A simplified overview of the World Health Organization classification of central nervous system tumors 2021

BACKGROUND: Building on the 2016 updated fourth edition and the work of consortium to inform molecular and practical approach to CNS tumor taxonomy, the major dramatic change occurs in 2021 fifth edition by advancing the role of molecular diagnostics in CNS tumor classification. The present review summarizes the major general changes in the 2021 fifth edition classification and the specific changes in each taxonomic category. METHODS: The review was designed in accordance to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis. Articles published in PubMed Central, Medline, and Embase databases till now were all searched. Only nonexperimental and nonanimal clinical studies were included in the study. Articles written only in the English language were considered. RESULTS: All IDH mutant diffuse astrocytic tumors are considered in a single type “astrocytoma IDH mutant” and then graded as CNS WHO Grades 2–4. Pediatric-type diffuse gliomas are now classified as separate entity. Anatomical site is also taken into consideration to classify ependymoma. The “Desmoplastic myxoid tumor of the pineal region, SMARCB1 mutant” and “Atypical neurofibromatous neoplasm of unknown biological potential” are new tumor type added to pineal and neurofibroma group, respectively. Mesenchymal tumor is now termed as only solitary fibrous tumor. Adamantinomatous and papillary subtype of craniopharyngioma are now classified as distinct tumor type. The new term “Pituitary neuroendocrine tumor” has been coined for pituitary adenoma. CONCLUSION: The WHO CNS-5 introduces a new knowledge into the classification with progressive manner by introducing newly recognizing entities, by obsoleting tumor type, and by adjusting the taxonomic structure.