What are Brain Tumors in Pets?


Brain tumors are more common in dogs than any other domestic species.  The incidence is reported to be 14.5 per 100,000 dogs.  Brain tumors occur in any age, breed and sex, but most commonly middle aged and older dogs (>5 years) are affected.  Some breeds that appear to be over-represented include the golden retriever, boxer, Doberman Pinscher, Scottish terrier, and the Old English Sheepdog.  

Tumors originating in the brain or primary brain tumors arise from cells that are normally found within the brain and meninges.  The most common primary brain tumors of dogs are gliomas and meningiomas.  Gliomas and pituitary tumors occur more commonly in brachycephalic breeds such as Pugs and boxers.  Meningiomas occur more commonly in dolichocephalic breeds such as Collies and German Shepherd dogs.  

In cats, brain tumors are much less common, occurring in approximately 3.5 per 100,000 cats. Meningiomas are most common and there does not appear to be a breed predisposition.

Brain tumors cause compression, decreased circulation and subsequent necrosis of the brain tissue surrounding the tumor resulting in neurologic dysfunction.  While primary brain tumors are slow-growing, they have serious effects because the brain is contained within a fixed space.  Therefore edema and hemorrhage that often surround a tumor can lead to increased intracranial pressure.

When neurologic symptoms are suggestive of an intracranial lesion, CT (computed tomography) or MRI (magnetic resonance imaging) is necessary to make a diagnosis of a brain tumor.  A minimum data base (Chemistry, CBC, urinalysis and chest radiographs) should be done prior to advanced diagnostic imaging.  Cerebrospinal fluid (CSF) analysis may be useful to differentiate inflammatory disease from cancer, but in the light of advanced imaging availability, it is rarely useful as a primary diagnostic modality.  CSF is recommended for patients with intracranial signs but a negative CR or MRI.  While a biopsy is the only method available to definitively diagnose the tumor type, it is rarely done unless the patient has a surgical lesion.

It must be kept in mind that CT or MRI characteristics of a tumor alone are not enough to make a definitive diagnosis.  Without the benefit of a biopsy, it can ultimately be difficult to predict the outcome for patients treated with radiation alone.  A CT-guided stereotactic brain biopsy system has been described by Dr. Richard LeCouteur at the University of California at Davis.  Because this procedure is not readily available, most patients with non-surgical lesions are treated without the benefit of histopathology.  

Once a CT or MRI identifies a tumor, a decision regarding treatment can be made.  For those tumors surgically accessible, removal is recommended.  Tumors more peripherally located are usually more amenable to surgical resection.  Meningiomas are typically more accessible surgically due to their meningial origin.    This image is a weighted post contrast MRI of a dog's brain showing a lesion in the cerebellum.  This is most likely a meningioma.

In cats, meningiomas tend to be slower growing than their canine counterparts and are almost always well defined and have a clear demarcation between the rumor and normal brain tissue.  Surgical excision is often curative, and when surgical excision is not complete, the tumors tend to be so slow growing that radiation therapy often not recommended.  Tumors may take 1-2 years to regrow, and can be reoperated on at that time.  In contrast, canine meningiomas tend to be more locally infiltrative and invariably lack demarcation from the normal brain tissue.  For this reason, complete surgical excision can be more difficult and radiation therapy is often used as an adjunct to surgery.  There is disagreement among neurologists and radiation oncologists as to whether canine meningiomas should be treated with surgery plus radiation therapy, or with radiation therapy lone.  Tumors located deep in the brain tissue or in the brainstem tend to be more difficult to approach surgically; therefore radiation therapy is often the more appropriate choice.                                      

Radiation therapy for the treatment of primary brain tumors is a well established treatment modality. CT or MRI based radiation planning is essential.  Improvement in treatment planning related to the use of CT and MRI have resulted in improved local control of tumors and a decrease in radiation-related problems to the CNS.  

Careful treatment planning by a qualified and experienced radiation therapist is essential to the success of radiation therapy.  The selection of a radiation dose is based on considerations of tumor type, tumor location, and on the tolerance of the normal tissues surrounding the tumor.  In general, brain tumor patients treated with radiation therapy tolerate treatment extremely well.  Dosage of 3 Gy per fraction are well tolerated and do not appear to be associated with significant delayed reactions such as brain necrosis that can be seen at higher dosages.  At the Animal Cancer and Imaging Center, we have treated a large number of patients with brain tumors with radiation therapy.  Three patients had suspected pituitary tumors, 1 patient had suspected astrocytoma and the remaining were suspected meningiomas.  The reported outcomes have been favorable for most patients with pituitary macroadenomas and meningioma patients showing minimal neurologic symptoms at presentation.  Three-dimensional CT based radiation therapy is performed on all brain tumor patients at ACIC.

Published studies regarding treatment and outcomes for dogs and cats with brain tumors are limited and highly variable.  Critical evaluation of many of these studies leaves a reader confused.  Different types of radiation equipment and scheduled of radiation therapy used, and limited numbers of patients in studies make interpretation difficult.  In one study of 41 dogs with either primary or secondary brain tumors, all dogs received some form of definitive treatment (such as surgery, megavoltage radiation, chemotherapy, etc).  The factor most associated with survival was the mode of therapy.  Dogs treated with radiation therapy, with or without any other treatment, lived significantly longer.  Reported median survivals for brain tumors treated with radiation vary.  In general, dogs with meningiomas have a median survival time of 1-2 years.  Pituitary tumors in cats have an excellent response rate to radiation therapy.  Long-term survival of greater than 2 years is often seen.