TECTAL GLIOMA VERSUS PILOCYTIC ASTROCYTOMA: REVISITING TUMOR CLASSIFICATION IN LIGHT OF MOLECULAR HETEROGENEITY

Two studies originating from the same team have previously suggested the existence of tectal glioma (TG) as a distinct pediatric low grade circumscribed astrocytic tumor type enriched in KRAS alterations or KRAS and BRAF concomitant alterations, and characterized by a distinct methylation profile. Although the majority of these tumors display a pilocytic astrocytoma (PA) morphology, the imprecise terminology of “glioma” was preferred by these authors. The inclusion of the entity “tectal glioma with KRAS mutation” in a future WHO classification is still under discussion. In this context, we performed a comprehensive analysis of 35 glial tumors located in the tectum including clinical, radiological, histopathological, and molecular data. The majority of tumors encountered were pilocytic astrocytomas. The other tumor types included gangliogliomas, rosette-forming glioneural tumors and a diffuse midline glioma, H3K27-altered. Surprisingly, none of these tumors showed any mutation in the KRAS gene. MAPK gene alterations were diverse: KIAA1549::BRAF, FGFR1 alterations (fusions and duplications), other RAF fusions, NF1 mutations and BRAF mutations. Furthermore, when we compared the epigenetic profile of our tumors with those available from the study by Liu et al., they clustered close to or alongside them.