JPC SYSTEMIC PATHOLOGY

NERVOUS SYSTEM

February 2023

N-N04

 

 

SIGNALMENT (JPC #1078510): 7-year-old spayed female boxer

 

HISTORY: This dog presented with progressive deterioration of the CNS. The animal was eventually unable to right itself.

 

HISTOPATHOLOGIC DESCRIPTION: Cerebrum: Effacing and replacing approximately 75% of the neuroparenchyma is a poorly circumscribed, unencapsulated, infiltrative neoplasm composed of spindle to fusiform cells loosely arranged in indistinct interlacing streams or packed into dense, solidly cellular areas that blend into the surrounding neuroparenchyma. Neoplastic cells have variably distinct cell borders and a scant amount of wispy, eosinophilic, fibrillar cytoplasm. Nuclei are oval to elongate with finely stippled chromatin and 1 to 2 nucleoli. There is moderate anisocytosis and anisokaryosis, and mitotic figures average 1-2 per individual HPF. There are multiple large, serpiginous areas of variably coagulative to liquefactive necrosis with foci of retained architecture and loss of differential staining blending into areas of abundant karyorrhectic and cellular debris admixed with numerous degenerate neutrophils and erythrocytes, fewer gitter cells, fibrin, edema, and necrotic blood vessels. Surrounding these necrotic areas are bands of pseudopalisading neoplastic cells oriented perpendicular to necrotic foci. There are numerous vascular proliferations at the periphery, which often form glomeruloid-like structures lined by hypertrophied (reactive) endothelium that occasionally pile up to 3-4 cell layers thick. Multifocally, neoplastic cells invade or obliterate vessel walls, and there is mild to moderate hemorrhage extending into the surrounding neuropil. There is mild gliosis of the neuroparenchyma surrounding the neoplasm.   

 

MORPHOLOGIC DIAGNOSIS: Cerebrum: Astrocytoma, diffusely infiltrative, high-grade, boxer, canine. 

 

SYNONYMS: Glioblastoma, Glioblastoma multiforme (GBM)

 

GENERAL DISCUSSION:

• Astrocytoma is the most most common intracranial primary tumor in dogs
• Most commonly occur in middle-aged or older dogs (17% of all primary nervous system tumors); also reported in cats, cattle, horses, pigs 

• Brachycephalic breeds are overrepresented

• Most commonly affected locations include cerebral hemispheres (particularly the temporal and pyriform lobes), thalamus, hypothalamus, and midbrain; less common locations include cerebellum, spinal cord, and retina 
• The Comparative Brain Tumor Consortium recently published a revised diagnostic classification of canine gliomas (Koehler Jour Neuropathol Exp Neurol 2018)

• In canines, high grade astrocytoma was previously known as glioblastoma or glioblastoma multiforme; this nomenclature is still in use for other species

 

PATHOGENESIS:

• Arise from astroglia and glial precursor cells
• Canine glioblastomas overexpress epidermal growth factor receptor (EGFR), platelet-derived growth factor-α (PDGFR-α), and insulin-like growth factor binding protein 2 (IGFBP-2).

 

TYPICAL CLINICAL FINDINGS:

• Specific nervous signs vary depending on site and size of the tumor
• Non-metastatic but highly locally invasive and diffusely disseminates into the brain parenchyma

 

TYPICAL GROSS FINDINGS:

• Astrocytomas exhibit varied gross features depending on degree of malignancy

• Well differentiated tend to be firm, grey-white, and poorly demarcated (may be difficult to detect, especially if involving the white matter
• Poorly differentiated (i.e. high-grade) tend to be more readily apparent, with increased demarcation, soft to friable texture, and typically have areas of hemorrhage, necrosis, and cavitation

• Considerable brain swelling (edema of surrounding neuropil) and often fatal herniation

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• WHO classification system, currently utilized for all but canine gliomas; astrocytomas are morphologically classified based on differentiation of tumor cells:

• Diffuse astrocytomas (well differentiated, low-grade; WHO grade I and II)
  • Uniform tumor cells
• Anaplastic astrocytomas (medium-grade; WHO grade III)
  • Increased cellular pleomorphism
  • High mitotic rate
  • Areas of necrosis
• Glioblastoma (N-N04; high-grade; WHO grade IV)
  • Glomeruloid-like vascular proliferation
  • Serpinous tracts of necrosis lined by neoplastic cells (pseudopalisading)
  • Hemorrhage common

 

• Features of malignancy: Two essential hallmarks areas of serpentine necrosis and microvascular proliferation; also presence of mitoses and universal features of malignancy (nuclear pleomorphism, anisokaryosis, anisocytosis, cellular atypia)
• “Glomeruloid” microvascular proliferation of adventitial and endothelial cells – resemble vascular tufts of nephron

 

• Canine glioma diagnostic classification scheme recently developed and published by the Comparative Brain Tumor Consortium (Koehler, Jour Neuropathol Exp Neurol 2018)

• 1. Glioma type:

• Astrocytoma if morphology of >80% of the tumor is consistent with astrocytic origin, i.e.:

• Oval to elongate nuclei (angular); open-faced chromatin pattern; naked nuclei; random, disorganized pattern; spindle cell morphology; pleomorphic cells (large nucleoli, multinucleate cells); mineralization
• Tumor cell variations:
  • Gemistocytic: Eosinophilic, abundant cytoplasm
  • Pilocytic: Elongate cells
  • Well-defined: Rare mucin microcysts
  • Fibrillary: Eosinophilic stroma 

• Oligodendroglioma if morphology of >80% of the tumor is consistent with oligodendroglial origin (see N-N02)
• Undefined glioma if morphology is consistent with neither of the above

• 2.  Level of infiltration as assessed at low magnification: 

• No infiltration: compact tumor growth or growth pattern and/or rare foci of infiltration
• Focal infiltration: Compact with focal/multifocal regions of infiltration
• Diffuse infiltration
• Note: Grade is not influenced by degree of infiltration

• 3.  Tumor is assigned a grade:

• Low grade:   
  • No necrosis (other than single cell necrosis)
  • No microvascular proliferation (must differentiate from reactive vasculature) or pseudopalisading 
  • No mitotic figures seen in 10 HPF
  • No overt features of malignancy 
  • Often are poorly demarcated and infiltrative
• High grade: any of the following:
  • Large areas of necrosis
  • Microvascular proliferation
  • Pseudopalisading (tumor cells oriented perpendicular to areas of necrosis)
  • Any mitotic figure(s) present in ten 400x fields
  • Overt features of malignancy (nuclear pleomorphism, anisokaryosis, anisocytosis, cellular atypia)

• Recent study correlated survival time in dogs with glial tumors to various putative markers of malignancy (MC, glomeruloid vascularization, necrosis) across glial tumors and grades (Merickel Vet Pathol 2021):
  • Dogs with astrocytic tumors had longer survival times than oligodendrogliomas and undefined gliomas 
  • Mitotic count was the only histologic feature on biopsy that significantly correlated with survival
  • Analysis of the three features of malignancy showed that lower values on all three criteria were significantly correlated with survival

 

ULTRASTRUCTURAL FINDINGS:  

 

ADDITIONAL DIAGNOSTIC TESTS:

• Fine needle aspirate or squash preparation: Pleomorphic plasmacytoid to histiocytic cytomorphology with eccentric nuclei, prominent nucleoli and moderate amount of eosinophilic to basophilic cytoplasm (+/- vacuolation) 
• Majority of astrocytomas exhibit positive immunoreactivity for glial fibrillary acidic protein (GFAP), nestin, S-100, vimentin

• GFAP immunoreactivity varies from sparse to abundant (GFAP is most reliable for confirmation)

• Negative cytokeratin immunoreactivity

 

DIFFERENTIAL DIAGNOSIS:

• Differentials of glial origin are dependent on grading scheme utilized (i.e. WHO vs. Comparative Brain Tumor Consortium); see chart below “comparative pathology”
• Many rare neoplasms of glial origin using WHO grading scheme are classified as “undefined gliomas” under Comparative Brain Tumor Consortium grading scheme
• Gross:

• Undefined glioma: Similar features
• Oligodendroglioma (N-N02): Usually well-demarcated; soft, red-pink to grey; gelatinous 
• Neuroblastoma: Well-demarcated, pink-grey neoplasm with areas of hemorrhage, necrosis, and calcification
• Medulloblastoma (N-N05): Cerebellum of puppies, calves, and adult dogs
• Primitive neuroectodermal tumor (PNET) (N-N05): Soft, grey-pink
• Metastatic brain tumors
• Inflammatory lesions

• Micro:

• Low grade astrocytoma (N-N02): Lack necrosis, microvascular proliferation, and easily identifiable mitotic activity
• Undefined glioma: >30-40% of each phenotype OR undifferentiated cellular morphology
• Primitive neuroectodermal tumor (N-N05): Densely packed round to polygonal cells with often hyperchromatic carrot-shaped nuclei, rosettes, pseudorosettes, and palisading of neoplastic cells
• Additional neoplasms of glial cell origin described under WHO grading scheme:

• Gliosarcoma:   Biphasic neoplasm with areas of highly anaplastic glial cells with glioblastoma features (serpentine foci of necrosis with a pseduopalisading tumor cells, glomeruloid vascular proliferation) and abundant sarcomatous components; GFAP positive
• Gliomatosis cerebri:  Diffuse infiltrate of cells reminiscent of astrocytes rather than a distinct mass; infiltrates often present bilaterally but asymmetrically in the brain and may also be present in discontinuous areas in the spinal cord; normal neural structures remain intact, with only slight damage to axons and neurons; GFAP negative
• Astroblastoma
• Giant cell glioblastoma
• Anaplastic oligiodendroglioma: Neoplastic oligodendroglial cells are round with clear to lightly stained cytoplasm and hyperchromatic nuclei; anaplastic variants exhibit high cellularity, glomeruloid vessel proliferation, nuclear pleomorphism, meningeal infiltration, mitoses and/or necrosis

 

COMPARATIVE PATHOLOGY:

• Horses: Primary brain tumors are rare. One recent report of intracerebral astrocytoma in an Anglo-European gelding (Cavasin J Comp Pathol 2020)
• Cats: One report of an angiocentric astrocytoma where neoplastic cells were confined to perivascular spaces with palisading (Rissi J Vet Diagn 2019)
• Cattle: Recent retrospective case series identified gliomas as the most common intracranial neoplasia in cattle, with astrocytomas the second most common tumor, after oligodendrocytomas. (Jahns Vet Pathol 2022)
• Rats: Malignant astrocytoma was most frequent CNS tumor in a survey of Sprague-Dawley rats (8 cases from 670 rats); most are GFAP negative and positive for macrophage markers indicating they may be of monocytic origin (Bertrand Tox Pathol 2014)
• Atlantic spotted dolphin: Case report of high grade astrocytoma
• African Hedgehogs: Astrocytomas are one of the most common CNS neoplasms, along with gangliogliomas (Muñoz-Gutiérrez J Vet Diagn Invest 2018)
  • All were gemistocytic and within the cerebrum; no metastasis observed
  • GFAP, S100, CD34 positive
• NHPs: rare reports of astrocytoma and glioblastoma multiforme, some associated with induction by viruses (simian virus 40 [SV40], concurrent polyomavirus and SIV infection) and radiation (macaques, baboons)

 

 

 

WHO Astrocytoma Classification Table for Non-Canine Species

Astrocytoma	WHO Grade	Histologic features	Diagnostics
Low-grade (4 variants)	I	Unencapsulated, expansile, subtly invasive, increased population of fibrous astrocytes; no mitosis   4 Variants –   - Fibrillary astrocytoma: neoplastic cells have scant cytoplasm, abundant fibrillary process and filaments - Protoplasmic astrocytoma: neoplastic cells have scant cytoplasm, few short processes and filaments - Pilocytic astrocytoma: neoplastic cells are bipolar, elongated (piloid or hair-like) astrocytes and have Rosenthal fibers - Subependymal giant cell astrocytoma: originates from subependymal tissue of lateral and fourth ventricles, composed of spindloid to polygonal large eosinophilic cells arranged in short interlacing streams and irregular nests
Medium-grade	II	Denser cell population, larger and darker nuclei w/ slight variation in size & shape, no mitosis;  vessel walls may be slightly thickened Variant: Gemistocytic astrocytoma: neoplastic cells have abundant acidophilic cytoplasm and eccentric, oval to round nuclei	Recognizable astrocytes show GFAP & vimentin immunoreactivity
Anaplastic	III	Nuclear atypia, mitotic figures, high proliferative index
High-grade astrocytoma or glioblastoma (formerly glioblastoma multiforme)	IV	Hemorrhage and necrosis; neoplastic cell pseudopalisade around necrotic areas; proliferation of vascular adventitial & endothelial cells forming glomeruloid blood vessels - Few cell recognizable of astrocytes - Pleomorphism (marked), giant nuclei, multinucleated giant cells are common - Increased cell density, marked nuclear atypia, high mitotic rate  - Infiltrative growth pattern	Canine glioblastomas over-express: - Epidermal growth factor (EGFR) - Platelet-derived growth factor (PDGFR- α) - Insulin-like growth factor binding protein 2 (IGFB-2)
Rare variants reported in dogs includes: Astroblastoma, Giant cell Glioblastoma, Gliomatosis cerebri & Gliosarcoma   - Gliomatosis cerebri: diffuse, infiltrating     disease of dogs (predominately brachycephalic breeds) and humans    - Gliosarcoma: rare tumor		- Gliomatosis cerebri:  diffuse infiltrates involve brain, often bilaterally but asymmetrically & discontinuous areas also in spinal cord; No tumor mass just diffuse enlargement of affected regions w/ cells insinuating among normal structures that remain intact with only slight damage to axons and neurons; composed mainly of cells reminiscent of fibrillary astrocytic cells w/ elongated,  hyperchromatic nuclei and oligodendrocytes, cells of transitional character & small unclassified cells; pattern of infiltration is unexplained, but involves participation of cell adhesion molecule    - Gliosarcoma: composed of highly anaplastic glial cells w/ abundant sarcomatous components.	- Gliomatosis cerebri: origin of neoplastic cells controversial, as they do not stain with glial markers such as GFAP    - Gliosarcoma: Positive GFAP staining differentiate this tumor from other spindle cell, tumors, i.e., fibrosarcoma

 

References:

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