Page 232 - Atlas of Small Animal CT and MRI

222 Atlas of Small Animal CT and MRI

enhancement of meninges adjacent to the tumor, often necrosis, microvascular proliferation, and sometimes
referred to as a dural tail sign, is a common imaging intratumoral hemorrhage, which contribute heterogene­
feature of meningiomas, although not pathognomonic ity to their MR imaging appearance in both humans and
for the disorder (Figure 2.8.2). 15,18 This feature may be dif­ animals. Astrocytomas may be globoid or irregularly
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ficult to recognize on contrast‐enhanced CT images shaped, and peritumoral edema is variable but usually
because of the hyperattenuation of adjacent bone. minimal to moderate. Intratumoral hemorrhage may
also occur in high‐grade tumors. MR features of gliomas
Granular cell tumor and presumed cerebrovascular accidents can be similar,
Granular cell tumors (GCT) are uncommon central although gliomas tend to be distributed primarily in the
nervous system neoplasms and are of uncertain origin. cerebrum, whereas vascular lesions are more likely to be
Astrocytic, pituicytic (modified astrocyte), meningeal located in the cerebellum, thalamus, midbrain, and
cell, glial cell, and glial precursor cells have all been sug­ brainstem. Diffusion‐weighted imaging can be used to
gested as originating cell lines, and there is evidence that discriminate between these two disorders, with vascular
granular cell tumors represent a common phenotype of a lesions more likely to result in diffusion restriction. 24
variety of neoplasms. Granular cell tumors are included Astrocytomas are generally hypoattenuating on unen­
in this section because of the many imaging features they hanced CT images, and mass margins may be ill defined,
have in common with meningiomas. GCTs are usually particularly when surrounded by peritumoral edema or
well defined and extraaxial with a plaque‐like, sessile when biological grade is low (Figure 2.8.9).
distribution involving the meninges. They are preferen­ Astrocytomas typically appear mildly to moderately
tially located along the convexity of the cerebrum, the T1 hypointense and moderately and heterogeneously T2
falx cerebri, or the floor of the cranial vault, and those hyperintense. Surrounding edema may mask tumor
involving the cerebrum can be quite extensive. 19 margins on both T1 and T2 images.
Peritumoral edema and mass effect associated with With both CT and MR, the intensity of tumor
these tumors can be seen on both CT and MR images. enhancement following contrast administration reflects
Granular cell tumors are mildly hyperattenuating on microvascular proliferation and blood–brain barrier
unenhanced CT images and mildly T1 hyperintense disruption and tends to increase with astrocytoma grade.
and T2 iso‐ to hyperintense on MR images. Granular Low‐grade astrocytomas typically do not enhance, or
cell tumors intensely and uniformly enhance on both CT enhance minimally, whereas high‐grade astrocytomas
and MR images following contrast administration, and are more likely to show moderate or marked, nonuni­
tumor margins are usually well defined (Figure 2.8.8). 19,20 form or peripheral contrast enhancement, although the
degree of enhancement is not a reliable indicator of
Neoplasms of neuroepithelial origin tumor grade (Figures 2.8.9, 2.8.10). 21,22,25–28
Astrocytoma Oligodendroglioma
Astrocytomas are one of the most common of the Oligodendrogliomas occur with similar frequency to
intraaxial central nervous system (CNS) neoplasms. astrocytomas and affect older dogs, particularly Boxers
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Boxers and some other brachycephalic breeds are highly and other brachycephalic breeds. Oligodendrogliomas
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predisposed. Older dogs are most frequently affected, but most frequently arise supratentorially in the frontal,
astrocytomas also occur in young animals. Although piriform, and temporal lobes of the cerebrum and, less
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astrocytomas can originate from either white or gray commonly, more caudally.
matter, those that occur within the cerebrum appear to Oligodendrogliomas may be globoid or irregularly
arise predominantly from white matter. The frontal, shaped, typically have a central mucinous core, and
4,5
piriform, and temporal lobes are the most common sites. often encroach on or breach the ependymal lining of the
The current human WHO classification scheme ventricles. Canine oligodendrogliomas are categorized
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grades astrocytomas based on cytological characteris­ as either low‐grade (grades I or II) or high‐grade (grade
tics. Grade I and II astrocytomas (diffuse astrocytomas) III), with grade III tumors predominating. Peripherally,
are considered the least biologically aggressive forms low‐grade oligodendrogliomas have a well‐ differentiated
and consist of a uniform, well‐differentiated infiltrative cell population and a well‐defined interface with
cell population without mitotic activity. Grade III adjacent brain parenchyma, whereas the high‐grade
( anaplastic) astrocytomas have more nuclear atypia, a oligodendrogliomas are composed of more anaplastic
much higher cell density, and mitotic activity. Grade IV cells with necrosis and microvascular proliferation.
astrocytomas (glioblastoma multiforme) are the most Intratumoral hemorrhage may also be present. As with
malignant and infiltrative, frequently having regions of astrocytomas, microvascular proliferation is associated

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