Page 683 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 31 Tumors of the Nervous System 661

features of gliomas, cerebrovascular accidents, and inflamma- may not be a possible or optimal, minimally invasive brain biopsy
tory lesions results in frequent misdiagnosis of these categories of (MIBB) techniques are a viable alternative.
52,53
The addition of diffusion-weighted, spectro-
intraaxial lesions.
Several MIBB techniques have been used in dogs and cats
VetBooks.ir scopic, and perfusion-weighted imaging sequences (see Fig. 31.3) with brain tumors including endoscopic-assisted, free-hand, and
56–58
When MIBB is performed in ani-
image-guided procedures.
to conventional MRI sequences has shown promise in improving
the ability of MRI to discriminate between neoplastic and non- mals with brain tumors by operators experienced with contempo-
neoplastic lesions, and differentiation of tumor grades. 54,55 rary CT and MRI stereotactic systems, diagnostic yields approach
Choroid plexus tumors (see Fig. 31.1F) and ependymomas 95%, and serious adverse events occur in approximately 5% of
are the most common tumors found in an intraventricular loca- cases. 8,58 The histologic classification and grading of anaplastic
tion, and both of these tumors types often uniformly contrast CNS tumors can be challenging when evaluating the limited sam-
enhance. 21,56 Other intraventricular tumors that are occasionally ple sizes associated with MIBB, but often can be facilitated with
or rarely seen include meningiomas arising from the tela choroi- additional immunohistochemical evaluation of tumors. 59,60
dea of the third ventricle, oligodendroglioma, PNET, and central Not surprisingly, as intracranial tumors are more common in
2,3
neurocytoma. The identification of intraventricular or sub- dogs, a wider variety of histologic types and subtypes have been
arachnoid metastatic tumor implants on MRI studies is a reliable described in dogs compared with cats. There are multiple histo-
means to clinically discriminate grade III choroid plexus carcino- logic subtypes of grade I meningiomas including angiomatous,
mas (CPC) from grade I papillomas (CPP). meningothelial, transitional, fibroblastic, psammomatous, micro-
Finally, although MRI is sensitive for the detection of intracra- cystic, and papillary variants. Meningothelial, transitional, and
nial neoplasms, a normal MRI does not rule out a brain tumor. psammomatous subtypes of meningiomas are the most common
Lymphomatosis and gliomatosis cerebri are notable for their pro- in both species. 20,29 The majority of feline meningiomas are grade
29
pensity to be occult on imaging studies of the brain. I tumors. Atypical (grade II) meningiomas account for a sig-
nificantly higher proportion (40%) of meningiomas in dogs. In
20
Tumor Biopsy and Pathology addition, all grades of canine meningiomas can display invasion
In practice, it is common to make clinical decisions in patients into the neural parenchyma, a mechanism that may contribute to
with presumptively diagnosed tumors based on imaging derived therapeutic failure. Anaplastic (grade III) meningiomas are rare
20
data. However, it is important to recognize the potential conse- in humans, dogs and cats, and account for about 1% of all canine
quences that the previously described limitations of imaging can and feline meningiomas. 20
have on individual patients. Histopathologic examination of rep- The histopathologic features of canine gliomas are remarkably
resentative tissue is required for the definitive diagnosis and grad- similar to what is seen in humans. 60–64 Their propensity to locally
ing of nervous system tumors. Excisional biopsy performed during or distantly infiltrate brain tissue contributes to their malignant
curative-intent surgery remains the most frequently employed clinical behavior. 32,60–64 A diverse spectrum of types and grades
biopsy technique for PBTs, but these procedures have been his- of astrocytomas, oligodendrogliomas, and oligoastrocytomas
torically limited to patients with superficially located, extraaxial that occur in people have been documented in dogs. However,
forebrain or cerebellar tumors. In cases in which surgical resection the frequency of glioma types encountered in dogs differs from

A C E G I

B D F H J

• Fig. 31.3 Comparative magnetic resonance image (MRI) features of canine gliomas and other ring enhancing
lesions. Top panels are all transverse, T1-weighted postcontrast images. (A, B) Nonenhancing grade II astro-
cytoma in the left piriform lobe. MRI (C, E) and CT (D) images of ring-enhancing grade III oligodendrogliomas
causing marked attenuation of the lateral ventricles. (E) Ring-enhancing glioblastoma (F) Dynamic contrast MRI
perfusion map of the tumor depicted in panel ((G, H) and hemorrhagic infarction with peripheral revasculariza-
tion (I, J) in the left occipital lobes of two dogs. The glioblastoma (G) appears hyperintense on the apparent
diffusion coefficient map (ADC; H) generated from DWI sequences, whereas restricted diffusion in the infarction
(J) appears as a hypointense region on the ADC map.