====== The World Health Organization grading system ======

The [[World Health Organization]] grading system is contained in the volume Histological Typing the [[Central Nervous System tumor]]s, whose first edition dates back to [[1979]], the second to [[1993]] the third [[2007]], [[2016]], [[2021]].
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The 2016 World Health Organization (WHO) histological grading system is similar to older 3-tier systems (the St. Anne/Mayo system, the Ringetz system, and previous WHO systems).

Grade I is reserved for special types of [[astrocytoma]]s that are more circumscribed, including [[pilocytic astrocytoma]]s, while [[diffuse astrocytoma]]s are graded II through IV. The frequencies of occurrence among 287 astrocytomas were: grade I=0.7% (very rare tumors), grade II=16%, grade III = 17.8%, and grade IV = 65.5%.
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The WHO grade has four categories of tumors:

[[World Health Organization grade 1]] 

[[World Health Organization grade 2]]

[[World Health Organization grade 3]]

[[World Health Organization grade 4]]