Magnetic resonance imaging (MRI) is the primary tool for the initial diagnosis of a meningioma, often providing strong evidence regarding its biological behavior. While a typical appearance on an MRI suggests a slow-growing, benign tumor, imaging alone cannot confirm its grade. The definitive determination of whether a meningioma is benign or aggressive relies on microscopic examination of the tumor tissue. Even tumors that look harmless on the scan can sometimes harbor higher-grade, aggressive cellular features.
Understanding Meningiomas
Meningiomas are the most common type of primary central nervous system tumor, accounting for about 37.6% of all such tumors. They originate from the arachnoid cap cells, which are part of the meninges, the protective layers of tissue covering the brain and spinal cord. These tumors typically grow slowly, often arising near the surface of the brain or spinal cord.
The World Health Organization (WHO) classifies meningiomas into three grades based on their potential for growth and recurrence. Grade I tumors are considered benign, are the most common, and represent approximately 80% of all cases. Grade II tumors are classified as atypical, showing a faster growth rate and a higher chance of recurrence.
Grade III tumors are malignant, or anaplastic, and are the most aggressive, though they are rare, making up only 1% to 3% of cases. The assigned WHO grade is the most important factor in determining the treatment plan and predicting the long-term outlook for the patient.
How MRI Visualizes Meningiomas
Magnetic resonance imaging is highly effective for visualizing soft tissues like the brain and nearby tumors. A standard MRI protocol includes using a contrast agent, typically gadolinium, injected into a vein. The contrast agent highlights the tumor because meningiomas have a rich blood supply and a compromised blood-brain barrier, allowing the agent to accumulate.
A classic, benign-appearing meningioma presents on an MRI as a well-defined, broad-based mass attached to the dura mater. The tumor often shows intense and uniform enhancement after the contrast agent is administered. A characteristic feature is the “dural tail sign,” which appears as a linear thickening and enhancement of the dura mater tapering away from the main tumor mass.
The dural tail sign is seen in 60% to 72% of cases and represents a reactive, hypervascular change in the adjacent meninges rather than a sign of tumor invasion. While this appearance strongly supports a meningioma diagnosis, similar features can occasionally be seen with other conditions, such as lymphoma or metastasis.
MRI Characteristics Suggesting Tumor Behavior
Radiologists analyze imaging characteristics to determine if a meningioma is likely lower-grade (benign) or higher-grade (atypical or malignant). Features associated with a likely benign Grade I tumor include smooth, well-defined borders and a uniform, homogeneous enhancement pattern following contrast. These tumors often show minimal or no surrounding brain edema, indicating compression rather than invasion. Calcification within the tumor is also seen as a favorable sign, suggesting slow growth.
In contrast, several features suggest a more aggressive Grade II or Grade III tumor, guiding the neurosurgeon’s approach. Atypical tumors may display irregular or lobulated margins, suggesting a less contained growth pattern. A heterogeneous, or patchy, enhancement pattern after contrast can indicate areas of necrosis or variable cellularity.
Significant peritumoral edema, which is swelling in the brain tissue surrounding the tumor, is a strong indicator of higher-grade pathology. Other suspicious findings include evidence of the tumor eroding adjacent bone or imaging suggesting invasion into the brain parenchyma. A significantly lower apparent diffusion coefficient on specialized MRI sequences can also suggest higher cellular density, which correlates with higher-grade tumors. These radiological findings are used for risk assessment and surgical planning.
Why Biopsy is Necessary for Definitive Diagnosis
Despite the information provided by MRI, a definitive determination of a meningioma’s grade requires a histopathological examination of the tissue. The WHO grading system, which guides treatment and prognosis, is based purely on the microscopic appearance of the tumor cells. This analysis cannot be performed using imaging alone.
A pathologist examines the tissue sample, obtained either from a biopsy or surgical removal, to look for specific cellular features. Grade I tumors lack features like increased cellularity, high rates of cell division, and necrosis. The pathologist counts the mitotic rate, or the number of dividing cells, which is a primary criterion for assigning a higher grade.
A mitotic rate of 4 to 19 mitoses per ten high-power fields, or the presence of three or more specific histological features, automatically classifies a tumor as Grade II. The confirmed pathological grade dictates the final treatment plan, determining if the patient is managed with observation, surgery alone, or surgery followed by radiation therapy. The MRI indicates likelihood, but the microscopic evaluation provides the necessary confirmation for clinical decision-making.