WHO grading of CNS tumors
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WHO (World Health Organization) grading of CNS tumors depends on the specifics of each entity; see WHO classification of CNS tumors.
Historically grading was based on histological characteristics such as cellularity, mitotic activity, pleomorphism, necrosis, and endothelial proliferation, however, over the past decade, molecular markers have become increasingly important both in terms of determining a specific diagnosis and grading.
For example, a diffuse adult-type glioma that is IDH wildtype and demonstrates TERT promoter mutation is considered a glioblastoma grade 4 regardless of histological features.
In the 5th edition (2021) of the WHO classification of CNS tumors, the traditional use of Roman numerals (I, II, III, IV) to denote grade has been replaced by Arabic numerals (1, 2, 3, 4) 1.