Often, the thought of the presence of brain tumor evokes a dramatic reaction in both the client and clinician.  At times, just the mention of the possibility of a brain or spinal cord tumor is enough for an owner to lose hope in the treatment of their pet.   Emotional, financial, and information constraints are the major factors in an owner decision to not pursue further diagnostic testing and therapy in pets with suspected nervous system tumors.  With the advent of improved diagnostic and therapeutic options, many dogs and cats with brain tumors can have a long–term survival.

The incidence of brain tumors is not well–documented in veterinary medicine as many cases are not definitively diagnosed.   One estimate is that brain tumors occur at an approximate rate of 14.5 per 100,000 dogs and 3.5 per 100,000 cats.  Genetic and environmental influences may alter this number according to the breed of dog and local exposure to potential carcinogens.   For example, brachycephalic dogs (Boxers, bulldogs) have the highest relative risk for intracranial glial cell tumors.  Nervous system neoplasms can be either primary or secondary.  Primary tumors arise from cells within the brain.   Secondary tumors arise from either surrounding structures of the skull or metastasize from distant sites.  Primary tumors are probably the most frequently diagnosed tumors.  There are three major classification schemes for tumors of the CNS:  histological, molecular genetics, and anatomical classifications.  The first and second schemes are the most commonly used in neuro–oncology in people, as therapy and prognosis are often related to tumor type.  Benign tumors are slow growing and usually non-invasive to brain tissue, while malignant brain tumors are invasive and rapidly growing.

Pets with brain tumors can have either acute or more insidious onset of neurologic signs.  In both situations, the disease process is progressive.  The degree of neurologic impairment is dependent upon the rate of tumor growth, interference with brain blood flow, presence of brain swelling, and location.  Tumors that are rapidly growing or that hemorrhage can induce acute, severe neurologic dysfunction.   Brainstem involvement results in a limited amount of room for tumor expansion before critical functional areas are affected.  In contrast, mass lesions of the cerebral or cerebellar hemispheres that are slower growing allow the brain to compensate, and thus, the onset of neurologic signs may be prolonged.   Specific historical problems are listed separately for each section of the nervous system presented.

The majority of brain tumors are diagnosed in older animals.  Typically, dogs over 7 years of age and cats over 10 years of age are at higher risk for developing brain tumors.  Animals can, however, be affected at any age.  Meningiomas are benign tumors and the most common type found in both cats and dogs.  There are four major areas of testing that are useful for the diagnosis of nervous system neoplasms.  Going from the least to the most specific testing modalities, the groups are:  1) Electrophysiology; 2) Cerebrospinal fluid (CSF) analysis; 3) Imaging; and 4) Tissue biopsy.  MRI scanning is the definitive diagnostic test of choice to provide information on tumor location and often tumor type.

The goals of therapy in affected patients are to reduce the size of the tumor, reduce the secondary mass effects, and ultimately, to improve neurologic function.  Complete elimination of the tumor is not necessary for improvement of the clinical condition of the animal over the short–term.  Long–term survival, however, does dictate significant tumor reduction.  Four major treatment modalities are used to accomplish these goals:  medical, surgical, radiation, and chemotherapy. 

Surgical management of brain tumors offers the advantages of debunking the mass, obtaining a histologic diagnosis, and the potential of complete excision.   Brain tumors most amenable to surgical debulking are those with surface contact. In particular, meningiomas in the dog and cat are very accessible with minimal disruption of neurologic function.  In cats, complete removal is often possible.  Craniectomy is advised in patients with meningiomas, as tumor infiltration into the bone can occur.  Radiation therapy is a proven treatment modality in dogs and cats with superficial, deep-seated, and infiltrative brain neoplasms.  Specific tumor–directed chemotherapy is becoming more available for our patients.

Michael Podell MSc, DVM

 

Diplomate ACVIM (Neurology)

 

BRAIN TUMORS

 

MRI scan of frontal lobe meningioma in a dog that was successfully surgically removed