Diagnosis

Suspicion of a primary brain tumor is warranted in any elderly dog or cat with slowly progressive neurologic dysfunction. Primary brain tumor should be included as a differential diagnosis for any animal over the age of 5 years that has recent onset of seizure activity, particularly in certain breeds (e.g., Golden Retrievers). A normal interictal period does not rule out a primary brain tumor, as the neurologic signs are determined by the location of the tumor within the neuraxis, as well as by tumor size.

As with any form of neoplasia, a definitive diagnosis of brain tumor cannot be made without histopathology. Several veterinary referral centers now have stereotactic computed tomography (CT)-guided biopsy available in an effort to obtain a definitive diagnosis at the time of imaging without the need for major intracranial surgery.²⁸ A tentative diagnosis can be made with advanced imaging modalities, including magnetic resonance imaging (MRI) and CT. MRI is currently the preferred imaging modality for primary brain tumors. Blood work (complete blood count and chemistry panel), in addition to a urinalysis, three-view thoracic radiographs, and an abdominal ultrasound, should be performed before advanced imaging, in an effort to rule out metastatic disease and/or concurrent unrelated neoplasia.

Certain characteristic features appreciated on MRI and CT can help to distinguish tumor types.30,31,39 Since meningioma arises from the dura and results in extra-axial compression, they are typically characterized by a broad-based, extra-axial mass effect. Additionally, meningioma typically demonstrates distinct tumor margins and uniform contrast enhancement (Figure 36-1).^12,30,31,39 The normal brain parenchyma is usually displaced by the tumor. Certain meningioma subtypes calcify and thus can be detected on noncontrast CT images.³⁰ Another characteristic feature of meningiomas appreciated on MRI is the “dural tail” sign.²⁵ This contrast-enhancing meningeal “tail” extends from the primary tumor.²⁵ Occasionally, meningiomas may have a cystic component extending from or contained within the primary tumor mass.⁴

In contrast to meningiomas, gliomas tend to arise from within the brain parenchyma (intra-axial) and grow toward the periphery of the brain. They often are highly infiltrative and invade the normal brain parenchyma, leading to a lack of distinct tumor margins and poor, nonuniform contrast enhancement (Figure 36-2).* Choroid plexus tumors and ependymomas tend to be intraventricular in location and often uniformly demonstrate contrast enhancement (Figure 36-3).^12,30,31,39 “Ring enhancement” is characterized by peripheral contrast enhancement of a lesion which surrounds a nonenhancing central area of the lesion.⁵⁹ This nonspecific imaging finding is associated with both neoplastic (gliomas) and nonneoplastic brain diseases.⁵⁹ It is essential to recognize that imaging features are not pathognomonic for specific tumor types. For example, falcine meningiomas (originating from the falx cerebri) and those arising from the choroid plexus can appear to be intra-axial on MRI. Similarly, peripherally located gliomas can be mistaken for meningiomas. In a recent study, the accuracy of predicting primary tumor type in 20 dogs on the basis of MRI was 65%.⁴⁷

Cerebrospinal fluid is often abnormal in patients with brain tumors; however, white blood cell counts and protein levels are variable and nonspecific for neoplasia.⁶ Because of this fact, the authors often do not pursue cerebrospinal fluid analysis if CT or MRI strongly suggests a brain neoplasm. Despite the fact that the risk of obtaining a cerebrospinal fluid sample under elevated intracranial pressure in a patient with a brain tumor is not high, the nonspecific result often does not alter the therapeutic plan or prognosis and is not worth the small risk to the patient. It is essential that imaging be performed to rule out concurrent cerebellar herniation before a cerebrospinal fluid tap is performed.

Treatment of Primary Brain Tumors

Two main treatment categories exist for the management of primary brain tumors: palliative and definitive. The goals of palliative therapy are to reduce the secondary effects of the tumor, such as decreased peritumoral edema, and to treat seizures if present.6,14,40 Most palliative therapy consists of an antiinflammatory dose of oral prednisone (0.25 to 0.5 mg/kg q12h) that can be adjusted according to patient response. Prednisone therapy decreases intracranial pressure by relieving tumor-associated brain edema and by decreasing cerebrospinal fluid production.¹⁴ If the tumor results in seizure activity, anticonvulsant drugs are also recommended. Administration of standard phenobarbital doses in dogs with rostral forebrain tumors can lead to profound sedation.¹⁴ As an alternative, drugs without appreciable side effects, such as zonisamide (5 mg/kg PO q12h if patient is not concurrently on phenobarbital, or 10 mg/kg PO q12h if patient is on phenobarbital) and/or levetiracetam (20 mg/kg PO, IV q8h), are used for dogs with brain tumors, unless cost prohibits usage.¹⁴

Definitive therapy consisting of surgical removal/debulking, chemotherapy, and megavoltage radiation therapy is aimed at diminishing tumor volume or eliminating the tumor.⁴⁰ Several features such as tumor size, degree of invasiveness, and tumor location can help the clinician determine whether surgical resection is an option.^{5,14,15,20,42} As expected, intra-axial tumors are significantly more difficult to remove than extra-axial tumors. Benefits of surgery include removal of cancerous tissue, an immediate decompressive effect, and a definitive diagnosis via histopathology.^{5,14,15,20,42} A definitive diagnosis provides prognostic information and, in the case of subtotal resection, allows further treatment planning.²⁰ Various surgical approaches have been described, including rostrotentorial, caudotentorial, transfrontal, and suboccipital craniectomy.^{5,15,20,24,42} The surgical approach can consist of one of the aforementioned techniques, or it can combine several different approaches with the goal of removing the entire mass. The approach is governed by tumor size, location, and degree of invasiveness.

Most brain tumors in dogs and cats are removed via standard rostrotentorial craniectomy (Figure 36-4). Suboccipital craniectomy is discussed in Chapter 37. For a rostrotentorial craniectomy, the authors prefer a midline incision on the head from approximately the level of the lateral canthi to several centimeters caudal to the external occipital protuberance. The cutaneous colli muscles are sharply incised and the cervicoscutilaris muscle is transected near midline, leaving about a centimeter for reattachment during closing. The temporalis fascia is sharply incised with a #11 blade, leaving at least several millimeters attached to the external sagittal crest for reattachment purposes. The temporalis musculature is reflected ventrally by elevation off the side of the skull with a Freer elevator (cats and small dogs) or a more sturdy instrument such as a Sayre, AO style, or Langenbeck periosteal elevator (large dogs). An oval outline of the intended defect in the calvaria is created with a high-speed air drill. The edges of this outline are deepened until a thin layer of periosteum is left. The ventral aspect of the defect does not have to be drilled full thickness. Once the bone along the dorsal, rostral, and caudal aspects of the craniectomy defect has been completely drilled to the inner periosteum, a periosteal elevator, positioned in the dorsal margin of the defect, is used to lever off the bone flap. The craniectomy defect can be further enlarged with the use of rongeurs once the bone flap has been removed. After removal of the bone flap, any bleeding meningeal vessels are cauterized using bipolar cautery.