A "tumor" has several definitions. One is simply any swelling, which is one of the hallmark signs of inflammation. Another definition of a tumor is a new growth of tissue in which cell multiplication is uncontrolled and progressive. This is also called cancer, or neoplasia. Cancer and neoplasia are interchangeable. When a tumor is cancerous or neoplastic, it is composed of new, rapidly growing cells whose growth continues even after the normal stimuli for cell growth is no longer present and serves no useful purpose for the animal.
Tumors are classified medically in several ways. The simplest classification is by the tissue of origin and whether the tumor is metastatic or benign. Tumors of the bone, fat, blood vessels and lymph tissue are typically called "sarcomas," and they may be benign or metastatic. Tumors of the glandular tissue and organs of the mammary glands, stomach, uterus or skin are typically called "carcinomas," which also may be benign or metastatic. "Metastatic" means that the tumor cells can transfer from one part of the body to a distant part and at the same time carry with them their ability to reproduce abnormally – in other words, they can transfer disease to areas of the body that they are not in direct contact with. "Benign" means not malignant – in other words, benign tumors do not spread disease to distant parts of the body.
Most of us think we know what the "brain" is, and in the most general sense we do. However, a bit more specific medical information will be useful to gain an understanding of brain tumors. The brain is defined as that part of the central nervous system contained inside the skull (the cranium). It is made up of the forebrain, the midbrain and the hindbrain. It is connected at its base to the spinal cord, which as its name implies runs through the bony spinal column. In the simplest of terms, the brain is a mass of soft, fleshy, pinkish and greyish nerve tissue. In the more accurate of modern terms, the brain is the most highly specialized computer that has ever existed.
Brain tumors, then, are either benign or malignant masses contained within brain tissue. They can be either primary or secondary. Primary brain tumors are those that originate from cells that are normally found within brain tissue. These include meningiomas, gliomas, choroid plexus tumors, ependymomas, medulloblastomas, olfactory neuroblastomas and primitive neuroectodermal tumors, among others. The specific details of each of these types of primary brain tumors are beyond the scope of this article.
Secondary brain tumors are either malignant neoplasms that started in tissue somewhere else in the body and ended up in the brain, usually through the blood circulatory system, such as lymphosarcoma and hemangiosarcoma. Secondary brain tumors can also be the result of local invasion of cancerous cells into the brain from areas that are adjacent to the brain, such as from primary pituitary gland tumors or from bone tumors in the skull or nasal cavity. What most people do not understand is that when their dog is diagnosed with a "brain tumor," it does not necessarily mean that the tumor is of brain tissue origin. For example, breast cancer (neoplasia of the mammary glands) can metastasize to the brain. That is not "brain cancer." It is "breast cancer" that has spread into the brain.
Causes of Brain Tumors
Since there are a number of different types of primary and secondary tumors that can affect the brain, there is no responsible or accurate way to describe the causes of brain tumors in general terms. In most cases, the causes of cancer remain uncertain. Heredity is thought to play a role in many tumors of the brain and other organs or tissues. Other things that have been suggested as causing or contributing to brain tumors in dogs include dietary, environmental, viral, bacterial, parasitic, chemical, immunologic and traumatic factors. Most brain tumors in dogs are solitary masses, although this is not always the case.
While the underlying reasons for brain tumors are poorly understood, what actually causes the observable effects of those tumors has been fairly well identified. Once a tumor develops in the brain, either from primary brain tissue or as a result of metastasis from a distant location, the normal brain tissue will be physically compromised, compressed and/or displaced. As a result, brain tissue in the surrounding areas may begin to die (this is called necrosis). The pressure of the blood and cerebral spinal fluid within the brain and skull can rise to extremely dangerous levels, to the point of causing brain hemorrhage (bleeding), herniation (bulging out at abnormal places, such as at the base of the skull) and/or excessive accumulation of fluid on, in or around the brain (hydrocephalus; cerebral edema). Obviously, when these conditions occur in the brain, the consequences can be devastating, irreversible and eventually fatal. At a minimum, masses in the brain can cause changes in a dog's behavior, attitude and mentation.
Prevention of Brain Tumors
There is no reported way to prevent brain tumors in domestic dogs. Cancer of any type, in any species, remains one of the most researched of all medical disorders.
The brains of domestic dogs are a more common site of primary tumors (tumors of brain tissue) than are the spinal cord or peripheral nerves, which are also parts of the nervous system. Tumors of the middle or inner ear are much less likely to metastasize to or invade brain tissue than are tumors of the nasal cavities.
How Brain Tumors Affect Dogs
It is hard to say, generally or specifically, how brain tumors affect any given animal. The symptoms that they experience will depend upon the location of the mass, its size and its aggressiveness. Certainly, depending upon those things, dogs may experience any range of effects, from none to extreme pain and distress. Unfortunately, the outcome for a dog with either a benign or a malignant brain tumor is not all that different in most cases... The effects of the tumor are caused by the space-occupying mass, no matter where it comes from.
Symptoms of Brain Tumors
Brain tumors, depending upon their location, size, aggressiveness and type, can cause any number of clinical signs in dogs and in other mammals. These can come on slowly (be insidious) or come on suddenly (acutely). They may include:
Seizures (seizures are the most commonly recognized sign of brain tumors in dogs and cats, especially after they reach 5 years of age)
Blindness or vision deficits
Abnormal ocular (eye) reflexes
Abnormal behavior and mentation, especially unusual aggression
Lack of coordination (ataxia)
Lack of appetite (inappetence; anorexia)
Nose bleeds (epistaxis)
Difficulty breathing (dyspnea)
Panting (open mouth breathing)
Dogs at Increased Risk
Dogs seem to have a higher incidence of brain tumors than do most other domestic species. The reason for this association is not well understood. Brachycephalic breeds (those with broad top skulls, short-to-nonexistent muzzles and flat faces) are predisposed to developing glial cell tumors and pituitary tumors, which are two specific types of brain tumors. Scottish Terriers, Old English Sheepdogs, Boston Terriers, English Bulldogs, Golden Retrievers, Boxers and Doberman Pinschers seem to be overrepresented among the population of domestic dogs that develop brain tumors. While tumors in the brain can occur in dogs of any age, older dogs are more likely to present with brain tumors – typically dogs over 5 years of age.
How Brain Tumors Are Diagnosed
Any dog presenting with seizures, sudden vision problems or other signs of abnormal mentation or behavior will be given a thorough physical examination and a complete neurological work-up. Of course, a complete history of the dog's symptoms and progression of the behavioral abnormalities will be taken from the owner. Most general veterinary practitioners can skillfully perform a basic neurological examination. In many cases, how the dog presents to the veterinarian, together with the history and results of the physical and neurological examinations, will enable the veterinarian to localizer the problem to the brain, and maybe even to a particular location within the brain. A urinalysis and routine blood work, including a complete blood count and serum biochemistry profile, can be extremely useful to rule out causes of the dog's symptoms that may not be neurological in nature. Survey radiographs (X-rays) and ultrasound of the chest and abdomen may help to identify any primary masses in those areas that may have metastasized to the brain. Skull X-rays are usually not very helpful in identifying primary brain tumors, but they may disclose masses in adjacent areas, such as the nasal cavities or nearby bone that possibly metastasized into brain tissue.
More advanced diagnostics include computed tomography (CT scan) and magnetic resonance imaging (MRI). These procedures are only offered at certain specialized veterinary facilities and at veterinary teaching hospitals. They can provide extremely accurate identification of the presence, location and size of masses in the brain and elsewhere. The attending veterinarian can also take a tap (sample) of the dog's cerebrospinal fluid (CSF), which is the fluid that coats the brain, spinal cord and associated tissues. The results of laboratory analysis of CSF can identify inflammation, infection and sometimes the presence of tumor cells. The CSF tap must be done very carefully – usually only after advanced radiographic imaging such as a CT scan and/or MRI is performed – to avoid any adverse consequences associated with rapid changes in the pressure inside the brain (intracranial pressure) that may result from withdrawing a sample of cerebral spinal fluid.
The most (and perhaps only) definitive way to diagnose the type of tumor in the brain is by doing a biopsy. This is not commonly done, due to the cost and invasiveness of the procedure. However, it is considered to be the gold standard and probably should be done or at least considered before any therapy is considered.
Treatment of Brain Tumors
Obviously, treatment of any brain tumor must be specifically tailored to the type of tumor, whether it is primary or secondary, its location, and the extent of its invasion into surrounding brain tissue. The primary goals of treating brain tumors are to manage the potential secondary side effects of increased pressure inside the brain/skull and to try to reduce or eliminate the tumor entirely. Three general therapeutic approaches are currently available to treat brain tumors in domestic dogs. These are surgery, chemotherapy and radiation.
Surgical removal of a brain tumor is no simple task. However, it probably is one of the most important initial considerations for owners and veterinarians to consider. Surgery can be used for a biopsy, for partial removal of the mass or, hopefully, for complete resection (removal) of the cancerous tissue. If the tumor in the brain is from metastasis from another location, a surgical "cure" is probably not possible by removing the mass in the brain. However, surgical excision can be used to debulk the tumor and obtain tissue for a laboratory definitive diagnosis of the tumor type.
Radiation therapy can be used alone or in combination with surgery and/or chemotherapy for both primary and secondary brain tumors. This is a very sophisticated procedure that requires a skilled veterinary oncologist (cancer specialist) and specialized veterinary technicians and facilities.
Chemotherapy basically is treatment with high-powered drugs designed to kill rapidly growing cells. Chemotherapy targets not only cancer cells, but also cells of the body that normally are rapidly growing or reproducing, such as cells of the intestinal lining and hair follicles. This is what accounts for many of the unpleasant side effects of chemotherapy, including nausea and hair loss, in both dogs and in people. Nonetheless, chemotherapy is an important part of cancer therapy considerations for owner of affected dogs. Unfortunately, most chemotherapeutic drugs do not cross what is called the "drug-brain barrier", which means that the drugs may not enter the brain and/or the spinal cord and therefore may not have their intended effect. However, there are some chemotherapeutic agents that can help to reduce brain tumors.
Chronic treatment for dogs with brain tumors usually involves treatment for seizures. Drugs such as phenobarbital and other anticonvulsant medications may be prescribed by the attending veterinarian. Certain oral or injectable medications that are not chemotherapeutic agents may also be used in conjunction with one or more of the above treatment protocols to support a dog in treatment for a brain tumor. These may include steroids to reduce inflammation and drugs to help control seizures.
There is no realistic way to generalize about what the prognosis may be for a dog that has one or more brain tumors. The outlook will depend upon the type of tumor, its size and location, and whether it is primary or secondary in nature. However, the prognosis is probably best if the mass in the brain can be removed or reduced, either through surgery, radiation and/or chemotherapy.