Glioneuronal and neuronal tumors: A review with special reference to the new types in the WHO CNS5 classification

Glioneuronal and neuronal tumors (GNTs) are slow-growing lower-grade neuroepithelial tumors with mature neuronal and, less consistently, glial differentiation. Their identification has relied solely on histological proof of neuronal differentiation, which was considered to represent the well-differentiated nature of GNTs. However, after discovering the genetic alterations in GNTs, particularly those in the MAP-kinase pathway, it became evident that histological diagnoses are not always concurrent with genetic alterations and vice versa. Furthermore, since several inhibitors mediating the MAP-kinase pathway are available, at least for clinical trials, molecular-based classification is now warranted. Thus, the upcoming WHO Classification of Central Nervous System Tumors, 5^th edition (WHO5CNS) applied DNA methylation profiling to segregate low-grade neuroepithelial tumors. This review gives an overview of the pathological features of GNTs with particular reference to the newly listed tumor types in WHO5CNS. The knowledge and awareness of each tumor type are essential to make a correct diagnosis and avoid unnecessary radical resection and chemoradiotherapy, as GNTs are relatively indolent and have a prolonged clinical course. In addition, being distinctive in location, age group, and histology, the integration of clinicopathological information will help identify relevant tumor types of GNTs without genetic testing, even in resource-limited settings.