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Background
Neuroradiologists play a key role in brain tumor diagnosis and management. Staying current with the latest classification systems and diagnostic markers is important to provide optimal patient care. Publication of the 2016 WHO Classification of CNS introduced a paradigm shift in the diagnosis of CNS neoplasms. For the first time, both histologic features and genetic alterations were incorporated into the diagnostic framework, classifying and grading brain tumors. The newly published 2021 WHO Classification of CNS Tumors, May 2021, 5th edition, has added even more molecular features and updated pathologic diagnoses.

Educational Goals / Teaching Points
We aim to emphasize the major changes of 2021 WHO Classification of CNS Tumors that mainly affect the children and young adults age group, present the clinical and molecular characteristics of these tumors, and demonstrate the neuroimaging features these tumors on multidisciplinary basic as well as advanced MRI modalities.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Pediatric type diffuse glioma is mainly classified into low-grade and high-grade groups. Low-grade group (diffuse astrocytoma, MYB-or MYBL1-altered; angiocentric glioma; diffuse low-grade glioma MAPK pathway-altered; and PLNTY). High-grade group (diffuse midline glioma (DMG), H3 K27-altered; diffuse hemispheric glioma (DHG), H3 G34-mutant; diffuse pediatric-type high grade glioma, H3-wildtype and IDH-wildtype; and infant-type hemispheric glioma). Neuronal and Glioneuronal tumors (MGNT and MVNT were newly recognized in 5th edition of WHO CNS tumors classification). Ependymal tumors (supratentorial ependymoma, YAP1 fusion-positive; posterior fossa ependymoma, group PFA; posterior fossa ependymoma, group PFB; and spinal ependymoma, MYCN-amplified were newly recognized in 5th edition of WHO CNS tumors classification). Other CNS embryonal tumors, aside from medulloblastoma (CNS neuroblastoma, FOXR2-activated and CNS tumor with BCOR internal tandem duplication were newly recognized in 5th edition of WHO CNS tumors classification). Mesenchymal, non-meningothelial tumors (intracranial mesenchymal tumor, CIC-rearranged sarcoma; and primary intracranial sarcoma, DICER1-mutant were newly recognized in 5th edition of WHO CNS tumors classification).

Conclusion
2016 WHO CNS tumor update started to incorporate molecular features and adopted an integrated approach of layered tumor reports and the 2021 WHO CNS tumor update introduces a more sophisticated classification enabling clinicians to group tumors into more biologically and molecularly defined entities. This new classification may change subject enrollment, treatment assignment and outcome stratification in the clinical trials. Neuroradiologists must familiarize themselves with the updated 2021 WHO Classification of CNS neoplasms especially for pediatric age group, to function appropriately as part of the modern neuro-oncology clinical team.