External beam radiation therapy for canine intracranial meningioma

While patient signalment, history, and clinical signs can be highly suggestive of an intracranial tumor, cross-sectional imaging is an essential step in diagnosis. There is an increasing availability of advanced imaging modalities and treatment options in Canada for dogs with intracranial tumors. Computed tomography (CT) and magnetic resonance (MR) imaging are now available to animals in most major cities in Canada, and should be recommended when an intracranial tumor is suspected. Radiation therapy is offered at veterinary hospitals in western and in eastern Canada.

The most common intracranial tumor in dogs is meningioma. When meningiomas are not surgically resectable, or when tumor cells are left behind after surgical resection, radiation therapy is the treatment of choice. This article summarizes the imaging features of intracranial meningiomas in dogs, as well as the use of radiation therapy in the treatment of this tumor type.

Anatomical origin

Intracranial tumors are anatomically categorized as extra-axial (arising outside the brain), and intra-axial (arising within the brain). Common extra-axial intracranial tumors include meningioma, pituitary tumors, and intracranially invading nasal tumors. Of these tumors, meningioma is by far the most common (1). The meninges refers to 3 protective layers that envelop the brain, spinal cord, and spinal roots. The dura mater is the outermost layer of the meninges, the arachnoid membrane lines the inner surface of the dura mater, and the pia mater is the innermost layer (2). Meningiomas arise from meningothelial cells of the arachnoid membrane and the pia mater (3).

Clinical signs and imaging

The prevalence of meningiomas increases with age in dogs, with most meningiomas occurring in dogs over 7 years of age (3). Dolichocephalic, or long, narrow-headed, breeds may have a higher risk of meningioma than other breeds (4). The clinical signs associated with meningiomas are usually slowly progressive but in some cases, especially with hemorrhage or edema, acute changes may be seen. The most common location for meningiomas in the dog is the cerebrum, and the most common clinical sign is seizure (4). Clinical signs are dependent on the location of the tumor, and may include ataxia, blindness, behavioral changes, head tilt, and other abnormalities. Neurological examination may help with anatomical localization of the tumor. However, as the disease progresses, generalized signs relating to multiple anatomical sites may also be present due to increased intracranial pressure.

Computed tomography and MR imaging are the most sensitive and specific techniques for evaluation of intracranial tumors, and referral for advanced imaging is recommended for dogs with a suspected intracranial tumor (5). Magnetic resonance imaging is extremely sensitive to the soft tissue alterations associated with tumors, and is generally accepted as being superior to CT for visualization of intracranial tumor-related changes. While a definitive diagnosis of meningioma requires histological examination, CT or MR image features such as anatomic site, margins, shape, pattern of growth, edema, and contrast enhancement can provide strong support for a diagnosis of meningioma (5). Canine meningiomas are typically solitary, broad-based masses that originate peripherally, displace or compress normal structures, and have strong and uniform contrast enhancement (Figure 1). Administration of contrast medium is always recommended, as meningiomas, and other intracranial tumors, may have the same intensity or density as surrounding normal brain and therefore may be missed if contrast material is not used (Figure 2). A dural tail sign, or an enhancing linear region of dura mater extending from the main mass lesion, may be present and supports the diagnosis of meningioma (Figure 3) (5).

Figure 1.

Post-contrast transverse computed tomographic image of an 11-year-old golden retriever with a meningioma (arrow) involving the left cerebrum and cerebellum. The mass has a broad base and originates peripherally.

Figure 2.

T1-weighted pre-contrast (A) and post-contrast (B) transverse magnetic resonance images of a 12-year-old Shetland sheepdog with a left cerebral meningioma (arrows). Strong contrast enhancement is present, and demonstrates the value of contrast media in imaging of intracranial masses.

Figure 3.

T1-weighted post-contrast sagittal magnetic resonance image of the same dog as in Figure 2, showing a dural tail (arrow).

Radiation therapy

The size, location, and invasiveness of an intracranial meningioma may limit the ability of a surgeon to perform a complete resection. Radiation therapy is indicated for meningiomas that are not surgically resectable, and for meningiomas in which surgical excision is incomplete. In addition to diagnostic CT or MR imaging, a treatment planning CT may be performed prior to radiation treatment. The patient is positioned for the planning CT in the treatment position, and three-dimensional computerized planning is performed to select an arrangement of radiation beams that provide maximum dose to the tumor cells, while minimizing dose to the surrounding normal tissues. At the Western College of Veterinary Medicine, most patients are treated on an outpatient basis 5 d a week for approximately 4 wk.

The outcome of 31 dogs with meningiomas treated with surgery alone or surgery followed by radiation therapy was reported by Axlund et al (4). The mean age of dogs in this study was 10.5 y, and 90% of the meningiomas were located in the cerebrum. The most common breeds affected were boxers and golden retrievers. Ninety-six percent of the dogs with cerebral tumors presented with seizures. Eighteen of the dogs had surgery alone, and 13 dogs had surgery followed by radiation therapy. The median survival for dogs treated with surgery alone was 7 mo (range: 0.5 to 22 mo), while the median survival for dogs treated with surgery and postoperative radiation therapy was 16.5 mo (range: 3 to 58 mo). The survival of dogs treated with radiation therapy was significantly longer (P < 0.005) than that of dogs treated with surgery alone. The findings of this study suggest that the use of radiation therapy as an adjunct treatment for intracranial meningiomas in dogs can extend life expectancy.

In another study, 20 dogs with intracranial meningiomas were treated with radiation therapy after incomplete resection of their tumors (6). The mean age of dogs in this study was 9 y, with a range in age from 1 to 18 y. The most common breeds were again golden retrievers and boxers. The median progression free survival of all dogs was 30 mo. This study also examined proliferating cell nuclear antigen (PCNA) assay as a predictor of outcome after radiation therapy. The PCNA assay provides a measure of proliferative activity of tumor cells, and was performed on tumor tissue samples. The authors found that meningiomas with a low proliferative fraction were 9 times more likely to be controlled with radiation therapy than meningiomas with a high proliferative fraction. This assay may be useful as a prognostic indicator if a biopsy is performed prior to radiation treatment.

One final study reported a median survival of 24 mo for 20 dogs with extra-axial intracranial tumors treated with radiation therapy alone (7). Tumor type was not confirmed in this group of dogs before treatment. In 85% of the dogs with intracranial tumors in this study, neurologic status improved during the course of radiation treatment or within the first 3 wk after completion of radiation therapy. Quality of life after radiation treatment was judged to be good to excellent by the dogs’ families.

Side effects of radiation therapy

Side effects of radiation therapy include early effects, which appear within weeks of starting treatment in most dogs receiving curative radiation therapy, and delayed effects, which appear months to years after treatment. Side effects are limited to the tissues within the radiation fields. Because radiation is targeted deep to the skin in dogs undergoing treatment for meningiomas, most dogs experience no to minimal early skin side effects. Depending on the location of the tumor and radiation treatment fields, early effects are most commonly seen in the ear canals, the mucous membranes of the pharyngeal region, the ocular conjunctiva, and the tear-producing glands. Early side effects are treated symptomatically and are generally resolved by 3 to 4 wk after the completion of radiation treatment. Exacerbation of pre-radiation therapy neurologic deficits may be seen early in the treatment period but are transient. Common side effects after completion of treatment include hair loss that may or may not be permanent, hypo- or hyperpigmentation of the skin, and alteration of hair coat color in the radiation field. Uncommonly, transient neurologic deterioration, often similar to initial presenting signs, may be seen 2 to 12 wk after treatment, with a peak incidence at about 8 wk after radiation therapy is completed (8). This early-delayed side effect generally resolves with time, and may respond to systemic steroids. It is important to be aware that recurrence of initial neurologic signs may be due to this temporary radiation side effect, and not to tumor recurrence. The risk of serious delayed effects is generally less than 5% with current treatment protocols.

In summary, prompt referral for advanced imaging is recommended when an intracranial tumor is suspected. Radiation therapy is an effective treatment option that should be discussed with clients for canine meningiomas that are nonresectable or incompletely resected. The early side effects of radiation therapy for meningiomas tend to be mild and self-limiting in most cases.