Meningiomas are common tumors originating from the meninges, the protective membranes surrounding the brain and spinal cord. Though often non-cancerous, they can cause symptoms by pressing on brain tissue, nerves, or blood vessels. Tumor classification helps predict behavior, guiding treatment and understanding the condition’s probable course.
Understanding WHO Meningioma Grades
The World Health Organization (WHO) provides a grading system for meningiomas, based on the 2021 WHO Classification of Tumors of the Central Nervous System. This system categorizes meningiomas into three grades, reflecting their biological behavior and recurrence potential. Grades are determined by tumor cell appearance under a microscope, indicating aggressiveness.
Grade 1 meningiomas are benign and the most common type, accounting for 75-90% of cases. These tumors grow slowly, have well-defined borders, and their cells resemble normal meningeal cells. They have a low recurrence likelihood after removal.
Grade 2, or atypical, meningiomas constitute 15-20% of diagnoses. Not malignant, they show moderate growth and a higher recurrence risk than Grade 1. Histological features include increased cellularity, higher mitotic activity, and sometimes brain tissue invasion.
Grade 3 meningiomas are classified as malignant or anaplastic tumors, making up about 1% to 4% of cases. These are fast-growing and aggressive, with a substantial likelihood of recurrence and potential to spread into healthy brain tissue. They show highly abnormal cells, significant mitotic activity, and features like brain invasion.
Implications of Each Grade
The WHO grade significantly influences meningioma management, dictating prognosis, treatment, and follow-up. Lower grades mean a more favorable outlook; higher grades indicate more aggressive tumor behavior.
For Grade 1 meningiomas, prognosis is favorable, with 5-year survival rates of 85-95% and 10-year rates of 80-90%. Treatment often involves observation for small, asymptomatic tumors, or curative surgical removal. Post-surgery, regular MRI scans are recommended, initially annually, then less frequently if stable.
Grade 2 meningiomas carry a higher recurrence risk, with 5-year survival rates of 65-75% and 10-year rates of 50-60%. Surgical removal is typical, often followed by radiation therapy to reduce recurrence. Patients undergo more frequent monitoring, with MRI scans at six months post-treatment, then annually for several years.
Grade 3 meningiomas have a less favorable prognosis due to their aggressive nature, with 5-year survival rates of 55-60% and 10-year rates of 25-30%. Management involves aggressive surgical removal, followed by radiation therapy. Systemic therapies might also be considered. High recurrence risk necessitates intensive follow-up, often including scans every six months for three years, then annually.
Determining Meningioma Grade
Determining a meningioma’s WHO grade relies on detailed tumor tissue examination. This begins with obtaining a tissue sample, typically via surgical resection or biopsy. The tissue is then sent to a neuropathologist for microscopic analysis.
Under the microscope, the neuropathologist assesses cell morphology, density, and atypical characteristics. Key grading indicators include cell division rate (mitotic activity), spontaneous cell death (necrosis), and brain tissue invasion. More mitotic figures or brain invasion often indicates a higher grade.
Molecular characteristics are increasingly considered alongside histological features to refine grading and predict tumor behavior. The 2021 WHO classification incorporates specific genetic alterations, like TERT promoter mutations and CDKN2A/B deletions, which can classify a meningioma as Grade 3 regardless of histological findings. Other molecular markers (e.g., NF2, AKT1, TRAF7, SMO, PIK3CA mutations) also provide insights into a tumor’s likely course.