Though relatively uncommon, a brain tumor has always made a grim prognosis for any unfortunate animal that is diagnosed with one. Traditionally they were often assumed but seldom confirmed, but since MRI and CT scanning has become more mainstream they can be diagnosed correctly. Here we discuss the different types of brain tumor that affect dogs and cats, the clinical investigations that can be performed, the treatments available and the likely outcomes.
Brain tumors seem to be more common in dogs than cats, and certain breeds are over represented such as Boxers, Golden Retrievers, Dobermans, Scottish Terriers and Old English Sheepdogs.
Primary vs Secondary
Brain tumors can be primary or secondary (metastasis from other sites). Primary brain tumors are usually solitary, the most common ones in the dog being gliomas and meningiomas. In cats, the most common type are meningiomas and these can occur at multiple locations.
Secondary tumors in dogs include extension of a nasal tumor, metastases from breast, lung or prostate cancer, hemangiosarcoma or extension of a pituitary gland tumor. Nerve sheath tumors and skull tumors have also been reported.
Secondary tumors in cats include pituitary gland tumors, metastatic carcinomas, local extension of nasal tumors, skull tumors and middle ear cavity tumors.
What causes a brain tumor?
The cause of brain tumors is not known. Diet, environment, chemical, genetic, viral, immunologic and trauma have all been considered. In cats with meningiomas, because they often occur in very young animals, a genetic element is suspected.
Benign vs Malignant
The terms benign and malignant must be used with care when referring to brain tumors. Normally these terms apply to various characteristics on a cellular level, but on a biological level, even benign brain tumors can kill the animal due to the secondary effects like increased intracranial pressure or cerebral edema. In short, any brain tumor can kill.
What are the symptoms?
There can be huge variety here. Many animals will present with vague signs, such as one or several of the following:
1. Loss of trained habits
2. Decreased levels of activity
3. Decreased frequency of purring in cats
More specific symptoms are dependent upon where exactly the tumor is located within the brain, the size of the tumor and how quickly it is growing. As a tumor enlarges, symptoms tend to become more severe. These can include:
6. Seizures (often indicate a tumor in the cerebral cortex)
7. Facial paralysis (may indicate a brainstem tumor)
8. Tremors (may indicate a tumor in the cerebellum)
9. Wobbliness (may indicate a tumor in the cerebellum)
10. Full or partial blindness (may indicate tumor in hypothalamus or optic nerve)
11. Loss of smell (may indicate tumor of olfactory system)
The physical presence of the tumor can cause knock on effects due to inflammation and edema of the surrounding area. This can cause symptoms such as:
12. Changes in behaviour or temperament (irritability, lethargy)
13. Compulsive walking
15. Pressing head against a wall or hard surface
Animals can sometimes carry brain tumors for several years before presenting to a veterinary clinic, if the tumor is slow growing. In these cases the symptoms develop gradually, and the owner tends to get used to them so that by the time the animal is examined, the tumor has reached a considerable size.
How is a brain tumor diagnosed?
History and Clinical Examination
The first step for a veterinarian is to take a thorough history of all of the clinical signs, and when they developed. This is followed by a full general clinical examination and a full neurological examination.
After that, blood should be taken for routine haematology and biochemistry profiles. This is to look for any disease outside the brain. Results will be normal for brain tumors, with the possible exception of some pituitary gland tumors.
Plain skull radiographs (xrays) under general anesthetic have little value in detecting a brain tumor, but they can be useful if there is a tumor in the nasal cavities or the middle ear which could extend into the skull. On rare occasions, they can identify bony changes in the skull which can accompany a brain tumor, or mineralization within the tumor itself. Radiographs and ultrasound of the chest and abdomen are useful to look for a tumor elsewhere in the body, in cases where the brain tumor is a secondary metastasis.
MRI and CT Scans
Confirmation of a brain tumor can is usually only achieved using the advanced imaging techniques, CT scans or MRI. Both of these have pros and cons when compared to one another. CT is better for bony changes, while MRI is better for soft tissue definition, for the detection of many of the knock on effects of brain tumors such as edema, cysts and bleeding. MRI is the preferred option for diagnosing primary brain tumors.
This is the only way to definitively diagnose a brain tumor. The advanced imaging techniques above offer much information, but they can occasionally confuse a tumor with a non cancerous mass or a cyst, and they also do not tell us the exact type of tumor present, and therefore the appropriate treatment and prognosis. The best type of biopsy is the CT guided stereotactic brain biopsy system, which is rapid, accurate and quite safe.
Since exploratory surgery is high risk, it is not usually attempted unless there is a reasonable chance of removing the whole tumor with minimal collateral damage. Many brain tumors in cats and dogs are not categorized on a cellular level until post mortem.
Cerebrospinal Fluid (CSF) Analysis
CSF analysis is useful for ruling out inflammatory causes of the symptoms, but tumor cells are rarely identified here. Increased levels of white blood cells and increased protein levels may be present in the CSF with many brain tumors, though this is not diagnostic. This test can be high risk when intracranial pressure is increased, as brain herniation can occur.
Treatment is aimed at being either curative or palliative. Curative treatment eradicates the tumor or reduces its size, whilst palliative therapy reduces the surrounding cerebral edema and slows down the growth of the tumor. Palliative therapy also involves administering antiepileptic drugs, if seizures are occurring as a result of the tumor.
Whether this is an option depends on the general health of the animal, and the precise location, size, extent, invasiveness and nature of the tumor. Tumors such as meningiomas in cats can be removed successfully by surgery. However, surgery to remove tumors in certain locations such as the brainstem can be extremely dangerous, possibly resulting in death. Even partial removal can benefit the animal though, particularly if the tumor is slow growing.
This is probably the most widely used form of treatment for brain tumors. Radiation therapy can be used alone or in combination with other treatments. It is also useful in the treatment of secondary brain tumors. The aim is to destroy the tumor without harming the normal tissue too much.
The main problem with chemotherapy for brain tumors is that many drugs do not cross the blood brain barrier. In addition, the tumor may only be sensitive to high doses, doses which are toxic to normal brain tissue and therefore unsuitable for use. However, several drugs have been used for this purpose that can cross the blood brain barrier with reported success, including cytosine arabinoside, lomustine and carmustine.
Studies of animals that receive palliative treatment (corticosteroids) for brain tumors show a survival range post diagnosis of 64 to 307 days. This demonstrates the inability to accurately predict life expectancy in these cases. What is certain is that the survival times significantly increase with surgery, radiotherapy or chemotherapy. Radiation therapy seems to offer the best results, alone or in combination with other treatments. Generally, the more severe the symptoms, the shorter the life expectancy.