Page 811 - Clinical Small Animal Internal Medicine

71 Disorders of the Forebrain 779

oligodendrogliomas plexus carcinoma). They can also seed along CSF path-
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VetBooks.ir ● ● choroid plexus papillomas ways and implant in other parts of the neuraxis.
pituitary macroadenomas
Pituitary macroadenomas arise from the hypophyseal
ependymomas
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primitive neuroectodermal tumors (PNETs). stalk and expand dorsally into the thalamus/hypothala-
mus, and on imaging appearance commonly look like
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“mushroom clouds.”
Meningiomas are the most common brain tumors seen Ependymomas and PNETs occur relatively uncom-
in both dogs and cats (approximately 45% of all pri- monly; ependymomas arise from ependymal cells lining
mary brain tumors in dogs). They are thought to arise the ventricular system, whilst the PNET’s origin is cur-
from arachnoid cap cells of the meninges and occur rently unknown.
anywhere along a meningeal surface (e.g., cerebral con- Secondary tumors most commonly include adenocar-
vexity, falx cerebri, at the cerebellopontine angle, in the cinomas arising from the nasal epithelium and invading
olfactory lobe), or within a ventricle. They can occur as a the brain, and osteosarcomas, fibrosarcomas or multi-
discrete focal mass, as an en plaque form or with large lobulated tumors of bone arising from the skull that
cystic components within them. On imaging, they have a compress the brain.
broad‐based dural attachment and are often associated Metastatic tumors arise from tumors that spread hema-
with hyperostosis of the overlying bone (approximately togenously (since there is no lymphatic drainage within
50% of the time in cats). Contrast enhancement is typi- the brain). Common candidates include lymphoma, met-
cally homogenous. Noncontrast‐enhancing areas associ- astatic carcinoma (from mammary gland, lung, and GI
ated with necrosis are not uncommon in dogs. tract, most commonly) and hemangiosarcoma.
Histopathologically, they appear benign; in cats, they are Histiocytic sarcoma can also occur in the brain (both as a
often well encapsulated and removable surgically but in metastatic tumor and as a primary brain tumor).
dogs tend to creep into the Virchow‐Robin spaces Clinical signs associated with tumors reflect their site
surrounding meningeal vessels so microscopic disease of origin. In the case of meningiomas, it is common for
usually remains after debulking. tumors to have been present for several months before
Astrocytomas and oligodendrogliomas together are the onset of clinical signs as these tumors grow slowly,
often referred to as gliomas. Astrocytomas are thought allowing the brain to accommodate. Tumors arising in
to arise from an astrocyte precursor cell while oligoden- the prosencephalon often cause seizures and cortical
drogliomas are thought to arise from an oligodendrocyte blindness. Seizures without interictal abnormalities may
precursor, although this is by no means certain. They be seen with tumors arising on the ventral midline or in
occur with approximately equal frequency – approxi- the rostral parts of the prosencephalon.
mately 17% and 14% of all primary brain tumors in dogs, Diagnosis requires either MRI or computed tomogra-
respectively. Astrocytomas in humans are graded using a phy (CT) scanning. CSF taps may show albuminocyto-
four‐grade classification scheme created by the World logic dissociation (increased protein with normal cell
Health Organization, WHO Grade I being a very small counts) in tumors with significant necrosis, and very
subset (pilocytic astrocytomas), grade II encompassing occasionally may reveal neoplastic cells (with lymphoma
most astrocytomas, Grade III anaplastic astrocytoma, most commonly, but also metastatic carcinomas and
and Grade IV glioblastoma multiforme. very occasionally with oligodendrogliomas or choroid
Oligodendrogliomas may occur more frequently in plexus papillomas).
boxers and more commonly in periventricular white mat- Treatment requires surgical debulking (where loca-
ter. They can also spread along CSF pathways. Imaging tion allows) followed by irradiation with megavoltage
features of both types suggest an intraaxial tumor (arising sources. Typical hyperfractionated dose schedules
from within the brain tissue). On MRI, oligodendroglio- include 2–3 Gy daily doses up to a maximum of approxi-
mas are frequently hypointense on T1‐weighted scans mately 50 Gy. Survival times in dogs are reported to
and hyperintense on T2‐weighted scans, as befitting their approach a median of two years, with some patients liv-
gross mucinous appearance. Astrocytomas tend to be ing three or four years following diagnosis. At present,
more solid in appearance on imaging with greater and there is little evidence to support different treatment
more heterogenous contrast uptake. modalities for different tumor types, with the exception
Choroid plexus papillomas arise from the choroid of meningiomas in cats, for whom surgical debulking
plexus of the lateral, third or fourth ventricles. Since they alone often results in survival times of up to 4–5 years.
arise from essentially a collection of blood vessels, they However, in people some tumor types (particularly oligo-
tend to enhance very strongly and uniformly following dendrogliomas) are chemosensitive, associated with par-
contrast administration on imaging studies. Most are ticular patterns of genetic rearrangements in these tumors
histologically benign but a few can be anaplastic (choroid (loss of heterozygosity of the short arm of chromosome