Six-year-old, castrated male, boxer, (Canis familiaris).The dog presented with a 20-day history of seizure-like activity (lateral recumbency, drooling, and paddling) with increasing frequency. Phenobarbital administration controlled the symptoms for several weeks, but the dog gradually became lethargic and inappetant with head twitching behavior. The owners elected euthanasia due to concern about the dog’s quality of life.

Gross Description:  

The rostral aspect of the right frontal lobe of the brain was enlarged, with shallow sulci and flattened gyri, and was deviated left of the midline. The brain was fixed in formalin and sectioned for examination. The cranioventral portion of the right cerebral hemisphere contained a gray, translucent, gelatinous, 2cm x 2cm x 4cm mass, extending from the frontal lobe of the cerebrum into the mesencephalon.

Histopathologic Description:

Within the white matter of the cerebral cortex, there is a poorly demarcated, unencapsulated, in-filtrative neoplastic mass. The neoplasm is composed of round to polygonal cells arranged as a loose meshwork or occasionally as closely packed sheets with a honeycomb appearance and scant fibro-vascular stroma. Neoplastic cells are 10-14 microns in diameter and have distinct cell borders; scant to moderate amounts of eosinophilic, fibrillar cytoplasm; a prominent perinuclear clear zone (peri-nuclear halo); and small, irregularly round, hyperchromatic nuclei with indistinct nucleoli. The mitotic rate is less than one per ten 400x high power fields. Capillary blood vessels within the mass are prominent with marked endothelial hypertrophy. Other variable features that are present in some slides: scattered foci of hemorrhage, glomeruloid microvascular proliferation, and pseudocystic cavities containing homo-geneous, lightly basophilic (mucinous) material. Neoplastic cells were negative for GFAP on immunohistochemical staining.

Morphologic Diagnosis:  

Brain (cerebrum): Oligodendroglioma.

Lab Results:  

Bloodwork was unremarkable.



Contributor Comment:  

This case had a classic signalment and presentation for canine oligodendrogliomas. Oligo-dendrogliomas occur most commonly in dogs, particularly in older animals and brachycephalic breeds (boxers, Boston terriers, and bulldogs), and rarely in other species including humans, cats, cattle, and horses.1,2,9 In a study involving 173 dogs, it was found that dogs with oligo-dendrogliomas are 3.6 times more likely to have seizures compared with dogs with other types of primary brain tumors.9 Other common presenting clinical complaints include mentation change, vision loss, neck pain, and vestibular syndrome.9 Bloodwork is usually unremarkable, as it was in this case. The most frequent location for oligo-dendrogliomas is the white or gray matter of the cerebral hemispheres, particularly the olfactory area and rostral lobes, but can occur caudally or in the spinal cord.5,9 Grossly, they appear as large, soft, gelatinous or mucoid, well-demarcated masses with grayish-blue matrix and gray to pink stroma on cut section.1,2,5 Histologically, oligodendrogliomas are moderately to highly cellular, and characterized by dense sheets of uniform cells. 1,5 Delayed fixation causes an arti-factual "honeycomb" cell pattern with a perinuclear halo effect. 1,2,5 Prominent microvascular proliferation is common, often with formation of a delicate "chicken-wire" pattern or vascular loops with glomerular-like tufts, as seen in this case. 1,2,5 Foci of hemorrhage, mineralization, or microcystic areas containing blue-staining mucinous material may also be present.1,2,5 Tumors can extend along or through the leptomeninges or ependymal surfaces; intraventricular growth may be associated with widespread intraventricular metastases.5 Anaplastic oligodendrogliomas are characterized by focal or diffuse anaplasia with prominent proliferation of glomeruloid vessels at the tumor margins, nuclear polymorphism, and increased mitotic index (1-2 per HPF), necrosis, and/or meningeal infiltration.1 Intermingled astrocytes are common, and in some tumors polymorphic multinucleated giant cells may be numerous.1 Often, areas of necrosis with peripheral glial cell palisading can be found.5 Due to the lack of malignant features, in this case, the tumor was diagnosed as a benign oligodendroglioma.

Oligoastrocytomas are rare mixed glial tumors composed of neoplastic astrocytes and oligodendroglia, in which the two cell populations may be diffusely intermingled or in geographically distinct clusters. 1,2,5 Canine oligoastrocytomas are predominantly composed of oligodendroglial cells with at least 25-30% neoplastic astrocytes; lower percentages of astrocytic elements are interpreted as reactive proliferating cells within oligodendrogliomas.1,5 Anaplastic (malignant) oligoastrocytomas are characterized by increased cellularity, nuclear atypia, high mitotic activity, vascular proliferation, and necrosis, and may be difficult to differentiate from high-grade astrocytomas.1

Ultrastructurally, oligodendrogliomas have no obvious distinguishing features, appearing as cells with sparse microtubules and few organelles in the cytoplasm and frequent desmosomal junctions between cells.5

Historically, diagnosis of oligo-dendrogliomas has relied on gross and microscopic tumor morphology and negative GFAP staining of cells, due to lack of specific markers.2,4,5 In recent years, several markers have been investigated, including doublecortin, olig2, and CNPase.3,4 Doublecortin is a cytoplasmic neuronal precursor marker which is frequently expressed in oligodendrogliomas and embryonal neoplasms such as neuro-blastomas and PNETs, but infrequently expressed in astrocytomas.3 Olig2 is a nuclear transcription factor that is required for oligodendrocyte differentiation but not astrocyte development, and has been shown to stain all oligodendrogliomas in one study.4 Non-neoplastic astrocytes do not stain with Olig2; however, astrocytomas can have positive nuclear staining.4 CNPase is a cytoplasmic phosphodiesterase that is expressed early in myelination.4  It stains normal and neoplastic oligodendrocytes, but weak cytoplasmic staining of canine astro-cytomas can also occur.4 A combination of negative GFAP staining and positive Olig2 staining may be most helpful in identification of oligodendrogliomas. Positive staining for factor VIII-like antigen and smooth muscle actin highlights the neoplasm's characteristic microvascular pro-liferations.5 In our case, the neoplasm stained negatively for GFAP. No staining for the other potential oligodendroglioma markers was attempted.

JPC Diagnosis:  

Brain, cerebrum: Oligo-dendroglioma, boxer, Canis familiaris.

Conference Comment:  

Despite some moderate slide variability, the contributor provides a great example and com-prehensive review of oligodendrogliomas in the canine central nervous system (CNS). Oligo-dendrogliomas are the third most common primary neoplasm in the dog brain, after meningiomas and astrocytomas.6 These soft gelatinous neoplasms typically arise in the telencephalic or diencephalic cerebral white matter, but can uncommonly occur in the brainstem, spinal cord, or within the ventricular system as a single tumor or as multiple concurrent oligodendrogliomas as a result of metastasis through the cerebro-spinal fluid.2,6,8 Oligodendrogliomas originate from oligo-dendrocytes, which normally function to provide support and myelination to axons within the white matter tracts of the CNS.7 Each oligodendrocyte can envelop up to 50 axons based on the thickness of the myelin sheath needed. Oligodendrocytes also serve a role in the production of neurotropic factors important for the remyelination of demyelinated axons in the CNS.7 Schwann cells provide an equivalent function in the peripheral nervous system (PNS); however, unlike oligodendrocytes, Schwann cells can only envelope one part of a single axon. Schwann cells are also extremely important in axonal regeneration in the PNS.10

As mentioned by the contributor, positive immunohistochemical staining for oligo-dendrocyte transcription factor-2 (Olig2) in combination with negative glial fibrillary acidic protein (GFAP) has been shown to be an extremely useful tool in the diagnosis of oligodendrogliomas. The proportion of Olig2-positive cells has been shown to be significantly higher in oligodendrogliomas when compared with other glial tumors such as astrocytoma and oligoastrocytoma, mentioned above.4,6 Prior to the conference, an Olig2 and GFAP stains were performed by the Joint Pathology center.  In this case, neoplastic cells showed strong and diffuse intranuclear immunoreactivity to Olig2. Expression of intranuclear Olig2 immuno-reactivity is restricted to the neoplasm and resident oligodendrocytes within the unaffected section of cerebrum. Additionally, neoplastic cells are diffusely immunonegative for GFAP. The GFAP-positive cells noted by conference participants at the periphery and within the neoplasm are interpreted as reactive astrocytes, which can occur due to tumor-induced activation of the residential astroglial network. This staining pattern is consistent with the contributor’s diagnosis of an oligodendroglioma, in this case.2,4,6,8


1. Burger PC, Scheithauer BW: Tumors of neuroglia and choroid plexus. In: AFIP Atlas of Tumor Pathology, Series 4, Tumors of the Central Nervous System. ed. Silverberg SG. Washington DC: ARP Press; 2007:225-233.

2. Cantile C, Youssef S. Nervous system. Maxie MG ed. In: Jubb Kennedy and Palmer's Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA:  Elsevier Saunders; 2016:400.

3. Ide, Uchida K, Kikuta F, Suzuki K, Nakayama H. Immunohistochemical characterization of canine neuro-epithelial tumors. Vet Pathol. 2010; 47(4)741-750.

4. Johnson GC, Coates JR, Wininger F. Diagnostic immunohistochemistry of canine and feline intracalvarial tumors in the age of brain biopsies. Vet Pathol. 2014; 51(1)146-160.

5. Koestner A, Higgins RJ. Tumors of the nervous system. In: Tumors in Domestic Animals. ed. Meuten DJ. 4th ed. Ames, IA: Iowa State Press; 2002:703-706.

6. Kovi RC, Wünschmann A, Armién AG, Hall K, Carlson T, Shivers J, Oglesbee MJ. Spinal meningeal oligo-dendrogliomatosis in two boxer dogs. Vet Pathol. 2013; 50(5):761-764.

7. Kremer D, Gottle P. et al. Pushing forward: Remylination as the new frontier in CNS disease. Trends Neurosci. 2016; 39(4):246-263.

8. Rissi DR, Levine JM, et al. Cerebral oligodendroglioma mimicking intra-ventricular neoplasia in three dogs. J Vet Diagn Invest. 2015; 27(3):396-400.

9. Snyder JM, Shofer FS, Van Winkle TJ, Massicotte C: Canine intracranial primary neoplasia: 173 cases (1986-2003). J Vet Intern Med. 2006; 20:669-675.

10. Toy D, Namgung U. Role of glial cells in axonal regeneration. Exp neurobiol. 2013; 22:68-76.

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2-1. Cerebrum, dog .

2-2. Cerebrum, dog .

2-3. Cerebrum, dog .

2-4. Cerebrum, dog .

2-5. Cerebrum, dog .

2-6. Cerebrum, dog .

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