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Atypical imaging appearances of intracranial meningiomas
Clinical Radiology (2007) 62, 10e17
PICTORIAL REVIEW
Atypical imaging appearances of intracranial meningiomas S. O’Leary, W.M. Adams, R.W. Parrish, W. Mukonoweshuro* Radiology Department, Derriford Hospital, Plymouth, UK Received 23 September 2005; received in revised form 1 August 2006; accepted 11 September 2006
Meningiomas are the commonest primary, non-glial intracranial tumours. The diagnosis is often correctly predicted from characteristic imaging appearances. This paper presents some examples of atypical imaging appearances that may cause diagnostic confusion. ª 2006 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction Meningiomas account for 13e26% of all intracranial tumours and represent the commonest non-glial primary tumours of the brain.1 They occur more commonly with increasing age and have an estimated annual incidence of 6 per 100,000.2 They are twice as common in females as in males, and have an increased prevalence in AfricaneAmerican females.3,4 The aetiology of the tumours is unknown in the majority of cases,5 although some are related to previous exposure to radiation and others occur as a manifestation of genetically inherited conditions such as neurofibromatosis type 2.5,6 There is also thought to be an association between previous severe head injuries and an increased incidence of meningiomas.6e8 Imaging has an important role in identifying the lesions, detecting tumour-related complications and in presurgical differential diagnosis, which is essential for optimizing treatment strategies. Meningiomas have characteristic appearances on imaging, but some have atypical appearances that may be confused with alternative histological * Guarantor and correspondent: W. Mukonoweshuro, Radiology Department, X-ray East; Level 6, Derriford Hospital, Derriford Road, Plymouth PL6 8DH, UK. Tel.: þ44 1752 517 698; fax: þ44 1752 763 277. E-mail address: [email protected] (W. Mukonoweshuro).
diagnoses. This paper presents some examples of unusual imaging appearances.
Typical imaging appearances Plain films The role of plain film imaging in the investigation of meningiomas has been superseded by other techniques. Plain films are normal in the majority of cases although they may demonstrate hyperostosis, calcification, and osteolysis associated with the tumours.9,10
Computed tomography (CT) Meningiomas are most likely to be detected initially on CT. CT is ideally suited to demonstrating the tumour and tumour-related characteristics such as tumour calcification, haemorrhage, oedema and calvarial changes.7 On unenhanced CT, meningiomas are typically seen as sharply circumscribed, homogeneous, hyperdense extra-axial mass lesions with a broad dural base. They are multiple in only 1e2% of cases.10 Sixty percent are hyperdense solid lesions on unenhanced images and up to 20% are calcified.11 Enhancement with intravenous contrast medium is intense, and usually homogeneous.
0009-9260/$ - see front matter ª 2006 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2006.09.009
Atypical appearances of intracranial meningiomas
11
A variety of associated skull vault changes may be demonstrated including hyperostosis, osteolytic changes and pneumosinus dilatans. Hyperostosis occurs in up to 20% of cases9 (Fig. 1). The site of hyperostosis may indicate the meningeal attachment of the tumour, but there is no relationship between the degree of hyperostosis and the size of the soft tissue tumour.11 In some cases hyperostosis is due to the tumour directly invading bone,12 whereas in other cases it represents reactive changes. One possible mechanism by which meningiomas produce hyperostosis without bone invasion was suggested by Heick et al.’s study of hyperostosing and non-hyperostosing meningiomas, which demonstrated significantly higher levels of alkaline phosphatase in the hypersostosing tumours.13 Osteolytic changes may be associated with meningioma invading the skull vault or with primary intraosseous meningiomas.14 Pneumosinus dilatans refers to abnormal enlargement of air-filled paranasal sinuses, which may be associated with intracranial meningiomas.15 Approximately 60% of meningiomas are associated with perilesional brain oedema.20 Some histological subtypes such as secretory meningiomas are more likely to excite a disproportionate amount of oedema.21 Some small studies have also suggested an association between the degree of oedema and the likelihood of brain invasion.18
However, it is not possible to predict the tumour histological type, vascularity or the likelihood of venous structure occlusion simply from the degree of peritumoral oedema11,16,17,19 (Fig. 2). Brain invasive meningioma is suggested when there is lack of clarity in the interface between the brain and the tumour. This is often associated with extensive oedema.
Figure 1 Unehanced CT image showing extensive right frontal meningioma (arrows). The skull vault adjacent to the tumour is hyperostotic.
Figure 2 Axial T2-weighted MRI image showing small right frontal meningioma exciting extensive oedema in the adjacent frontal lobe.
Magnetic resonance imaging (MRI) MRI is the mainstay of meningioma imaging because of its superior soft-tissue resolution and multiplanar capabilities. Meningiomas range from isointense to hypointense on T1-weighted images, and from isointense to hyperintense on T2weighted images. They can be difficult to detect without the use of intravenous contrast media. They enhance avidly and homogeneously after gadolinium injection, with the exception of the cystic and very heavily calcified lesions. Gadolinium is particularly good at delineating en-plaque lesions9 (Fig. 3). Up to 72% of meningiomas also have an associated dural tail. However, the dural tail is not specific for meningiomas as it may be associated with other tumours such as schwannomas, lymphoma and peripheral gliomas11,16,22,23 (Fig. 4).
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Figure 3 Post-gadolinium axial T1-weighted MRI image showing en-plaque left sphenoid wing meningioma (black arrowheads) associated with extensive hyperostosis of the sphenoid wing (white arrows).
The presence of overlying bone changes may help differentiate en-plaque meningiomas from other neoplastic and inflammatory causes of dural thickening, although histological confirmation is still required in the majority of cases. Most meningiomas have characteristic findings on proton MR spectroscopy such as the presence of an alanine peak. This may enable differentiation of meningiomas from alternative diagnoses such as schwannomas.24 In addition, there is a growing body of evidence showing the potential utility of diffusion-weighted (DWI) and perfusion MRI in differentiating between typical and atypical meningiomas. Filippi et al.25 found atypical and malignant meningiomas tended to have a lower average diffusion constant (Dav) than benign meningiomas on DWI. Yang et al.26 demonstrated a difference in the volume transfer coefficient (Ktrans) on MRI perfusion studies of typical and atypical meningiomas suggesting it may be possible to differentiate them from each other preoperatively.
Angiography Meningiomas are usually very vascular tumours, which demonstrate a prominent tumour blush and delayed ‘‘washout’’ on catheter angiography. They
Figure 4 Left parietal high convexity meningioma. (a) Axial T2-weighted MRI image shows the tumour is slightly hyperintense to cerebral cortex. The extra-axial location is confirmed by the CSF surrounding the tumour (arrows) and the compression and medial displacement of the cortex. (b) Post-gadolinium coronal T1-weighted image shows intense, uniform enhancement of the tumour and a dural tail (arrow).
Atypical appearances of intracranial meningiomas
13
are usually supplied by meningeal branches of the external carotid artery, the internal carotid artery and/or the vertebral artery. Additional supply may also arise from pial vessels.11 Meningiomas may encase or displace major vessels, and may occlude large veins and cerebral venous sinuses. Preoperative identification of these features is essential to minimize the risk of intra-operative haemorrhage or postoperative infarction due to vascular damage.11
This appearance may simulate glioma or even a cerebral infarct. Hypodensity on CT is usually due to cysts associated with the meningioma (Fig. 6). The cysts often have slightly higher attenuation characteristics than cerebrospinal fluid (CSF) on CT. The cysts may be intrinsic to the tumour, related to cystic degeneration, or the result of CSF trapped within the tumour. This appearance may mimick schwannomas or intrinsic brain tumours.11,29 Alternatively, the associated cysts may represent reactive arachnoid cysts, which are estimated to occur in up to 5% of meningiomas.11 Low density on CT may also be due to fatty transformation in the meningiomas. The mechanisms for this are poorly understood. Meningoepithelial cells undergo metaplasia, which leads to adipocyte formation in the tumour producing a rare subgroup of meningiomas known as lipomatous meningiomas.31,32 These are often heterogeneous on imaging with heterogeneous enhancement patterns. On T1-weighted MRI, the fatty components return a high signal intensity.11,16 Meningiomas may have unusual enhancement patterns, including heterogeneous enhancement due to haemorrhage (Fig. 7), cysts and necrosis; ring enhancement or absence of enhancement.27,33e35
Atypical imaging features The imaging features of meningiomas may be atypical in terms of signal characteristics, tumour location or behaviour. However, atypical imaging appearances do not necessarily predict atypical histology. Atypical appearances Meningiomas may be hypodense on CT before contrast medium administration and may remain hypodense on post-contrast CT11,16,28,30 (Fig. 5).
Figure 5 (a) Post-contrast CT image showing hypodense mass lesion predominantly affecting the left frontal lobe with extension into the paramedian right frontal lobe. There are several enhancing septae within it and the lesion appears to be located within the brain parenchyma, simulating a glioma. The tumour was confirmed at histology to be a meningioma.
Figure 6 Pre-contrast coronal T1-weighted MRI image showing a parafalcine meningioma (arrows) associated with a large right frontal cyst.
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Figure 8 Unenhanced, coronal T1-weighted MRI image showing a meningioma in the trigone of the left lateral ventricle (arrows).
Unusual locations The majority of meningiomas arise from specialized meningothelial cells in the arachnoid granulations
Figure 7 (a) Unenhanced CT image showing haemorrhagic right temporal tumour. (b) Post-gadolinium, axial T1-weighted MRI image shows peripheral ring-enhancing area (arrow) with enhancement of the adjacent dura. This was histologically confirmed to be a meningioma.
Figure 9 Unenhanced CT image showing tramline calcification of the optic nerve sheath consistent with an isolated optic nerve sheath meningioma.
Atypical appearances of intracranial meningiomas
15
known as the arachnoid cap. However, meningothelial cells in any location can potentially give rise to meningiomas. Intraventricular meningiomas account for 0.5e3% of all intracranial meningiomas (Fig. 8). Makoto et al.36 reviewed 16 intraventricular meningiomas and found 80% occurred in the lateral ventricles mainly in the trigone. McDermott37 noted that they more commonly occurred on the left than the right. Meningiomas involve the orbit in 0.4e1.3% of cases. Primary orbital meningiomas occur in 10e33% of the cases and are unilateral in the majority of cases.38 Bilateral orbital involvement occurs in 5% of cases. It is related to multiple meningiomatosis and there is an association with neurofibromatosis38 (Fig. 9). Meningiomas at the cerebellopontine angle sometimes have an intracanalicular component, simulating vestibular schwannomas (Fig. 10). Extradural primary meningiomas with no dural attachment account for approximately 2% of meningiomas.39 They have been reported in the calvaria, orbit, scalp, paranasal sinus, nasopharynx, neck, and skin. Rare meningiomas have also been reported in the lung, mediastinum, adrenal gland, paraspinal region, and finger. These extradural meningiomas are more likely to develop malignant features than intracranial meningiomas.14,39,40 Atypical behaviour Approximately 8% of non-neurofibromatosis patients with meningiomas develop multiple lesions41 (Fig. 11). Meningiomas occurring as a complication of previous exposure to radiation tend to affect a younger patient group. They are more likely to be multiple, to be malignant and are also more likely to recur after surgical treatment.42,46 Approximately 2.4% of meningiomas are malignant.43 Malignant meningiomas may metastasize to extracranial sites. The incidence of meningioma metastasis is thought to be about 1 in 1000. However, this figure has tended to include haemangiopericytomas, now recognized as a separate entity in the World Health Organisation (WHO) classification.9 Enam et al.44 reported meningioma metastasis to the vertebral bodies, liver, pelvis, long bones and spinal cord. The most common site of
Figure 10 Axial T2-weighted MRI images showing a large right cerebellopontine angle meningioma extending into the inferior portion of the internal auditory canal (arrow a), and spreading into the carotid canal (arrow b).
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metastasis is the lung, reported in 61% of cases.44,45 Tumours may also be implanted into the skin at the time of surgery.46 Meningiomas may rarely metastasize throughout the CSF.47 Other tumours, particularly lung and breast carcinomas, can rarely metastasize to intracranial meningiomas. Metastases to meningiomas should be suspected if there are multiple discrete enhancing foci within the meningioma.48
Conclusions Meningiomas are common intracranial tumours, which are usually easily recognizable by their radiological and histological appearances. However, there are unusual characteristics, which may cause them to mimic other brain lesions. It is important to be aware of these features in order to avoid misdiagnosis.
References
Figure 11 Post-gadolinium, axial T1-weighted MRI image showing bilateral occipital meningiomas and a meningioma extending into the left orbit (arrows). The left occipital lesion is invading the skull vault.
1. Louis DN, Scheithauer BW, Budka H, et al. Meningiomas. In: Kleihues P, Cavenec WK, editors. Pathology and genetics of tumours of the nervous system: World Health Organisation classification of tumours. Lyon: ARC Press; 2000. p. 176e84. 2. Longstreth WT, Dennis LK, McGuire VM, et al. Epidemiology of intracranial meningiomas. Cancer 1993;72: 639e48. 3. Bouchardy C, Wanner P, Parkin DM. Cancer mortality among sub-Saharan migrants in France. Cancer Causes Control 1995;6:539e44. 4. Grulich AE, Swerdlow AJ, Head J, et al. Cancer mortality in African and Carribean migrants to England and Wales. Br J Cancer 1992;66:905e11. 5. Surawicz TS, McCarthy BJ, Kupelian V, et al. Descriptive epidemiology of primary brain and central nervous system tumours in the United States. Results from CBTRUS 1990e1994. Neuro-Oncology Doc.1999; 98e113. 6. Sadetzki S, Flint-Richter P, Ben-Tal T, et al. Radiation induced meningiomas: a descriptive study of 253 cases. J Neurosurg 2002;97:1078e82. 7. Evans DGR, Watson C, King A, et al. Multiple meningiomas: differential involvement of the NF2 gene in children and adults. J Med Genet 2005;42:45e8. 8. Phillips LE, Koepsell TD, Van Belle G, et al. History of head trauma and risk of intracranial meningioma: populationbased caseecontrol study. Neurology 2002;58:1849e52. 9. Ricci PE. Imaging of adult brain tumours. Neuroimaging Clin N Am 1999;9:651e69. 10. Sheehy JP, Crockard HA. Multiple meningiomas: a long-term review. J Neurosurg 1983;59:1e5. 11. Buetow MP, Buetow PC, Smirniotopoulos JG. Typical, atypical, and misleading features in meningioma. RadioGraphics 1991;11:1087e106. 12. Pieper DR, Al-Mefty O, Hanada Y, et al. Hyperostosis associated with meningioma of the cranial base: secondary changes or tumor invasion. Neurosurgery 1999;44:742e6. 13. Heick A, Mosdal C, Jorgensen K, et al. Localized cranial hyperostosis of meningiomas: a result of neoplastic enzymatic activity? Acta Neurol Scand 1993;87:243e7.
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14. Nil TN, Oner YA, Kaymaz M, et al. Primary intraosseous meningioma: CT and MRI appearance. AJNR Am J Neuroradiol 2005;26:2053e6. 15. Miller NR, Golnik KC, Zeidman SM, et al. Pneumosinus dilatans: a sign of intracranial meningioma. Surg Neurol 1996; 46:471e4. 16. Kizanna E, Lee R, Toung N, et al. A review of the radiological features of intracranial meningiomas. Australas Radiol 1996;40:454e62. 17. Davis PC, Hudgins PA, Peterman SB, et al. Diagnosis of cerebral metastases: double dose delayed CT vs. contrast enhanced MR imaging. AJNR Am J Neuroradiol 1991;12:293e300. 18. Mantle RE, Lach B, Delgado MR, et al. Predicting the probability of meningioma recurrence based on the quantity of peritumoral brain edema on computerized tomography scanning. J Neurosurg 1999;91:375e83. 19. Domingo Z, Rowe G, Blamire AM, et al. Role of ischaemia in the genesis of oedema surrounding meningiomas assessed using magnetic resonance imaging and spectroscopy. Br J Neurosurg 1998;12:414e8. 20. Pistolesi S, Fontanini G, Camacci T, et al. Meningiomaassociated brain oedema: the role of angiogenic factors and pial blood supply. J Neurooncol 2002;60:159e64. 21. Probst-Cousin S, Villagran-Lillo R, Lahl R, et al. Secretory meningioma: clinical, histologic, and immunohistochemical findings in 31 cases. Cancer 1997;79:2003e15. 22. Goldsher D, Litt AW, Pinto RS, et al. Dural ‘‘tail’’ associated with meningiomas on Gd-DTPA-enhanced MR images: characteristics, differential diagnostic value, and possible implications for treatment. Radiology 1990;176:447e50. 23. Nagele T, Peterson D, Klose U, et al. The ‘‘dural tail’’ adjacent to meningiomas studied by dynamic contrast-enhanced MRI: a comparison with histopathology. Neuroradiology 1994;36:303e7. 24. Majo ´s C, Julia `-Sape ´ M, Juli Alonso J, et al. Brain tumor classification by proton MR spectroscopy: comparison of diagnostic accuracy at short and long TE. AJNR Am J Neuroradiol 2004;25:1696e704. 25. Filippi CG, Edgar MA, Ulug AM, et al. Appearance of meningiomas on diffusion-weighted images: correlating diffusion constants with histopathologic findings. AJNR Am J Neuroradiol 2001;22:65e72. 26. Yang S, Law M, Zagzag D, et al. Dynamic contrast-enhanced perfusion MR imaging measurements of endothelial permeability: differentiation between atypical and typical meningiomas. AJNR Am J Neuroradiol 2003;24:1554e9. 27. Seung Hyun K, Dong Gyu K, Chae-Yong K, et al. Microcystic meningioma: the characteristic neuroradiological findings. J Korean Neurosurg Soc 2003;34:401e6. 28. Savoiardo M, Passerini A, Allegranza A. The hypodense meningioma: report of two cases. Neuroradiology 1978; 16:558e60. 29. Carvalho GA, Vorkapic R, Biewener G, et al. Cystic meningiomas resembling glial tumours. Surg Neurol 1997;47:284e9. 30. Awada A, Scherman B, Palker V. Cystic meningiomas, a diagnostic and pathogenic challenge. Eur J Radiol 1997;25:26e9.
31. Fitt GJ, Kalnins R, Mitchell LA. Lipomatous meningioma: characteristic computed tomographic appearance. Australas Radiol 1996;40:84e7. 32. Bolat F, Kayaselcuk F, Aydin MV, et al. Lipidized or lipomatous meningioma, which is more appropriate? A case report. Neurol Res 2003;25:764e6. 33. Kubota Y, Ueda T, Kagawa Y, et al. Microcystic meningioma without enhancement on neuroimaging. Neurol Med Chir (Tokyo) 1997;37:407e10. 34. Roosen N, Deckert M, Lumenta CB, et al. Endotheliomatous meningioma with coalescing microcysts, presenting as a hypodense lesion with ring enhancement on computed tomography. Neurochirurgia 1989;32:160e3. 35. Masahiro Y, Masahiro S, Tsuyoshi O. Ring-enhanced malignant meningioma mimicking a brain metastasis from a renal cell carcinoma. Urol Int 2003;70:80e2. 36. Makoto N, Florian R, Bundschuh O, et al. Intraventricular meningiomas: a review of 16 cases with reference to the literature. Surg Neurol 2003;59:490e503. 37. McDermott MW. Intraventricular meningiomas. Neurosurg Clin N Am 2003;14:559e69. 38. Ortiz O, Schochet SS, Kotzan JM, et al. Radiologicepathologic correlation: meningioma of the optic nerve sheath. AJNR Am J Neuroradiol 1996;17:901e6. 39. Lang FF, Macdonald OK, Fuller GN, et al. Primary extradural meningiomas: a report on nine cases and review of the literature from the era of computerized tomography scanning. J Neurosurg 2000;93:940e50. 40. Swain RE, Kingdom TT, Delgado JM, et al. Meningiomas of the paranasal sinuses. Am J Rhinol 2001;15:27e30. 41. Huang H, Buhl R, Hugo HH, et al. Clinical and histological features of multiple meningiomas compared with solitary meningiomas. Neurol Res 2005;27:324e32. 42. Al-Mefty O, Topsakal C, Pravdenkova S, et al. Radiationinduced meningiomas: clinical, pathological, cytokinetic, and cytogenetic characteristics. J Neurosurg 2004;100: 1002e13. 43. Verheggen R, Finkenstaedt M, Bockermann V, et al. Atypical and malignant meningiomas: evaluation of different radiological criteria based on CT and MRI. Acta Neurochir 1996; 65:66e9. 44. Enam SA, Abdulrauf S, Mehta B, et al. Metastasis in meningioma. Acta Neurochir (Wien) 1996;138:1172e7 [discussion 1177e8]. 45. Kaminski JM, Movsas B, King E, et al. Metastatic meningioma to the lung with multiple pleural metastases. Am J Clin Oncol 2001;24:579e82. 46. Akai T, Shiraga S, Iizuka H, et al. Recurrent meningioma with metastasis to the skin incision. Neurol Med Chir (Tokyo) 2004;44:600e2. 47. Ramakrishnamurthy TV, Murty AV, Purohit AK, et al. Benign meningioma metastasising through CSF pathways: a case report and review of literature. Neurol India 2002;50: 326e9. 48. Lee A, Wallace C, Rewcastle B, et al. Metastases to meningioma. AJNR Am J Neuroradiol 1998;19:1120e2.
PICTORIAL REVIEW
Atypical imaging appearances of intracranial meningiomas S. O’Leary, W.M. Adams, R.W. Parrish, W. Mukonoweshuro* Radiology Department, Derriford Hospital, Plymouth, UK Received 23 September 2005; received in revised form 1 August 2006; accepted 11 September 2006
Meningiomas are the commonest primary, non-glial intracranial tumours. The diagnosis is often correctly predicted from characteristic imaging appearances. This paper presents some examples of atypical imaging appearances that may cause diagnostic confusion. ª 2006 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.
Introduction Meningiomas account for 13e26% of all intracranial tumours and represent the commonest non-glial primary tumours of the brain.1 They occur more commonly with increasing age and have an estimated annual incidence of 6 per 100,000.2 They are twice as common in females as in males, and have an increased prevalence in AfricaneAmerican females.3,4 The aetiology of the tumours is unknown in the majority of cases,5 although some are related to previous exposure to radiation and others occur as a manifestation of genetically inherited conditions such as neurofibromatosis type 2.5,6 There is also thought to be an association between previous severe head injuries and an increased incidence of meningiomas.6e8 Imaging has an important role in identifying the lesions, detecting tumour-related complications and in presurgical differential diagnosis, which is essential for optimizing treatment strategies. Meningiomas have characteristic appearances on imaging, but some have atypical appearances that may be confused with alternative histological * Guarantor and correspondent: W. Mukonoweshuro, Radiology Department, X-ray East; Level 6, Derriford Hospital, Derriford Road, Plymouth PL6 8DH, UK. Tel.: þ44 1752 517 698; fax: þ44 1752 763 277. E-mail address: [email protected] (W. Mukonoweshuro).
diagnoses. This paper presents some examples of unusual imaging appearances.
Typical imaging appearances Plain films The role of plain film imaging in the investigation of meningiomas has been superseded by other techniques. Plain films are normal in the majority of cases although they may demonstrate hyperostosis, calcification, and osteolysis associated with the tumours.9,10
Computed tomography (CT) Meningiomas are most likely to be detected initially on CT. CT is ideally suited to demonstrating the tumour and tumour-related characteristics such as tumour calcification, haemorrhage, oedema and calvarial changes.7 On unenhanced CT, meningiomas are typically seen as sharply circumscribed, homogeneous, hyperdense extra-axial mass lesions with a broad dural base. They are multiple in only 1e2% of cases.10 Sixty percent are hyperdense solid lesions on unenhanced images and up to 20% are calcified.11 Enhancement with intravenous contrast medium is intense, and usually homogeneous.
0009-9260/$ - see front matter ª 2006 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.crad.2006.09.009
Atypical appearances of intracranial meningiomas
11
A variety of associated skull vault changes may be demonstrated including hyperostosis, osteolytic changes and pneumosinus dilatans. Hyperostosis occurs in up to 20% of cases9 (Fig. 1). The site of hyperostosis may indicate the meningeal attachment of the tumour, but there is no relationship between the degree of hyperostosis and the size of the soft tissue tumour.11 In some cases hyperostosis is due to the tumour directly invading bone,12 whereas in other cases it represents reactive changes. One possible mechanism by which meningiomas produce hyperostosis without bone invasion was suggested by Heick et al.’s study of hyperostosing and non-hyperostosing meningiomas, which demonstrated significantly higher levels of alkaline phosphatase in the hypersostosing tumours.13 Osteolytic changes may be associated with meningioma invading the skull vault or with primary intraosseous meningiomas.14 Pneumosinus dilatans refers to abnormal enlargement of air-filled paranasal sinuses, which may be associated with intracranial meningiomas.15 Approximately 60% of meningiomas are associated with perilesional brain oedema.20 Some histological subtypes such as secretory meningiomas are more likely to excite a disproportionate amount of oedema.21 Some small studies have also suggested an association between the degree of oedema and the likelihood of brain invasion.18
However, it is not possible to predict the tumour histological type, vascularity or the likelihood of venous structure occlusion simply from the degree of peritumoral oedema11,16,17,19 (Fig. 2). Brain invasive meningioma is suggested when there is lack of clarity in the interface between the brain and the tumour. This is often associated with extensive oedema.
Figure 1 Unehanced CT image showing extensive right frontal meningioma (arrows). The skull vault adjacent to the tumour is hyperostotic.
Figure 2 Axial T2-weighted MRI image showing small right frontal meningioma exciting extensive oedema in the adjacent frontal lobe.
Magnetic resonance imaging (MRI) MRI is the mainstay of meningioma imaging because of its superior soft-tissue resolution and multiplanar capabilities. Meningiomas range from isointense to hypointense on T1-weighted images, and from isointense to hyperintense on T2weighted images. They can be difficult to detect without the use of intravenous contrast media. They enhance avidly and homogeneously after gadolinium injection, with the exception of the cystic and very heavily calcified lesions. Gadolinium is particularly good at delineating en-plaque lesions9 (Fig. 3). Up to 72% of meningiomas also have an associated dural tail. However, the dural tail is not specific for meningiomas as it may be associated with other tumours such as schwannomas, lymphoma and peripheral gliomas11,16,22,23 (Fig. 4).
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Figure 3 Post-gadolinium axial T1-weighted MRI image showing en-plaque left sphenoid wing meningioma (black arrowheads) associated with extensive hyperostosis of the sphenoid wing (white arrows).
The presence of overlying bone changes may help differentiate en-plaque meningiomas from other neoplastic and inflammatory causes of dural thickening, although histological confirmation is still required in the majority of cases. Most meningiomas have characteristic findings on proton MR spectroscopy such as the presence of an alanine peak. This may enable differentiation of meningiomas from alternative diagnoses such as schwannomas.24 In addition, there is a growing body of evidence showing the potential utility of diffusion-weighted (DWI) and perfusion MRI in differentiating between typical and atypical meningiomas. Filippi et al.25 found atypical and malignant meningiomas tended to have a lower average diffusion constant (Dav) than benign meningiomas on DWI. Yang et al.26 demonstrated a difference in the volume transfer coefficient (Ktrans) on MRI perfusion studies of typical and atypical meningiomas suggesting it may be possible to differentiate them from each other preoperatively.
Angiography Meningiomas are usually very vascular tumours, which demonstrate a prominent tumour blush and delayed ‘‘washout’’ on catheter angiography. They
Figure 4 Left parietal high convexity meningioma. (a) Axial T2-weighted MRI image shows the tumour is slightly hyperintense to cerebral cortex. The extra-axial location is confirmed by the CSF surrounding the tumour (arrows) and the compression and medial displacement of the cortex. (b) Post-gadolinium coronal T1-weighted image shows intense, uniform enhancement of the tumour and a dural tail (arrow).
Atypical appearances of intracranial meningiomas
13
are usually supplied by meningeal branches of the external carotid artery, the internal carotid artery and/or the vertebral artery. Additional supply may also arise from pial vessels.11 Meningiomas may encase or displace major vessels, and may occlude large veins and cerebral venous sinuses. Preoperative identification of these features is essential to minimize the risk of intra-operative haemorrhage or postoperative infarction due to vascular damage.11
This appearance may simulate glioma or even a cerebral infarct. Hypodensity on CT is usually due to cysts associated with the meningioma (Fig. 6). The cysts often have slightly higher attenuation characteristics than cerebrospinal fluid (CSF) on CT. The cysts may be intrinsic to the tumour, related to cystic degeneration, or the result of CSF trapped within the tumour. This appearance may mimick schwannomas or intrinsic brain tumours.11,29 Alternatively, the associated cysts may represent reactive arachnoid cysts, which are estimated to occur in up to 5% of meningiomas.11 Low density on CT may also be due to fatty transformation in the meningiomas. The mechanisms for this are poorly understood. Meningoepithelial cells undergo metaplasia, which leads to adipocyte formation in the tumour producing a rare subgroup of meningiomas known as lipomatous meningiomas.31,32 These are often heterogeneous on imaging with heterogeneous enhancement patterns. On T1-weighted MRI, the fatty components return a high signal intensity.11,16 Meningiomas may have unusual enhancement patterns, including heterogeneous enhancement due to haemorrhage (Fig. 7), cysts and necrosis; ring enhancement or absence of enhancement.27,33e35
Atypical imaging features The imaging features of meningiomas may be atypical in terms of signal characteristics, tumour location or behaviour. However, atypical imaging appearances do not necessarily predict atypical histology. Atypical appearances Meningiomas may be hypodense on CT before contrast medium administration and may remain hypodense on post-contrast CT11,16,28,30 (Fig. 5).
Figure 5 (a) Post-contrast CT image showing hypodense mass lesion predominantly affecting the left frontal lobe with extension into the paramedian right frontal lobe. There are several enhancing septae within it and the lesion appears to be located within the brain parenchyma, simulating a glioma. The tumour was confirmed at histology to be a meningioma.
Figure 6 Pre-contrast coronal T1-weighted MRI image showing a parafalcine meningioma (arrows) associated with a large right frontal cyst.
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Figure 8 Unenhanced, coronal T1-weighted MRI image showing a meningioma in the trigone of the left lateral ventricle (arrows).
Unusual locations The majority of meningiomas arise from specialized meningothelial cells in the arachnoid granulations
Figure 7 (a) Unenhanced CT image showing haemorrhagic right temporal tumour. (b) Post-gadolinium, axial T1-weighted MRI image shows peripheral ring-enhancing area (arrow) with enhancement of the adjacent dura. This was histologically confirmed to be a meningioma.
Figure 9 Unenhanced CT image showing tramline calcification of the optic nerve sheath consistent with an isolated optic nerve sheath meningioma.
Atypical appearances of intracranial meningiomas
15
known as the arachnoid cap. However, meningothelial cells in any location can potentially give rise to meningiomas. Intraventricular meningiomas account for 0.5e3% of all intracranial meningiomas (Fig. 8). Makoto et al.36 reviewed 16 intraventricular meningiomas and found 80% occurred in the lateral ventricles mainly in the trigone. McDermott37 noted that they more commonly occurred on the left than the right. Meningiomas involve the orbit in 0.4e1.3% of cases. Primary orbital meningiomas occur in 10e33% of the cases and are unilateral in the majority of cases.38 Bilateral orbital involvement occurs in 5% of cases. It is related to multiple meningiomatosis and there is an association with neurofibromatosis38 (Fig. 9). Meningiomas at the cerebellopontine angle sometimes have an intracanalicular component, simulating vestibular schwannomas (Fig. 10). Extradural primary meningiomas with no dural attachment account for approximately 2% of meningiomas.39 They have been reported in the calvaria, orbit, scalp, paranasal sinus, nasopharynx, neck, and skin. Rare meningiomas have also been reported in the lung, mediastinum, adrenal gland, paraspinal region, and finger. These extradural meningiomas are more likely to develop malignant features than intracranial meningiomas.14,39,40 Atypical behaviour Approximately 8% of non-neurofibromatosis patients with meningiomas develop multiple lesions41 (Fig. 11). Meningiomas occurring as a complication of previous exposure to radiation tend to affect a younger patient group. They are more likely to be multiple, to be malignant and are also more likely to recur after surgical treatment.42,46 Approximately 2.4% of meningiomas are malignant.43 Malignant meningiomas may metastasize to extracranial sites. The incidence of meningioma metastasis is thought to be about 1 in 1000. However, this figure has tended to include haemangiopericytomas, now recognized as a separate entity in the World Health Organisation (WHO) classification.9 Enam et al.44 reported meningioma metastasis to the vertebral bodies, liver, pelvis, long bones and spinal cord. The most common site of
Figure 10 Axial T2-weighted MRI images showing a large right cerebellopontine angle meningioma extending into the inferior portion of the internal auditory canal (arrow a), and spreading into the carotid canal (arrow b).
16
S. O’Leary et al.
metastasis is the lung, reported in 61% of cases.44,45 Tumours may also be implanted into the skin at the time of surgery.46 Meningiomas may rarely metastasize throughout the CSF.47 Other tumours, particularly lung and breast carcinomas, can rarely metastasize to intracranial meningiomas. Metastases to meningiomas should be suspected if there are multiple discrete enhancing foci within the meningioma.48
Conclusions Meningiomas are common intracranial tumours, which are usually easily recognizable by their radiological and histological appearances. However, there are unusual characteristics, which may cause them to mimic other brain lesions. It is important to be aware of these features in order to avoid misdiagnosis.
References
Figure 11 Post-gadolinium, axial T1-weighted MRI image showing bilateral occipital meningiomas and a meningioma extending into the left orbit (arrows). The left occipital lesion is invading the skull vault.
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