PATH-26. MIDLINE INVOLVEMENT IS ASSOCIATED WITH ABERRANT RB1 SIGNALING IN PATIENTS WITH GRADE 4 IDH-MUTANT ASTROCYTOMA

Abstract BACKGROUND The prognostic factors impacting the survival of grade 4 IDH-mutant astrocytoma treated with standard radiation/temozolomide are not well-described. METHODS A retrospective cohort was generated of all patients diagnosed at Mayo Clinic with grade 4 IDH-mutant astrocytoma (2021 WHO Classification) on initial diagnosis between 2002-2023, and who received standard radiation/temozolomide. The impact of patient, tumor and molecular variables on overall survival (OS) and progression-free survival (PFS) was evaluated using survival analyses and Cox regression models. RESULTS Fifty-eight patients were identified. The median age was 36 years, 64% were male and 93% were white. The median follow up was 3.3 years. The median OS and PFS were 6.9 years and 2.4 years, respectively. CDKN2A/B homozygous deletion significantly reduced both OS (2.5 years vs NR, HR=3.9, p=0.0168) and PFS (1.4 vs 4.5 years, HR=4.2, p=0.0005). Midline involvement (brainstem, basal ganglia, thalamus, cerebellum or corpus callosum) also significantly reduced both OS (2.4 vs 12.8 years, HR=7.0, p=0.0014) and PFS (1.4 vs 4.5 years, HR=4.8, p=0.0002). MGMT promoter hypermethylation showed a trend toward significance for PFS (p=0.082) but did not impact OS. Age, sex, tumor size, ATRX immunohistochemistry and tumor-treating field therapy did not impact survival outcomes. Multivariable analyses revealed that PFS remained significantly impacted by both CDKN2A/B homozygous deletion (HR=3.5, p=0.0051) and midline involvement (HR=3.1, p=0.0125). A significant association between CDKN2A/B homozygous deletion and midline involvement was unexpectedly observed (53% if midline, 24% if non-midline; χ2=4.3, p=0.037). Exploratory analyses revealed that 15/15 (100%) of patients with midline involvement demonstrated aberration in RB1 signaling (CDKN2A/B homozygous deletion, n=8; CDKN2A/B hemizygous deletion or loss of heterozygosity, n=6; CDK4 amplification, n=1). CONCLUSION CDKN2A/B homozygous deletion and midline involvement independently impact PFS in patients with grade 4 IDH-mutant astrocytoma treated with standard radiation/temozolomide. Furthermore, midline involvement may be associated with aberrant RB1 signaling.