Objectives: To evaluate the prognostic impact of the interval between surgery and the initiation of postoperative radiotherapy (RT) in patients with glioblastoma (GB).

Methods: This retrospective cohort study included patients diagnosed with GB and treated with surgical resection followed by adjuvant RT at a single institution between 21 April 21 and 20 December 2022. Patients were grouped according to the time interval between surgery and RT initiation (≤4, 4–6 and >6 weeks). Most patients received 60 Gy in 30 fractions; elderly or low-performance patients received 40.05 Gy in 15 fractions. Survival outcomes, including overall survival (OS) and progression-free survival (PFS), were assessed. Kaplan–Meier survival analysis and Cox proportional hazards modelling were performed.

Results: 132 patients were included. Median time from surgery to RT initiation was 32 days. Patients who began RT within 4–6 weeks had better OS and PFS compared to those treated earlier or later. On multivariate analysis, initiating RT after more than 6 weeks was associated with worse OS (HR: 1.72, 95% CI: 1.12–2.63, p = 0.013). Initiation within 3 weeks trended toward inferior outcomes, though not statistically significant. RT dose distribution: ~85% received 60 Gy/30 fr, and ~15% received 40.05 Gy/15 fr.

Conclusion: Timing of postoperative RT significantly impacts survival outcomes in GB patients. Initiating treatment between 4 and 6 weeks post-surgery may provide optimal benefit. These findings support careful planning of adjuvant RT to avoid undue delay or premature initiation.