orbital meningiomas account for less than 2% of cranial meningiomas, but constitute 10% of all intraorbital neoplasms, arising from the optic nerve sheath between the globe and the optic canal.

Intraventricular meningiomas Intraventricular meningiomas are the most common trigonal masses in an adult accounting for approximately 2–5% of intracranial meningiomas. About 80% are located in the lateral ventricle, 15% in the third ventricle, and about 5% within the fourth ventricle.

Subcortical meningiomas Subcortical meningiomas are mainly deep Sylvian meningiomas that arise from leptomeningeal infolding in the Sylvian fissure and involve branches of the MCA as they grow. The mean age of appearance is reported to be 29.3 years, which is earlier than for ordinary meningiomas. They are reported to be more frequent in Japanese populations.

Differential diagnosis There are multiple neoplastic and non-neoplastic entities that clinically and radiographically mimic meningiomas, including solitary fibrous tumors, hemangiopericytoma, gliosarcoma, leiomyosarcoma, dural metastases, Hodgkin’s disease, plasmocytoma, Rosai Dorfman disease, neurosarcoidosis, melanocytic neoplasms, plasma cell granuloma, Tolosa-Hunt syndrome, and pituitary macroadenoma.

The differential diagnosis of dural-based lesions in the brain varies from incidental and benign to symptomatic and malignant lesions. Careful vigilance in patients with a history of cancer, presenting with new symptoms or imaging evidence of dural-based lesions, should raise the possibility of dural metastasis.

Thank You.