BIOM-46. EXPLORING CLINICAL AND PATHOLOGIC CHARACTERISTICS IN GLIOBLASTOMA LONG -TERM SURVIVORS (LTS) VERSUS SHORT-TERM SURVIVORS (STS)

Glioblastoma (GBM) has a median survival of less than 18 months with infrequent long-term survival (LTS). Reports on LTS to date have not identified distinct molecular or clinical/imaging characteristic in long ( >3 years) versus short (< 3 year) survivors. We identified IDH wildtype GBM patients living ≥ 3 years post diagnosis (LTS, Nf25) and a control cohort (STS, Nf76) from our Natural History Study. Available tissue was analyzed with targeted panel sequencing (Nf25) and methylation analysis (Nf21) for classification, MGMT promoter (MGMTp) status and copy number changes. Median survival and age for LTS and STS was 54mo and 48yrs, and 16mo and 56yrs, respectively. LTS were more likely to be female (2.8x) or have a GTR (2.9x) but 83% less likely to be white. MGMTp was 10x more likely to be methylated in LTS tumors (95% CI [2.6, 39.6]), yet was unmethylated in 17%. LTS were 63% and 90% less likely to have TERTp or EGFR amplification (95% CI [0.14, 0.97] and [0.04, 0.29]). Core pathway review showed 21% of LTS had at least one alteration in p53, RB and RTK/PI3K with RTK/PI3K the most common (46%) . Methylation classifier identified 74% as GBM (STS=96%) with most the RTK II subtype (53%; STS=41%) and despite classic histologic features, indicated a non-GBM diagnosis in three cases (14%).Although uncommon, there are LTS with molecularly confirmed GBM. Remarkably, not all had MGMTp methylation, were young or had extensive tumor resection. Clinical estimates of outcome in GBM patients should consider the possibility of long-term survival even with established poor prognostic factors. Imaging characteristics in a subset of these cases are reported in a separate abstract. Additional interrogation of tumor from LTS may uncover additional determinants of response and outcomes.