<scp>cIMPACT</scp> ‐ <scp>NOW</scp> update 11: Proposal on adaptation of diagnostic criteria for <scp>IDH</scp> ‐ and <scp>H3</scp> ‐wildtype diffuse high‐grade gliomas and for posterior fossa ependymal tumors

The Consortium to Inform Molecular and Practical Approaches to Central Nervous System Tumor Taxonomy (cIMPACT-NOW) updates provide guidelines for the diagnosis of central nervous system (CNS) tumors and suggestions for future World Health Organization (WHO) classification. Following publication of the fifth edition WHO Classification of CNS Tumors (WHO CNS5) in 2021, the cIMPACT-NOW working group "Clarification" reviewed WHO CNS5 and prioritized two topics for further elucidation: (a) distinction of Glioblastoma, IDH-wildtype from Diffuse pediatric-type high-grade glioma, H3-wildtype, and IDH-wildtype and (b) clarification of subgroups of posterior fossa (PF) ependymal tumors. Recommendations regarding the IDH- and H3-wildtype diffuse high-grade gliomas include: (1) use caution assigning CNS WHO grade 4 (diagnosis of Glioblastoma, IDH-wildtype) to a "TERT promoter only", histologically low-grade, IDH-wildtype tumor; (2) EGFR gene amplification and +7/-10 chromosome copy number alterations should not be used as solitary defining features for diagnosing high-grade gliomas as Glioblastoma, IDH-wildtype in patients <40 years of age; (3) Diffuse pediatric-type high-grade glioma, H3-wildtype, and IDH-wildtype should be considered in the differential diagnosis in adults, especially those <40 years of age; (4) PDGFRA alteration, EGFR alteration, or MYCN amplification count as key molecular features of Diffuse pediatric-type high-grade glioma, H3-wildtype, and IDH-wildtype only in patients <25 years. Guidelines for improved diagnosis of posterior fossa ependymal tumors include: (1) immunohistochemical demonstration of nuclear EZHIP supports classification as PF group A ependymoma; (2) a PF ependymoma with retained nuclear H3 K27me3 expression and no nuclear EZHIP overexpression for which DNA methylation profiling is not performed should be considered as PF ependymoma, "not otherwise specified"; (3) for emerging tumors not included in WHO CNS5, "not elsewhere classified" (NEC) can be added to the diagnosis. Of note, these recommendations are not formal changes to the WHO definitions and diagnostic criteria but are intended to provide diagnostic guidance in advance of WHO CNS6.