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Introduction: Glioblastoma (GBM) often extends to deep supratentorial locations, which limits the extent of maximal safe resection. Deep supratentorial extension (DSE) may be a clinically convenient prognostic indicator following GBM resection.

Methods: 582 GBM resections from 2012-2018 were retrospectively reviewed. DSE was defined as tumoral extension to the basal ganglia, thalamus, corpus callosum, internal capsule, hypothalamus, caudate, or putamen as identified on preoperative imaging.

Results: DSE was identified in 32.9% cases (192), while 52.5% (306) involved only superficial supratentorial locations (frontal, parietal, temporal and occipital lobes). Within the DSE cohort, the most commonly affected anatomical locations were the corpus callosum (18.9%), basal ganglia (10.8%), and thalamus (5.5%). DSE was associated with a significantly higher rate of residual tumor (71.9%vs59.3%, p=.015), larger size (48.3 vs 43.6 mm, p=.007), and lower rate of radiological gross total resection (GTR) (55.6% vs 70.6%, p=.005). DSE was also associated with a worse progression free survival (PFS) (5.55vs8.32 months, p = .009) and overall survival (OS) (9.89vs14.23 months, p=.000). Kaplan-Meier curves showed worse OS with DSE (log rank=.003), and worse OS with involvement of 2+ DSE structures as compared to 1 or none (log rank=.000). DSE had no effect on OS among those achieving GTR (log rank=.626), but without GTR, DSE significantly worsened survival (log rank=.030) on Kaplan-Meier.

Discussion: DSE portended higher rates of residual tumor, lower rates of GTR, and worse PFS and OS, particularly among those not achieving GTR and with involvement of 2+ structures. DSE in GBM is a convenient and reliable negative prognostic factor.