Primary brain tumors

Last updated: May 11, 2025

Primary brain tumors arise from the brain parenchyma, surrounding meninges, or ventricular structures. The incidence of brain tumors in dogs and cats has been reported from as low as 14.5 in 100,000 dogs and 3.5 in 100,000 cats to as high as 2.6% in dogs and 2.2% in cats. Meningioma is the most common brain tumor overall for both dogs and cats.

Primary brain tumors

Meningioma
Glioma
- Astrocytoma
- Oligodendroglioma
Choroid plexus tumor
Ependymoma
Primary CNS lymphoma
Primitive neuroectodermal tumors (PNET)
- Olfactory neuroblastoma
- Medulloblastoma
Primary histiocytic sarcoma (a.k.a., malignant histiocytosis)
Vascular hamartoma

Benign or malignant?

Malignant brain tumors are more common than benign brain tumors. That being said, regardless of whether the tumor is benign or malignant, it is a space-occupying mass in or around a soft organ (brain) housed inside a non-compliant, rigid bony skull. As a result, the brain is affected and clinical signs can be severe, regardless of whether the tumor is benign or malignant.

In human medicine, tumors are frequently given a score of I to IV. Tumors with more normal cellular appearance are typically classified as Grade I or II and are generally considered to be benign, while tumors with more abnormal cellular features are classified as grade III or IV and considered to be malignant.

In general, brain tumors that are benign in human medicine are:

Slowly growing
Less likely to return if completely removed
Unlikely to metastasize to other areas of the CNS or body
May not require radiotherapy or chemotherapy

Brain tumors that are malignant in human medicine are:

Fast growing
Likely to return even if completely excised
May spread to other parts of the CNS or body
Will need radiotherapy and/or chemotherapy to try to decrease recurrence

Signalment

Brain tumors are most commonly diagnosed in older dogs and cats. The median age across multiple studies is approximately 9 years in dogs and 10 years in cats, although, as in human medicine, brain tumors are occasionally identified in juvenile and young adult dogs and cats. Meningioma has been reported in young cats with mucopolysaccharidosis I (MPS I). In one small study of MPS I in cats, 4 of 7 cats had a meningioma and were less than 3 years of age. Meningioma is slightly more common in male cats, but there is no significant difference between male and female cats. Golden Retrievers and Boxers are among the most common breeds of dogs to develop brain tumors. Meningioma is the most common brain tumor identified in Golden Retrievers and other dolicocephalic dog breeds, while glioma is more common in Boxers and other brachycephalic dog breeds. There does not appear to be a breed predilection in cats.

History

Historical information typically reflects lesion location. Clinical signs are often chronic with a slowly progressive nature. However, an acute or subacute onset of clinical signs is relatively common and may be due to sudden changes in intracranial pressure, acute hemorrhage, or obstructive hydrocephalus. In one large-scale retrospective study of dogs (Snyder JS, et al), the median duration of clinical signs was 26 days (range, 1-1068 days).

Seizures are the most common clinical complaint reported in dogs with brain tumors, while behavior change is more common in cats. Dogs with gliomas are 3.6 times more likely to have seizures than other tumor types. Non-specific clinical signs were very common in one large retrospective study of feline brain tumors, especially lethargy (20%), inappetence/anorexia (18.1%) and weight loss (6.2%).

Common clinical signs based on lesion location

Forebrain (cerebrum, thalamus)

Seizures
Behavior change / aggression
Altered mental status (dull, stupor, coma)
Compulsive pacing and/or circling
Head pressing
Getting stuck in tight corners/spaces
Staring off into space / zoning out
Hyperesthesia
Visual deficits
Urinating / defecating in the house

Brainstem

Altered mental status (dull, stupor, coma)
Cranial nerve dysfunction (CN III-XII)
Gait abnormalities (weakness, proprioceptive deficits)
Vestibular dysfunction (head tilt, vestibular ataxia, circling, nystagmus, etc.)

Cerebellum

Dysmetria – most often hypermetria
Cerebellar ataxia
Truncal sway
Intention tremors
Vestibular dysfunction

Neurologic exam

Neurologic exam findings reflect lesion location. See the neurolocalization page for more specific information.

In most cases, neurological exam findings reflect a single lesion location, but the tumor may be large enough to affect to different regions of the brain, leading to clinical signs suggestive of a multifocal disease process. Some patients will have multiple brain tumors identified. Multiple meningiomas have been reported in cats in several reports. In one large-scale retrospective study of feline brain tumors, 10% of cats had multiple meningiomas. Another 10% of cats had two or more tumors of different histologic origin (e.g., meningioma and pituitary tumor). In the study by Snyder et al., 4 of 173 dogs (4%) had 2 different intracranial neoplasms.

Diagnosis

Definitive diagnosis requires biopsy and histologic analysis. A fairly good tentative clinical diagnosis can be made following advanced imaging and other diagnostics.

Minimum database

Routine blood tests and thoracic radiographs should be obtained prior to performing MRI, both to look for a systemic cause for the neurologic signs and as a pre-anesthetic screen prior to advanced imaging. CBC, biochemical profile, and urinalysis are typically normal, assuming no other underlying systemic/metabolic conditions.

The patient’s blood pressure should be taken to screen for systemic hypertension as a potential cause for clinical signs. High blood pressure also can be a reflection of increased intracranial pressure (ICP; e.g., hypertension with concurrent bradycardia [Cushing reflex]).

Thoracic radiographs should be taken to screen for extracranial disease, including primary and metastatic neoplasia, pneumonia and other lung pathologies, and cardiovascular disease. In the study by Snyder et al., 19% of dogs with a primary brain tumor had thoracic cavity pathology, including presumptive metastatic neoplasia (8 dogs), pneumonia (5 dogs), megaesophagus (3 dogs), megaesophagus with aspiration pneumonia (1 dog), and heart failure (1 dog).

Abdominal imaging (radiographs, ultrasound) are infrequently performed as part of the standard workup plan for presumptive intracranial neoplasia. However, it should be performed if there is history or physical exam findings suggestive of abdominal disease.

Advanced imaging

Advanced imaging (MRI > CT) is recommended for all patients with clinical signs of intracranial disease. See the MRI vs. CT page for a description of the advantages and disadvantages of each imaging modality. Listed below are the common MRI features of individual tumor types. CT imaging characteristics of various brain tumors can be found elsewhere in the veterinary literature.

“Typical” imaging characteristics of various tumors

Meningioma (fig. 1)

Extraparenchymal (i.e., outside the brain)
Broad base along the skull, falx cerebri or tentorium calvaria
Have distinct tumor margins
Often display moderate to marked homogeneous contrast enhancement
May have calcification (hypointense on all MRI sequences)
May exhibit a “dural tail” – an extension of the tumor or contrast enhancement along the meninges away from the mass (Note: A dural tail can be seen with other neoplastic [e.g., lymphoma, histiocytic sarcoma] and non-neoplastic disorders.)
May have a cystic component, especially meningiomas in the olfactory/frontal region of the brain
Meningiomas can arise within the third ventricle in cats in association with the tela choroidea of the third ventricle in this species

Glioma (figs. 2 & 3)

Intraparenchymal location (i.e., inside the brain)
Variable shape, often with indistinct margins
Intratumoral hemorrhage may be present
Variable contrast enhancement, often poorly enhancing and heterogeneously; “classic” gliomas have peripheral enhancement (“ring enhancing”; Note: ring enhancement has also been associated other neoplastic and non-neoplastic diseases)
More common in the cerebrum and diencephalon

Choroid plexus tumor (fig. 4)

These are extraparenchymal tumors arising from the choroid plexus within the lateral, third, or fourth ventricles
Often have distinct, but frondular margins
Contrast enhancement is usually marked and homogeneous
Hydrocephalus may be observed due to either obstruction of CSF outflow (obstructive hydrocephalus) or excessive CSF production

Ependymoma

These are extraparenchymal tumors arising from the ependymal lining of the ventricular system
They usually have strong contrast enhancement

Lymphoma and histiocytic sarcoma

CNS lymphoma can look like anything, ranging from a solitary extraparenchymal or intraparenchymal mass lesion to multifocal or diffuse intraparenchymal disease
Histiocytic sarcoma is often extraparenchymal in origin
Both are frequently strongly contrast enhancing
Extraparenchymal lymphoma and histiocytic sarcoma can look like a meningioma

Fig. 1: Large left parieto-occipital lobe meningioma in a cat that presented for an acute onset of circling to the left. The mass was causing transtentorial and cerebellar herniation (not visible in these images). GE 1.5 T magnet

Fig. 2:Right olfactory bulb/frontal lobe astrocytoma confirmed by surgical excision via transfrontal craniotomy. The dog underwent radiation therapy after surgery and lived 3 1/2 years after surgery. GE 0.5 T magnet

Fig. 3: MRI of an oligodendroglioma confirmed at autopsy. Siemens 1.5 T magnet

Fig. 4: MR images from a dog with a necropsy-confirmed chroid pelxus carcinoma.

Cerebrospinal fluid (CSF) analysis

CSF results are variable and can range from normal to mildly abnormal. Moderate to markedly elevated nucleated cell counts (ie, white blood cells) are less common with neoplasia than with encephalitis. Elevated protein with normal white blood cell count (albuminocytologic dissociation) is relatively common but is not pathognomonic for intracranial neoplasia. Neoplastic cells are rarely identified. Lymphocytic pleocytosis is common with CNS lymphoma. Lymphoblasts are not often identified, but if they are observed, a definitive diagnosis can be made, although this should be confirmed by additional testing (e.g., PARR, flow cytometry). Meningioma and lymphoma frequently cause a neutrophilic pleocytosis.

Whether or not to obtain CSF for analysis is somewhat controversial because CSF results are variable and rarely provide a definitive diagnosis. There is a small risk that performing CSF analysis in patients can harm the patient, the theory being that if there is a sudden shift in ICP as CSF is drained it can lead to brain herniation. As a result, many veterinary neurologists do not routinely perform CSF analysis in patients with a presumptive brain tumor.

Histopathology

Histologic analysis of tissue from the mass is required for definitive diagnosis. A variety of freehand and stereotactic CT- and MRI-guided biopsy techniques have been described in the veterinary literature. The benefits of biopsy typically outweigh the risks of the procedure. Unless the mass is readily accessible via a surgical approach, brain biopsy is uncommonly performed in veterinary neurology due to the cost of stereotactic biopsy equipment, client concerns relative to the procedure, and the cost to the client.

Treatment

A variety of treatment options have been reported in the veterinary literature. Treatment options can be divided into definitive (e.g., surgery, radiotherapy, chemotherapy) and supportive measures (e.g., corticosteroids, anticonvulsants). Surgical debulking/excision and radiotherapy are the most frequently described definitive treatment options.

Surgery

Surgery is the most common treatment for brain tumors that are surgically accessible, primarily those outside the cerebral hemispheres, superficially located within the cerebral parenchyma, or caudodorsal to the cerebellum. Surgical excision of meningiomas is recommended for both dogs and cats. In cats, the meningioma typically sits on the surface of the brain and is easily “shelled out” (like a peanut coming out of a shell) completely. Gross excision of canine meningiomas can often be performed, but approximately 1/3 of canine meningiomas have microscopic invasion into the underlying brain parenchyma. Surgical excision of gliomas can be attempted for those that are superficially located. However, the surgeon has to go through “normal” brain tissue to reach the glioma and it can be difficult to differentiate neoplastic tissue from surrounding “normal” brain tissue, which is often abnormal in appearance/texture due to cerebral edema and pressure necrosis. Surgical excision of more deeply-seated tumors (e.g., glioma, third ventricular tumors, ventral brainstem tumors) have been reported, but carry a higher surgical risk and post-op morbidity and mortality.

Radiation therapy (RT)

RT is another common definitive treatment option in veterinary neurology for presumptive brain tumors. RT is recommended in the following circumstances:

Following surgical excision of any brain tumor in dogs
Following incomplete resection of meningioma or other tumors in cats
As a primary treatment in any surgically-inaccessible tumor
As a primary treatment if the owner is unwilling to accept surgical risks/morbidity/mortality

Megavoltage RT is generally preferred. RT protocols vary but are typically fractionated into daily doses of 2.5-4 Gy, usually once daily, Monday-Friday for 3 weeks. Radiation oncologists and RT centers are few and far between, although they are becoming more readily available at both university and private-practice referral centers. Most patients with CNS neoplasia that are treated with standard megavoltage RT do very well with minimal adverse effects.

Intensity modulated radiation therapy (IMRT) is a newer form of RT increasingly available in veterinary medicine. IMRT is a form of 3-D conformal radiation therapy that delivers variable intensity of radiation by a device that is able to rotate 360^o around the patient. This allows the delivery of many beams of radiation with more precise targeting of the tumor and less impact on surrounding normal brain tissue.

Gamma knife radiosurgery has also been reported in veterinary medicine. Essentially, this technique involves administration of a large number of low-dose radiation pulses on the target from numerous angles. The low-dose radiation has little effect on the tissue it passes through but has a cumulative effect where the radiation overlaps within the target lesion. The advantage of gamma knife therapy is that only 1-3 treatments are required, greatly reducing the number of anesthetic events, which is beneficial in patients with higher anesthesia risk (e.g., hyperadrenocorticism, diabetes mellitus, cardiovascular disease). The primary disadvantages are the small number of veterinary facilities that offer this form of RT and the very high cost (currently $8,000-12,000 for treatment at most locations). The other disadvantage is that the long-term survival is approximately the same for brain tumors compared to conventional RT.

Chemotherapy

Chemotherapy is another definitive treatment option for intracranial neoplasms, but is often considered ineffective for brain tumors due to inability of most agents to cross the blood-brain barrier (BBB). There are, however, several medications that are able to cross the BBB and may have some efficacy, including lomustine (CCNU), carmustine (BCNU), cytosine arabinoside (cytarabine, Cytosar), and hydroxyurea. Chemotherapy may provide a longer lifespan than palliative treatment alone.

CCNU (Lomustine) and BCNU (Carmustine) have been reported most commonly for the treatment of canine gliomas. Lomustine is used more commonly. The recommended dose is 60 mg/m² every 6-8 weeks. Most dogs tolerate this medication very well. Adverse effects include bone marrow suppression (anemia, thrombocytopenia, leukopenia) and hepatotoxicity (especially in patients being given other liver-metabolized medications). GI side effects (decreased appetite, vomiting, diarrhea), stomatitis, alopecia, and rare renal toxicity and pulmonary infiltrates/fibrosis have been reported). The CBC nadir occurs at 1-6 weeks after administration so a CBC should be performed weekly until the nadir has passed. Serum chemistries (especially liver chemistries) should be performed prior to at least every other treatment (ideally before every treatment).

Hydroxyurea is an oral chemotherapeutic agent most commonly used to treated polycythemia, but has also been shown to have efficacy against meningiomas in both people and dogs. In one small study presented at the ACVIM Forum in 2009, patients that were given hydroxyurea and glucocorticoids lived 2.97 times longer than patients that received glucocorticoids alone. The reported doses have been either 20 mg/kg PO once daily or 50 mg/kg PO three times weekly. A CBC with platelet count should be performed weekly for 1-3 weeks until it is stable and then every 3-4 months. BUN and creatinine should be measured prior to starting the medication and then every 3-4 months. This medication is generally well-tolerated, but adverse effects that have been reported include GI side effects (inappetence, vomiting, diarrhea), stomatitis, sloughing of nails, alopecia, and dysuria. Bone marrow suppression (anemia, thrombocytopenia, leukopenia) and pulmonary fibrosis have been reported.

The use of cytarabine (cytosine arabinoside, Cytosar) has not been reported in peer-reviewed veterinary literature, but there are anecdotal reports of its use in patients with CNS lymphoma. A wide variety of doses have been used, but a common dose is 100-200 mg/m² given intravenously over 12-24 hours. The total dose can be divided and given subcutaneously (50 mg/m² q12h for 4 doses).

Corticosteroids

Corticosteroids are recommended to reduce secondary effects of brain tumors. They can reduce the amount of peritumoral cerebral edema and reduce CSF production to secondarily reduce ICP. With the exception of CNS lymphoma and other myeloid neoplasms, corticosteroids will have no effect on the mass itself. Intravenous dexamethasone or dexamethasone sodium phosphate (0.25 – 0.5 mg/kg) at the time of diagnosis may result in rapid improvement. Patients are then maintained on prednisone long term (0.5 mg/kg PO q12h for 1-2 weeks, then reduced to the lowest effective dose). The dose can be raised and lowered as needed to control clinical signs.

Anticonvulsants

Anticonvulsants are recommended for brain tumor patients that are experiencing seizures. Phenobarbital is commonly used, but a significant number of patients with structural brain disease will experience more profound sedation on phenobarbital, even at standard doses. If using phenobarbital, consider reducing the dose slightly from the standard 2 mg/kg PO q12h dose. Less sedating anticonvulsants should be considered, such as zonisamide (5-10 mg/kg PO q12h) or levetiracetam (standard release: 20 mg/kg PO q8h; extended release: 30 mg/kg PO q12h).

Medications to reduce ICP

Acute increases in ICP are typically managed with mannitol (0.5 – 1.0 gram/kg IV over 15-20 minutes) and furosemide (1-2 mg/kg IV). Mannitol is a monosaccharide that does not cross the BBB. Elevated plasma osmolality due to infusion of hypertonic mannitol effectively removes fluid from the brain. Bolus administration of mannitol leads to expansion of circulatory volume, decreased blood viscosity, and increased cerebral blood flow, temporarily causing increased ICP. This effect is mitigated by giving the mannitol slowly. Many recommend giving furosemide prior to mannitol to help further mitigate initial hypertension when mannitol is given. Hypovolemia must be avoided when administering these medications to reduce the risk of acute tubular necrosis, especially in patients that have poor renal perfusion (e.g., shock), signs of sepsis, receiving nephrotoxic drugs, or with elevated serum osmolality.

Prognosis

With the exception of meningioma, there is little information in the veterinary literature regarding long-term prognosis for specific tumor types in large numbers of patients. For most brain tumors, patients treated with palliative measures have an average lifespan of 3-6 months. Surgery is the treatment of choice for feline meningiomas. The average survival time across multiple studies is 2 years, but surgery can be curative in cats if the entire meningioma can be removed.

Meningioma average survival times in dogs

Surgery alone: approximately 7 months
Radiation alone: approximately 7-8 months
Surgery followed by RT: approximately 18-24 months
Hydroxyurea: approximately 6-12 months