Meningioma in a 9-year-old chocolate Lab: Radiology perspective

Dr. Silke HechtAlthough magnetic resonance imaging (MRI) is the modality of choice for brain imaging, computed tomography (CT) can provide valuable information about the presence, number and location of intracranial lesions, especially if they are large or contrast-enhancing. Whether neoplastic or not, intracranial masses can be subdivided based on location into intra-axial (arising from within the brain axis) and extra-axial.1-4 They can be further characterized by number, location, size, margination, signal intensity, homogeneity, contrast enhancement and concurrent imaging findings (e.g. ventriculomegaly, changes associated with the cranium or meninges, hemorrhage, mineralization, mass effect, edema, cystic or necrotic component).5-8

Extra-axial masses arise from tissues other than actual brain parenchyma (e.g. meningiomas and choroid plexus tumors). Many are located in the periphery of the brain and compress rather than invade brain parenchyma. Some are in a typical location (e.g. pituitary tumors originating from the pituitary fossa), and most are strongly contrast-enhancing. Concurrent changes to the skull (e.g. lysis or hyperostosis), broad-based contact of the lesion with meninges and skull forming an obtuse angle, and meningeal thickening and enhancement adjacent to a mass (“dural tail sign”) also suggest extra-axial origin.

Intra-axial masses originating from brain parenchyma may be completely surrounded by normal brain tissue, in which case determination of mass origin is straightforward. Peripherally located or very large masses may be in contact with overlying meninges, which may make a diagnosis challenging. An acute rather than obtuse angle of the mass with the meninges and absence of adjacent meningeal enhancement may be helpful in classifying a lesion as intra-axial in those cases. Finally, while the presence and degree of contrast enhancement of intra-axial masses is highly variable, absent or poor enhancement is most consistent with intra-axial origin and protection of the mass by the blood-brain barrier.

Imaging findings with intracranial meningioma include a round to ovoid or plaque-like smoothly marginated mass associated with the brain, typically in broad-based contact with underlying bone (except meningiomas in a ventricular location) (Figure 1).2-4,9-17

Figure 1. Meningioma in a 13-year-old West Highland white terrier (not the patient in this case). Transverse T2-weighted (a), T2-FLAIR (b), and T1-weighted MRI images before and after contrast administration (c, d); and transverse CT images before and after contrast medium administration (e, f). A focal peripheral well-circumscribed mass is associated with the left parietal lobe (arrows). The lesion is faintly hyperintense on T2-weighted and T2-FLAIR MRI images (a, b), isointense to brain parenchyma on the T1-weighted MRI image (c), isoattenuating to brain parenchyma on the pre contrast CT image (e), and strongly homogeneously contrast enhancing on both post contrast MRI and CT images (d, f).

They are usually single, although multiple tumors are possible.9,15,18,19 On CT images, meningiomas are typically isoattenuating or hyperattenuating (Figures 1e and 1f). On MRI, meningiomas are usually hypointense to isointense on T1-weighted images (Figures 1c and 2d) and hyperintense on T2-weighted/T2-FLAIR images (Figures 1a, 1b, 2a, 2b, and 3a).

Figure 2. Cystic meningioma in a 13-year-old Dachshund (not the patient in this case). Transverse T2-weighted (a), T2-FLAIR (b), T2*-weighted (c), and T1-weighted MR images before and after contrast administration (d, e) and a sagittal T1-weighted MRI image after contrast medium administration (f). A peripheral plaque-like mass is associated with the right frontal and parietal lobes (a-e, arrows). This mass is mildly hyperintense to normal brain parenchyma on T2-weighted (a) and T2-FLAIR (b) images, isointense on T2*-weighted (c) and T1-weighted (d) images, and strongly homogeneously contrast enhancing (e). Associated with the ventral aspect of the mass is a large cyst, which is isointense to cerebrospinal fluid on all sequences (*). This cystic component is ring enhancing (e). Diffuse perilesional T2 and T2-FLAIR hyperintensity (a, b) is consistent with vasogenic edema, which results in a mass effect and midline shift toward the left. A focal punctate hypointense focus/susceptibility artifact seen on the T2*-weighted image (c; arrowhead) is consistent with focal mineralization or, less likely, focal hemorrhage. Thickened contrast enhancing meninges ("dural tail sign") extend peripherally from the mass (f; arrow).

Figure 3. Caudal fossa meningioma in an 11-year-old mixed-breed dog (not the patient in this case). Sagittal T2-weighted (a) and post contrast T1-weighted (b) MRI images. A large T2 hyperintense and strongly homogeneously contrast-enhancing mass is associated with the caudal ventral cranial vault (*), which results in severe displacement and compression of the adjacent brainstem and cerebellum. A tubular T2 hyperintensity in the cervical spinal cord (a; arrow) is consistent with syringomyelia.

Strong contrast enhancement is seen with both imaging modalities, with homogeneous, heterogeneous or ring enhancement patterns possible (Figures 1d, 1f, 2e, 2f, and 3b).1-3,6,9-13,17,20-22 Concurrent common findings include brain edema (Figures 2a and 2b), mass effect3,5,12,15 and a “dural tail sign” (thickening and enhancement of the dura adjacent to the mass) (Figure 2f).15,23 Less common findings include mineralization or hemorrhage associated with the mass causing hyperattenuating areas on CT and T2 hypointense foci and/or susceptibility artifact (signal void) on T2*-weighted MRI images, respectively (Figure 2c).4,13,15,24,25 Additional findings may include bone changes adjacent to the tumor including hyperostosis, pressure atrophy or tumor invasion of bone (more common in cats),13,15,26,27 single or multiple tumor-associated cystlike changes (more common in dogs and predominantly seen in the rostral fossa) (Figures 2a-2e),13,15,28-31 brain herniation and cervical syringomyelia (Figure 3a), especially secondary to tumors in the caudal fossa.32-34

Other tumor types that may affect the meninges and resemble meningiomas include disseminated histiocytic sarcoma, lymphoma, granular cell tumors and metastatic disease (meningeal carcinomatosis).35-40

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