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38 Section I: Diagnostics and Planning

considered. Definitive diagnosis requires tissue biopsy (stereotaxis, In addition to identifying the primary lesion, the brain should
free‐hand or open surgical biopsy). also be evaluated for secondary pathological effects. Hydrocephalus
The diagnosis of a brain tumor relies on diagnostic imaging. The is commonly seen with masses within the caudal fossa (e.g., brain-
aim of imaging an animal with a suspect brain tumor is to confirm stem masses) due to compression of the CSF pathways or due to
the diagnosis, screen for metastases either to or from the brain, and increased CSF production and protein concentrations seen with
to aid therapeutic planning. MRI also allows identification of the choroid plexus masses. With certain tumors (e.g., choroid plexus
secondary pathological effects of intracranial masses and inflam- tumors), treatment may need to be directed to the secondary patho-
mation (e.g., hydrocephalus, vasogenic edema, bleeding, or brain logical effects. Any disease that increases ICP may result in brain
herniation) [93,97,98]. herniation. This is easiest to see on T2‐weighted sagittal images,
Most primary brain tumors in dogs result in no changes to the
skull and are not visible on plain radiography. Meningiomas in cats
can be associated with hyperostosis of the calvaria (Figure 4.10),
which can occasionally be visualized on radiographs, as can areas of
mineralization of the tumor. As with any animal with a suspected
tumor, it is advisable to obtain thoracic radiographs to screen for
pulmonary metastases. While primary brain tumors rarely spread
to the lungs, metastasis does occur and this has consequences for
treatment and prognosis. It is more common for tumors to metasta-
size to the brain than vice versa. Metastases to the brain are less
common than primary brain tumors. When they do occur, they are
usually multiple, small, located at the junction between gray and
white matter (i.e., watershed zone), and surrounded by marked
edema (Figure 4.11).
Most intraaxial tumors (and other types of brain pathology)
result in an increase in water content, which appears hyperintense
on T2‐weighted images and hypointense on T1‐weighted
images [93,95,99]. Extraaxial masses are variable in appearance
depending on cellularity and mineralization [94]. Most extraaxial
masses are associated with marked contrast enhancement
(Figures 4.10 and 4.12). The contrast uptake by intraaxial masses is
variable, from none to marked [100]. While the mass effect pro- Figure 4.10 Transverse plane T1‐weighted postcontrast MRI of a cat with a
duced by brain tumors is obvious, delineating the mass is not always meningioma. Note the thickening and reduced signal of the calvarium over-
easy. In some cases of diffuse neoplasia (e.g., lymphoma or glioma- lying the mass due to hyperostosis (arrow). The dense homogeneous con-
tosis cerebri) the changes may mimic inflammatory disease. trast enhancement is common with meningiomas and other extraaxial
Definitive diagnosis may require brain biopsy. masses.

A B

Figure 4.11 Transverse plane T2‐weighted (A) and T2* GRE (B) MRI of a dog with cerebral metastases from hemangiosarcoma. Note the multiple small
hemorrhagic nodules (best seen on the T2* GRE image) with extensive perilesional edema.