Astrocytomas are probably the most common neuroectodermal brain tumor in dogs. They are usually found in adult dogs, but they have been reported in dogs less than 6 months old as well. They are common in brachycephalic (short nose) breeds but can be seen in any breed. The cells tend to be arranged around blood vessels.
Choroid plexus papilloma are common tumors in dogs, with reported frequency similar to that of glioblastomas (~12% of neuroglial tumors). Due to their cell of origin, they tend to arise within the ventricular system and can block drainage of CSF, thus a small tumor can cause very severe neurologic signs. These tumors are reddish, papillary growths that may bleed. Histologically, they are well defined, grow by expansion, and have a granular papillary appearance. In both benign and malignant variants of choroid plexus papillomas, dissemination to other areas of the brain or spinal cord via the CSF pathways may occur following exfoliation. Obstructive hydrocephalus may occur. Meningeal carcinomatosis may follow spread of the tumor in the subarachnoid space. Choroid plexus tumors are seen as well-defined, hyperdense masses with marked, uniform contrast enhancement on CT scans. Marked enhancement, potentially including hemorrhage and mineralization, is also seen with MRI. Choroid plexus papillomas have no apparent predilection for brachycephalic.
Ependymomas originate from the epithelium lining the ventricles and central canal of the spinal cord. They are rare, but have been reported most frequently in brachycephalic breeds. The gray to reddish, soft, lobular masses tend to invade the ventricular system and meninges, which may result in obstructive hydrocephalus. Mestastases within the CSF system may be observed. Ependymomas of the fourth ventricle may encircle the brain stem. Both epithelial and fibrillary varieties have been described.
Gangliocytomas are rare intracranial tumors reported in adult dogs of several breeds. Histologic findings include mature, neuronal-like cells with multiple processes, a central nucleus, and a nucleolus. Neuroblast-like immature cells may also be seen, and occasionally, newly formed myelin sheaths. They seem to be seen most often in the cerebellum.
Suprasellar germ cell tumors are located dorsal to the sella turcica at the base of the brain. They are often intimately associated with the pituitary gland, which may be trapped within or replaced by the germ cell tumor. They are thought to result from extensive migration of germ cells during embryogenesis. Neurologic signs may be acute in onset and may include lethargy; depression; bradycardia; dilated, non-responsive pupils; ptosis; visual deficits; and blindness. Affected animals are usually 3-5 yr old; Doberman Pinschers may be at higher risk than other breeds. Some germ cell tumors have been misdiagnosed as pituitary tumors or craniopharyngiomas.
Glioblastoma multiforme, considered to be equated with the more malignant forms of astrocytomas, has been reported with varying frequency in dogs. In one study, the incidence was 12% of 215 neuroglial tumors. Most are large and found in the cerebrum. They are locally invasive and destructive, well vascularized, and often contain necrotic zones. Glioblastomas and are most common in brachycephalic breeds.
Glioma – These tumors arise from the supporting cells of the brain and include astrocytomas, oligodendrogliomas, glioblastoma multiforme and ependymomas. They are common in certain breeds of dog, in particular breeds with short noses (brachycephalic breeds) such as the Boxer, the Boston terrier, and the French and English bulldog. Gliomas can range in malignancy from low grade and slow growing, to high grade, poorly differentiated malignant tumors (known as glioblastoma multiforme).
Hamartomas are formed by disorderly overgrowth of tissues normally present at a site. They are focal malformations resembling neoplasms and have been reported only rarely in dogs, usually as a subclinical finding.
Hematogenous metastatic brain tumors commonly originate from extracranial sites. In dogs, they often develop from carcinomas of the mammary glands, thyroid, bronchopulmonary epithelium, kidneys, chemoreceptor cells, nasal mucosa, squamous epithelium of the skin, prostate, pancreas, adrenal cortex, and salivary glands. Common sarcoma metastases in dogs include fibrosarcomas, hemangiosarcomas, lymphosarcomas, and melanoblastomas. Brain metastases may accompany intramedullary spinal cord metastasis in dogs with lymphosarcomas or hemangiosarcomas. Most CNS lymphomas, especially in dogs, are one part of a multicentric disease, with extensive infiltration of the choroid plexus and leptomeninges a common finding. Neoplastic angioendotheliomatosis in dogs is thought to be an angiotropic lymphoma, possibly of the B-cell line. Extraneural tumor cells sometimes localize in the meninges (eg, meningeal carcinomatosis), often in association with intestinal carcinoma or mammary adenocarcinoma.
Intracranial intra-arachnoid cysts have been reported in dogs. These rare malformation tumors seem to develop most often in the quadrigeminal cistern. Of the 6 dogs in one report, 3 were 1 yr old, 4 were males, and 5 of the 6 dogs weighed 11 kg. One dog had additional developmental anomalies (abnormal corpus callosum and block vertebrae). On CT scans and MRI the cysts were extra-axial, had sharply defined margins, contained fluid isodense to CSF, and did not show contrast enhancement.
Malformation tumors, including epidermoid and dermoid cysts and teratomas, originate from heterotopic tissue and are rare tumors in dogs. They typically lie close to embryonal lines of closure. Epidermoid and dermoid cysts result from inclusion of epithelial components of embryonal tissue at the time of closure of the neural tube. They reportedly have a predilection for young dogs (e.g, 3-24 mo old), although cysts have been found in older dogs.
Malignant histiocytosis, which has focal and diffuse forms, is rarely reported in dogs.
Medulloblastomas are highly malignant, uncommon neuroectodermal canine tumors that almost always develop in the cerebellum. The tumors tend to bulge into the fourth ventricle, often replacing part of the cerebellar vermis and compressing the midbrain rostrally and the brain stem ventrally. They may infiltrate the meninges, metastasize within the CSF pathways, and cause obstructive hydrocephalus. While most cases are seen in young dogs, a cerebellar medulloblastoma with multiple differentiation was recently noted in a 4-yr-old Border Collie.
Meningioangiomatosis is a rare, benign malformation of CNS blood vessels, characterized by proliferation of the vessels and spindle-shaped, perivascular meningothelial cells in the cerebral cortex and brain stem of juvenile and adult dogs.
Meningioma is the most common primary brain tumor in dogs (and in humans) probably the cause of most seizures in dogs over 6 years old. These tumors can also occur in dogs under 6 months old. It arises from the arachnoid mater of the meninges (the membranes that line the skull and vertebral column, effectively surrounding the central nervous system which consists of the brain and spinal cord) rather than the cells of the brain itself. As such, meningiomas are not strictly brain tumors, but tend to be grouped with them because they arise within the cranial cavity and compress or invade the brain. These tumors occur more commonly in long nosed (doliochocephalic) breeds of dog, such as the Golden retriever and collie.
Meningiomas are usually relatively slow growing and amenable to treatment, although more malignant forms do occur. Because the meningioma is a tumor of the meninges (the outer membranes) these tumors grow from the skull inward. This makes them much more surgically accessible (depending on their size).
Meningiomas rarely metastasize outside the brain, but may extend into paranasal regions and lungs or be seen as primary extracranial masses as a result of embryonic displacement of arachnoid cells or meningocytes. The reason they are a problem is that there is a limited amount of space within the skull. The brain and its bath of cerebrospinal fluid takes up almost all the room and when a tumor begins to grow, the brain tissue is compressed. Inflammation can result leading to more swelling and soon nerves of the brain are damaged.
Meningeal sarcomatosis are sarcomas which cause diffuse thickening of the meninges; extensive hemorrhages are common. These rare tumors tend to infiltrate nervous tissue and run along blood vessels. Cell types include lymphoid, plasmacytoid, mature plasma cells, immunoblastic cells, and multinucleate giant cells.
Oligodendrogliomas are common tumors in dogs, particularly in brachycephalic breeds. Most grow by infiltration and destroy invaded tissue.
Pituitary adenoma or adenocarcinoma – The pituitary gland lies beneath the forebrain and is connected by a stalk to an area of the brain called the hypothalamus. Pituitary tumors (adenomas) are common in dogs, with an apparent predilection for brachycephalic breeds. They cause hyperadrenocorticism (Cushing’s disease). Usually they do not cause any other neurologic signs and remain outside the actual cranial cavity. However, in some cases they expand rapidly and compress the overlying brain. These tumors are known as pituitary macroadenomas, or adenocarcinomas (if more malignant). There are several other more unusual tumor types that may be seen in this area such as the suprasellar germ cell tumor.
Primary skeletal tumors do not typically cause neurologic signs. Multilobular osteochondroma originates in the flat bones of the skull, usually in older medium- or large-breed dogs and appears as a firm, fixed mass. It may erode the cranium and compress, rather than infiltrate, underlying brain tissues. Radiographically, the tumor contains nodular or stippled areas of mineralization, resulting in a characteristic “popcorn ball” appearance. Local recurrence and metastasis are common. Vertebral osteochondroma is the spinal cord counterpart.
Vascular malformations are considered developmental lesions rather than true neoplasms and are uncommon in dogs. They may be located in the cingulate gyrus, pyriform-hippocampal area of the temporal lobe, basal ganglia, cerebellum, occipital lobe, or septum pellucidum and fornix and comprise variable combinations of arteries, veins, and capillaries. The vessels tend to be dilated, sinusoidal in
Whenever there are neurological changes in a dog, especially in those older than 5 years, a veterinarian should be consulted to rule out a brain tumor.