Neurology: Brain Tumors in Dogs and Cats
Cancer affecting the brain is not uncommon in older dogs and cats, although the need for advanced imaging of the brain (such as magnetic resonance imaging, or MRI) in order to detect a brain tumor means that they frequently go undiagnosed.
There is also a concerning trend for brain tumors to arise in young dogs of certain breeds, such as the Boxer or the Boston terrier. Developing a brain tumor has serious implications and many owners feel helpless when such a diagnosis is made for their pet.
However, these tumors vary widely in their level of malignancy and some can be treated effectively. Unfortunately, there is still a lot that we do not yet know about how different types of brain tumor behave in dogs and cats, and this can make it difficult to advise owners as to the best form of treatment for their pet. This web page provides information on what we do know about the diagnosis, treatment and prognosis of different types of brain tumor and describes ongoing research efforts at North Carolina State University College of Veterinary Medicine.
What is a brain tumor?
Strictly speaking, the term brain tumor simply means a mass in the brain. However, it is commonly used to describe a cancerous (or neoplastic) mass inside the cranial cavity (figure 1). Brain tumors may be primary, arising from the cells of the brain and its lining (figure 2), or secondary, arising elsewhere and spreading to the brain.
Primary brain tumors seen in dogs and cats include meningioma, glioma, choroid plexus papilloma, pituitary adenoma or adenocarcinoma, and others.
Meningioma – This is the most common primary brain tumor in dogs and cats (and in humans). It arises from the arachnoid mater of the meninges (the membranes that line the brain) 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. Figure 3 is a MRI of a meningioma. These tumors occur more commonly in long nosed (doliochocephalic) breeds of dog, such as the Golden retriever. Meningiomas are usually relatively slow growing and amenable to treatment, although more malignant forms do occur.
Secondary brain tumors – Secondary tumors represent spread (metastasis) of another tumor to the brain from elsewhere in the body. Examples of tumors that may spread to the brain include hemangiosarcoma, mammary carcinoma and melanoma. These tumors carry a very poor prognosis because they have already spread through the body. It is routine practice to take radiographs of the thorax and even to ultrasound the abdomen to check there is no evidence of cancer elsewhere in the body whenever a diagnosis of a brain tumor has been made or is suspected.
What signs do brain tumors cause?
Brain tumors cause signs by compressing or invading the brain. The resulting signs relate directly to the area of the brain affected and are not specific to a tumor: any disease affecting that area of the brain could produce similar signs. As a general rule, brain tumors cause progressive signs in older animals. Signs may start very suddenly or quite insidiously, and they can wax and wane in severity.
The Forebrain: The forebrain is responsible for “thinking,” behavior, and final integration of sensory information. Tumors in the forebrain may therefore cause:
- Behavioral abnormalities such as loss of learned behavior and depression
- Increased or decreased appetite and thirst
- Constant pacing or circling
- Decreased awareness and vision on one side of the body, causing misjudgement of openings to doorways and bumping of one side of the body
- Sometimes affected animals behave as if they are in pain
- Seizures are a classic sign of forebrain disease and in fact, new onset of seizures is the most common presenting sign of animals with tumors of the forebrain. These seizures can occur alongside any of the other signs listed above, or may occur as the only abnormality. Whenever a dog or cat that is greater than five or six years of age has a new onset of seizures, a brain tumor is a possible differential and ruled out by a full diagnostic workup.
The Brainstem: The brainstem plays various vital roles including regulation of motor function (the ability to walk), the level of wakefulness, and the respiratory and cardiovascular systems. The sense of balance originates in the brain stem. The brain stem is also the source of the nerves that control movement of and sensation to the face, the eyes, the throat, larynx and tongue, and the muscles of mastication (figure 7). Tumors in the brainstem can be rapidly fatal if, for example, they affect the control of breathing, but typically the first signs of brainstem disease are a loss of balance (vestibular signs), and weakness on one side of the body. There can be a wide variety of additional signs such as difficulty swallowing, change in voice and inability to move the eyes. Further progression of the signs can result in paralysis, coma and death.
Vestibular signs include:
- Head tilt
- Leaning and falling to the side of the head tilt
- Drunken gait with loss of balance (ataxia)
- Circling to the side of the head tilt
- Involuntary flicking of the eyes (nystagmus)
- Loss of appetite and vomiting
- Abnormal eye position (strabismus)
The Cerebellum – The cerebellum controls coordination of movements and interacts closely with the vestibular system to control balance and posture. Signs of cerebellar disease include:
- Uncoordinated gait characterized by dramatic goosestepping (hypermetria)
- Head tremors that are worst when the animal is intent on something (i.e., food) but disappear when the animal is relaxed (intention tremors)
- Swaying of the trunk
- Wide based stance
- Sometimes there can be vestibular signs such as a head tilt
- The animal’s strength remains normal
How is a brain tumor diagnosed?
A brain tumor should be suspected whenever there is new onset of neurological signs in an animal older than 5 years. It is important to understand, that with rare exceptions, brain tumors are tumors of the soft tissues of the brain and they cannot be seen on radiographs of the skull. The brain can be imaged using magnetic resonance imaging (MRI) or computed tomographic (CT) scans. The following diagnostic steps are recommended:
- Complete physical and neurological examination to identify any other health problems and to localize the neurological signs to a particular area of the brain
- Routine blood work to rule out a systemic problem and assess the anesthetic risk
- Thoracic radiographs to check there is no evidence of spread (metastasis) of cancer to the lungs (a common site of metastasis)
- CT or MRI of the brain. This has to be done under general anesthesia. As a general rule, MRI shows the brain in more detail than CT and is the test of choice when assessing for brain tumors. However, it is a more expensive test and less widely available. CT images will identify most meningiomas and choroid plexus papillomas but can fail to identify gliomas. CT images also have a lot of artifact when trying to assess the brainstem and cerebellum. We therefore strongly recommend an MRI if the animal has signs of brainstem or cerebellar disease, or if it is a breed of dog that is predisposed to gliomas, such as the Boston Terrier.
- Tumor type can be suspected from the appearance of the mass on CT or MRI, but can only be definitively identified by taking a sample of the tumor, either at surgery or by biopsy. Indeed, masses caused by infections (for example abscesses or fungal granulomas) can look like brain tumors on brain images (figure 8). It is therefore vital that a sample of the tumor is taken and examined with a microscope to identify the cell types involved. Not only will this identify the tumor type, but it will also grade the malignancy of the tumor. Many neurologists, particularly those working in university teaching hospitals, routinely perform CT guided biopsies of tumors.
How are brain tumors treated and what is their prognosis?
The options for treating brain tumors include surgical removal, radiation therapy, chemotherapy, and palliative treatment of the symptoms. Unfortunately, we have relatively little data to present on the outcome of tumors because it is common for owners to decide not to treat their pet, or because pets are treated with radiation therapy or chemotherapy without ever determining the tumor type. This mentality is changing as CT guided biopsies become more routinely offered, and one of our research aims is to generate a large data base of how different tumors respond to different types of treatment, and what happens if they are not treated at all (see research).
Brain tumors present unique problems related to their location and the tissue that they affect. Firstly they arise in the restricted space of the cranial cavity (Figure 1). The cranial cavity is formed by the bones of the skull and encloses and protects the brain. While a delicate structure like the brain must be protected in this way, it means there is no room for anything else within the cavity and when a tumor grows, it compresses the surrounding brain. Secondly, brain tissue cannot regenerate, and therefore removal of normal brain tissue surrounding a tumor can have unacceptable repercussions for the patient.