The optimal timing of adjuvant chemoradiation after surgical resection of glioblastoma remains unknown. The National Cancer Database was queried for all patients diagnosed with glioblastoma from 2004 to 2012 who underwent surgical resection followed by adjuvant chemoradiation. Cox proportional hazards modeling was used to determine factors influencing survival. Optimal thresholds for time to initiation (TTI) of adjuvant therapy were determined using recursive partitioning analysis (RPA). A total of 16,335 patients were included. The RPA model identified an optimal threshold of 61 days to initiation of adjuvant therapy which was confirmed with randomly selected training and validation sets using conditional inference trees repeated 1000 times for robustness. Compared to patients who initiated adjuvant therapy within 61 days (median overall survival: 14.1 months, 95% CI 13.8-14.3 months), patients who initiated adjuvant therapy after day 61 (MOS: 12.0, CI: 10.7-12.9) had higher risk of death (p < 0.0001). For patients with glioblastoma, large increases in time to the initiation of adjuvant therapy leads to inferior survival with a threshold of 61 days or less following surgical resection.
