Brain tumors in cats: Is MRI useful?

A considerable amount of information in the literature describes the occurrence of tumors in the central nervous system (CNS) in different animal species. Intracranial neoplasia is recognized less frequently in cats than in dogs. CNS tumors can be either a primary neoplasm, a secondary neoplasm from an extra-cranial primary site metastasizing to the CNS, or a non-neoplastic mass effect that can mimic neoplasia. Diagnosis of an intracranial tumor is a major challenge. Cerebrospinal fluid (CSF) analysis and skull radiographs rarely are specific for diagnosing CNS tumors. Lesions can be detected by using computed tomography (CT) or magnetic resonance imaging (MRI). Magnetic resonance imaging equipment now is widely available and is being marketed directly to veterinarians by manufacturers. However, little information is available regarding MRI features of intracranial tumors in cats. In a retrospective study (Magnetic Resonance Imaging Features of Feline Intracranial Neoplasia: Retrospective Analysis of 46 Cats. J Vet Intern Med 2004; 18:176-189), Troxel and coworkers reviewed the MRI features of histologically confirmed brain tumors in cats and determined whether or not these characteristics could be used to predict tumor type accurately. Table 1 shows the distribution of tumor cell types observed in this group.

The authors identified several patterns of MRI characteristics among tumor types that assisted with the prediction of histologic tumor type. Most extra-axial masses (those located outside of the brain parenchyma) that had moderate-to-marked contrast enhancement and a dural tail were found to be meningiomas. Extra-axial masses that caused mild edema and showed evidence of chronic hemorrhage or mineralization always were meningiomas. Most extra-axial masses with a hyperintense signal on T2-weighted images, isointense or hypointense signal on T1-weighted images, and moderate-to-marked contrast enhancement were meningiomas, whereas lymphomas and pituitary tumors accounted for only 10 percent of cases with these imaging characteristics. All intra-axial (within the brain parenchyma) masses with ring enhancement and cystic regions were gliomas.

Among meningiomas, peritumoral edema varied considerably but was mild most of the time. Although rare, cyst formation was seen. Transtentorial and cerebellar herniation occurred respectively in approximately 42 percent and 21 percent of cats with meningiomas, respectively.

Lymphomas were either intra-axial or extra-axial. Signal intensity generally was hyperintense and heterogenous on T2 images and isointense or hypointense on T1 images and was homogenous in all cats. Proton density weighted images were hyperintense and heterogenous. Peritumoral edema was seen in all cats. Mass effect was seen in half of the cat population with lymphomas in which an extra-axial mass was identified, and a dural tail sign was identified in one of the six cats in this group.

MRI images were obtained in one cat with an astrocytoma and three cats with oligodendrogliomas. These intra-axial tumors were round, hyperintense on T2, and hypointense on T1 images. All four tumors had ring enhancement. Mild to moderate peritumoral edema was noted in most cats. Some of the gliomas appeared to be cystic.

Olfactory neuroblastoma

Two cases of olfactory neuroblastoma were described by the authors. Both tumors were extra-axial, ovoid with irregular and indistinct borders, and appeared to break through the cribriform plate, causing a mass effect. T2 images were either hyperintense or isointense, and both had a heterogenous pattern. T1 images were either hyperintense or hypointense. A homogenous pattern was seen on T1 images. Isointense images were recorded on proton density images. There was a heterogenous enhancement in both tumors and moderate to marked peritumoral edema. Chronic hemorrhage or mineralization was suspected.

In one case, a pituitary tumor appeared round with regular and distinct margins. Images were hypointense and heterogenous on T2 images, and isointense and heterogenous on T1 images. There was moderate, heterogenous, ring-enhancing contrast enhancement.

In this study, the brain tumor was visualized via MRI in 98 percent of the cats reported. MRI features of intracranial tumors predicted the histopathologic diagnosis in 82 percent of the cases. Individual accuracy rate for each tumor type varied widely, from 0 for olfactory neuro- blastoma to 96 percent for meningioma. A high frequency of meningioma in the feline population was mentioned in another recent paper by Troxel and co-workers in 2003 [Feline Intracranial Neoplasia: Retrospective Review of 160 Cases (1985-2001]. J Vet Intern Med 2003; 17:850-859). Meningioma was the most common tumor type identified in 58.1 percent of cats with brain tumors. Multiple meningiomas were seen in 17.2 percent of cats affected with intracranial tumors. In fact, meningioma is the most-common extra-axial tumor in cats, and the odds are high that an extra-axial mass in a cat is a meningioma. Improved knowledge of feline brain tumors is required for accurate ante- mortem diagnosis. MRI appears to be extremely valuable in predicting tumor type before surgical treatment, biopsy, radiation therapy, chemotherapy or a combination of these measures can be appropriately employed.

Editor’s Note:Comments and questions can be directed to Pierre S. Bichsel, DVM, or Ronald Lyman, DVM at the Animal Emergency and Referral Center, 3984 S. US Highway 1, Ft. Pierce, Fla., 34982; (772) 466-0460.