[Evolution of Glioma Diagnosis: The WHO 2021 Classification and Beyond]

The World Health Organization (WHO) classification of brain tumors, which is based on the histogenetic concept, underwent a major transformation in the revised fourth edition published in 2016. Molecular genetic information was incorporated into the classification system, enabling the distinction of tumors based on the presence or absence of IDH mutations and 1p/19q co-deletion. This represents a significant paradigm shift in glioma diagnosis. In the fifth edition (WHO CNS5), the classification evolved by clearly separating adult- and paediatric-type gliomas and integrating genetic information directly into tumor grading. Molecular alterations are essential components of diagnosis. Consequently, the diagnostic criteria were refined to better reflect biological behavior and clinical outcomes. The forthcoming sixth edition may further advance this molecularly driven approach. Simultaneously, efforts are underway to establish a more coherent framework for using newly identified tumor entities and emerging molecular terminology. Thus, the WHO classification continues to evolve toward a more biologically meaningful and clinically relevant system for glioma diagnosis.