Four grades of the WHO grading system for CNS tumors
- Grade I tumors are typically well circumscribed , slow growing, nonmalignant, and associated with long-term survival.
- Grade II tumors are relatively slow growing but sometimes recur as higher grade tumors and are defined histopathologically by nuclear atypia (abnormal appearance of cell nuclei).
- Grade III tumors are malignant and often recur as higher grade tumors and are defined histopathologically by mitosis (a process in cell division by which the nucleus divides).
- Grade IV tumors are fast growing and defined histopathologically by microvascular proliferation (blood vessels that have multiple layers of enlarged endothelial cells) and/or necrosis (cell death).