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2.8

Neoplasia

Intracranial tumors can be characterized by anatomic or metastasis from a single primary site. Meningiomas
7–9
location, distribution (intraaxial, intraventricular, may occasionally have a mineralized, cystic, or hemor­
extraaxial), CT density or MR signal characteristics, rhagic component.
intensity and pattern of contrast enhancement, tumor Meningiomas are typically iso‐ to mildly hyperattenu­
margin definition, secondary mass effects, and the ating to cortical gray matter on unenhanced CT images
extent of associated brain edema. Although biopsy is and can produce considerable mass effect. Cystic
necessary for definitive diagnosis, this constellation of components appear hypoattenuating, as does surround­
imaging features can often lead to a specific or differential ing peritumoral edema, which can be extensive
clinical diagnosis. The tumor classification scheme (Figure 2.8.1). In some patients, hyperostosis of calvarial
1
used in this chapter is based on current WHO classifica­ bone adjacent to a meningioma will appear thickened
tion for central nervous system tumors in people. 2,3 and hyperattenuating.
Solid meningiomas are usually uniformly T1 isoin­
Neoplasms of the meninges tense on unenhanced MR images but are occasionally
hypo‐ or hyperintense. Approximately 70% of meningi­
Meningioma omas are T2 hyperintense, with the remainder being
Meningiomas are the most common of the primary isointense. Despite the relatively benign biological
intracranial, extraaxial neoplasms in dogs and cats. behavior of the majority of meningiomas, about 95%
German Shepherd Dogs, Collies, Golden Retrievers, and are accompanied by edema, which may be peritumoral
Boxers are overrepresented. Meningiomas are derived (40%) or diffuse (50%). Edema in T2 or FLAIR images
4,5
6
from meningothelial cells and are divided into three often clearly delineates the meningioma margin,
grades: WHO grade I, benign; WHO grade II (atypical), confirming its extraaxial origin (Figures 2.8.2, 2.8.3,
which have intermediate histologic features; and WHO 2.8.4, 2.8.5, 2.8.6, 2.8.7). Signal void of adjacent calvarial
grade III, malignant. In a report of 112 canine meningi­ bone due to reactive hyperostosis can sometimes be seen
omas, 56% were grade I, 43% were grade II, and less than (Figure 2.8.4). 10
1% were grade III. 6 On both CT and MR images, approximately 60–70% of
Meningiomas in dogs most frequently impinge on meningiomas show marked, uniform contrast enhance­
the olfactory bulbs and frontal lobes and are often ment, with the remainder being heterogeneous and often
of a macrocystic histological subtype. Other common associated with cystic, hemorrhagic, or mineralized com­
sites include the cerebral or cerebellar convexity and ponents (Figure 2.8.5). Contrast enhancement usually
cerebellopontine, basilar, tentorial, falcine, foraminal, or reveals well‐defined tumor margins; a globoid, plaque‐
intraventricular locations. Multiple meningiomas may like, or irregular shape; and a broad‐based superficial
6
be present simultaneously, particularly in older cats, and margin conforming to the meningeal plane. 6,9,11–17 On
it is unclear whether these represent multicentric disease contrast‐enhanced MR images, thickening and intense

Atlas of Small Animal CT and MRI, First Edition. Erik R. Wisner and Allison L. Zwingenberger.
© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.
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