Page 679 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

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Tumors of the Nervous System

JOHN H. ROSSMEISL, JR AND THERESA E. PANCOTTO

Nervous System Tumor Classification and are ongoing to assess if this type of methodological approach can
Grading provide clinically useful correlations between tumor histology,
biologic behavior, and therapeutic outcomes in a manner similar
Tumors of the nervous system can be broadly categorized into pri- to those that exist for other veterinary neoplasms, such as mast
mary and secondary varieties. 1–4 Primary brain (PBT) or spinal cell tumors. 12
cord (SC) tumors arise from the constitutive tissues of the brain Since the late 1990s, key discoveries have been made regard-
or SC, such as neurons, glia, and the meninges. Secondary brain ing the fundamental genetic aberrations that drive tumorigenesis
(SBT) or SC tumors represent those cancers that metastasize to in many types of human nervous system tumors. These studies
the brain or SC from a distant site or affect nervous tissue by direct have revealed that considerable molecular and genotypic hetero-
extension from an adjacent tissue (e.g., pituitary, nasal, or calvarial geneity exists between tumor types, even among of tumors of the
tumors). 4 same histology, and these features can have important prognostic
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In recent years, efforts have been made to model classification implications that transcend phenotype. This has resulted in a
of domestic animal nervous system neoplasms after the 2007 departure of the most recent human WHO classification of ner-
World Health Organization (WHO) criteria applied to human vous system tumors from a histology-based platform to an inte-
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tumors. This has been motivated principally by the remark- grated system that incorporates tumor genotypic, molecular, and
able clinical, diagnostic imaging, phenotypic, molecular, and classic phenotypic information into the definition of neoplastic
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genomic similarities observed between many canine and human entities. As the genetic characterization of canine and feline
PBTs, 6–10 as well as attempts to address limitations in the previ- nervous system tumors continues to advance, it is likely that this
11
ous system used to describe nervous system tumors of animals. approach to classification will also become relevant in veterinary
The 2007 WHO criteria are grounded in traditional principles medicine. 7–10
of histogenesis where light microscopic phenotypic similarities
define the various tumor types, each type putatively originat- Intracranial Tumors
ing from different cellular lineages (Table 31.1). The benefits
of using this WHO system include the availability of a more Epidemiology
comprehensive descriptive library of histologic tumor subtypes
and the inclusion of a tumor grading scheme applicable across Besides humans, dogs and cats are the only mammalian spe-
tumor types. cies in which spontaneous brain tumors are encountered fre-
The tumor grades represent the range of potential biologic quently. 1–4,14–19 The overall estimated incidences of canine and
behaviors displayed by tumors. Grades are assigned based on the feline nervous system tumors have been reported as 14.5 cases per
presence and degree of classic cytoarchitectural features of malig- 100,000 and 3.5 per 100,000 at risk, respectively. 14,18 Other stud-
nancy, such as cellular atypia, nuclear pleomorphism, mitotic ies indicate that intracranial neoplasms are observed in 2.0% to
rate, microvascular proliferation, and necrosis. For example, in 4.5% of dogs and 2% of cats at necropsy. 1,17–19
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humans, grade I tumors are well differentiated with low prolif-
erative potential that may often be cured by surgical excision, Dogs
whereas grade IV tumors are cytologically malignant, mitotically
active tumors that are refractory to therapy. Thus in humans, the In dogs, approximately 90% of PBTs (see Table 31.1) are rep-
tumor grade represents one piece of a composite system that uses resented by meningiomas (45%), gliomas (40%), and choroid
patient clinicopathologic data to guide therapeutic decision mak- plexus tumors (CPTs; 5%), although the distribution of specific
5
ing and predict long-term prognosis. Within the closed confines PBTs in individual studies varies substantially. 1,2,17–19 Less com-
of the calvarium or vertebral column, even slow-growing grade I monly reported PBTs in dogs include ependymomas, primary
tumors will cause progressive morbidity and eventual death of the central nervous system (CNS) lymphoma, primitive neuroecto-
patient in the absence of therapy regardless of presence of benign dermal tumors (PNETs), gliomatosis cerebri, and primary CNS
versus malignant characteristics. By prospectively applying WHO histiocytic sarcoma (HS) (see Table 31.1); numerous other rare
criteria in animal populations with nervous system tumors, efforts PBT have been described 1,2,14–19 SBTs comprise one-half of all

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