OUTC-02. CASE SERIES OF HYPERMUTATED PEDIATRIC CNS TUMORS

In pediatric central nervous system (CNS) tumors, high tumor mutational burden (TMB) (≥3 mutations/megabase (mut/Mb)) has been correlated with lower overall survival and higher rates of progression as compared to low TMB tumors (<3 mut/Mb). This study is the first to examine a subset of pediatric CNS hypermutated tumors (≥10 mut/Mb) in comparison to high TMB tumors. We conducted a retrospective analysis of 121 pediatric patients (61 female (50%)) with a median age of 9 years diagnosed with a primary CNS tumor (high TMB, N=34; low TMB, N=55) at four academic medical centers from 2008 to 2021. Hypermutated tumors (N=3) were exclusively high grade tumors; subtypes were ependymoma, medulloblastoma, and oligodendroglioma. Hypermutated tumors had an average TMB of 65 mut/Mb (range=11-132 mut/Mb). Most of the hypermutated tumors had TP53 mutations (N=2; 67%) as compared to 29% of high TMB patients (N=12). Sixty-seven percent of patients with hypermutated tumors experienced tumor progression (N=2) as compared to 44% (N=15) of patients with high TMB tumors. Average time to progression was 2.4 years in hypermutated tumors and 1.4 years in high TMB tumors. No patients with a hypermutated tumor experienced a death event, as compared to 41% (N=14) of patients with high TMB. Although we report a small subset of hypermutated pediatric CNS tumors, this multi-institution case series suggests that hypermutated tumors may be associated with higher rates of progression, slower time to progression, and lower mortality as compared to high TMB tumors. Future research with a larger population of hypermutated tumors is needed to better characterize outcomes to optimize treatment for this subset of patients.