Canine Brain Tumors
Brain tumors are uncommonly diagnosed in dogs. The cause for brain tumors is unknown. Although a broad spectrum of tumor types has been reported in dogs, the two most commonly diagnosed primary tumors are gliomas, which tend to occur more commonly in short-nosed breeds, and meningiomas, which tend to occur in medium nosed breeds. Brain tumors can also represent spread from other sites (i.e. metastatic hemangiosarcoma or melanoma) or local extension into the brain in the case of nasal tumors, pituitary adenomas, and nerve sheath tumors. Although many primary tumors are histologically benign when examined under the microscope, the location confers a more malignant biologic behavior. Since the brain is contained within a fixed space, the presence of a tumor exerts secondary effects such as increased intracranial pressure, cerebral edema, or brain herniation, which are more clinically devastating than the tumor itself.
The most common clinical signs that dogs will present with are seizures, changes in behavior/attitude, circling, difficulty walking or even blindness and other deficits in the sensory organs. A neurologic examination will help localize the lesion to the central nervous system and should be followed with bloodwork to rule out other causes of seizures and altered mentation. X-rays of the skull are usually of little yield unless an extensive amount of bony changes have occurred. Computed tomography (CT) and MRI are the primary imaging modalities used to diagnose brain lesions. MRI is excellent to look at soft tissue changes while CT is better for examining bony lesions. Even though the major tumor types have been reported to have a “classic” appearance on advanced imaging techniques, a biopsy remains the sole means for making a definitive diagnosis. Once a mass is noted additional diagnostics that can be useful (depending on the given situation) are chest x-rays and an abdominal ultrasound to rule out spread to the brain from other sites (lungs, prostate) or a CSF tap in which a sample of fluid is removed from around the spinal cord and examined for the total amount of protein and white blood cells present. Some studies have shown that a normal white blood cell count and increased protein content in the CSF is more consistent with cancer but the results must be interpreted with caution, as this is not a 100% sensitive tool.
The primary goal of therapy for brain tumors is to improve or even eliminate when possible the adverse secondary effects and resulting clinical signs and include a combination of medical management, surgery, radiation therapy, and chemotherapy.
Medical management is often required to palliate clinical signs prior to diagnosis and definitive therapy. Steroids (prednisone) are often used to decrease surrounding tumoral inflammation and edema. Anti-convulsants such as Phenobarbital and potassium bromide are required in some dogs to control seizures. Depending on the individual patient, these drugs may be discontinued following definitive treatment.
Surgery has the advantage of removing the structural mass which immediately relieves intracranial pressure in addition to providing a tissue sample for a definitive diagnosis—however, surgery is not without risk and post-surgical complications include infection, edema, and hemorrhage. The location, size, and invasiveness of the tumor will determine the possibility for both surgical removal and completeness of surgical margins.
Radiation therapy (RT) can be used alone or in combination with other treatment modalities such as surgery and is well established for the treatment of intracranial neoplasms. The goal of RT is to destroy the tumor while preserving surrounding tissues which is optimized by the use of a radiation plan based on the results of the previously performed CT or MRI. Although numerous protocols have been reported in the literature to treat brain tumors, most protocols involve the use of small doses of radiation delivered daily for several weeks. This protocol appears to improve clinical signs with minimal late-term effects on normal tissues.
Chemotherapy may be employed depending on the tumor type. The type of blood vessels in the brain act as a “blood-brain barrier” and prevent entrance of toxic or foreign substances, including most chemotherapy drugs, into the CSF. Hydroxyurea, CCNU, and Cytosar are chemotherapeutic agents that can penetrate the blood-brain barrier and have been shown to improve clinical signs and reduce tumor size in some instances. The benefit of chemotherapy for dogs with incompletely excised brain tumors is currently unknown.
Overall prognosis is highly dependent on the therapy chosen. For dogs who receive only palliative medication (steroids and anti-convulsants) the median survival time is around 40-60 days, and the results from several studies have shown a significant improvement in survival time when surgery, radiation, and chemotherapy are used alone or in combination. Surgical excision alone improves the survival time to 4-6 months especially for those dogs whose tumor is considered to be completely removed with surgery.
Several reports in the veterinary literature have proven the efficacy of radiation therapy for canine and feline brain tumors. In dogs, the overall median survival time of dogs with brain tumors with or without prior surgery was approximately 18 or 12 months, respectively. Dogs who presented with severe neurologic signs did worse than those presenting with mild neurologic signs (6 months vs 21 months). Other important prognostic factors were:
Type of tumor (meningioma = better)
Size of tumor (<2cm3 = better)
Radiation as part of the protocol (better)
Primary vs Secondary tumor (primary = better)