The World Health Organization grading system
The World Health Organization grading system is contained in the volume Histological Typing the Central Nervous System tumors, whose first edition dates back to 1979, the second to 1993 the third 2007, 2016, 2021.
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 astrocytomas that are more circumscribed, including pilocytic astrocytomas, while diffuse astrocytomas 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%.
The WHO grade has four categories of tumors:
World Health Organization grade 1
World Health Organization grade 2