HGG-11. HIGH-GRADE GLIOMAS IN ADOLESCENTS AND YOUNG ADULTS HIGHLIGHT HISTOMOLECULAR DIFFERENCES WITH THEIR ADULT AND PAEDIATRIC COUNTERPARTS

BACKGROUND Considering that paediatric high-grade gliomas (HGGs) are biologically distinct from their adult counterparts, the objective of this study was to define the landscape of HGGs in adolescents and young adults (AYAs). METHODS We performed a multicentric retrospective study of 112 AYAs from adult and paediatric Ile-de-France neurosurgical units, treated between 1998 and 2013 to analyse their clinicoradiological and histomolecular profiles. The inclusion criteria were age between 15 and 25-years, histopathological HGG diagnosis, available clinical data, pre-operative and follow-up MRI. MRI and tumoral samples were centrally reviewed. Immunohistochemistry and complementary molecular techniques such as targeted/next generation sequencing, whole exome sequencing and DNA-methylation analyses were performed to achieve an integrated diagnosis according to the 2016 WHO classification. RESULTS Based on 80 documented AYA patients, HGGs constitute heterogeneous clinicopathological and molecular groups, with a predominant representation of paediatric-subtypes (Histone H3-mutants, 40%) but also adult-subtypes (IDH-mutants, 28%) characterized by the rarity of oligodendrogliomas, IDH-mutant and 1p/19q co-deleted and the relative high frequency of “rare adult IDH mutations” (20%). H3G34-mutants (14%) represent the most specific subgroup in AYAs. In the H3K27-mutant subgroup, the non-brainstem diffuse midline gliomas are more frequent (66.7%) than diffuse intrinsic pontine gliomas (23.8%), contrary to children. We found that WHO grade has no prognostic value, but molecular subgrouping has major prognostic importance. CONCLUSIONS HGGs in AYAs could benefit from a more personalized neuro-oncological management, driven by molecular subtyping rather than age group. Collaborative efforts are needed from paediatric and adult neuro-oncology teams to improve the management of HGGs in AYAs.