2654 - Assessment of Subventricular Zone Irradiation in Glioblastoma Patients

Purpose/Objective(s): This study aims to analyze the association between incidental subventricular zone (SVZ) irradiation dose and its impact on tumor recurrence and overall survival in a well-defined cohort of glioblastoma (GBM) patients. Additionally, it seeks to explore the therapeutic potential of intentional SVZ irradiation as a feasible strategy for optimizing treatment efficacy and improving patient outcomes.

Materials/Methods: A single-arm, prospective analytical study with a total of twenty-eight patients with histopathologically confirmed glioblastoma, with KPS score >70 age 18-70yrs, post operative patients (GTR/STR) were selected. Target volumes contoured based on CT-MR Fusion according to NRG protocol. The ipsilateral, contralateral, and bilateral subventricular zones (SVZs) were delineated using co-registered MRI and computed tomography images. The ipsilateral SVZ (iSVZ), defined as the SVZ on the side where the bulk of the tumor was located. The contralateral SVZ (cSVZ) is the SVZ on the contralateral hemisphere. Post operative RT with total dose of 60 Gy ( 46Gy/23 fractions followed by 14Gy/7 fractions in 2 phases) 2 Gy/# in 6 weeks was delivered. Dose–volume histograms were calculated, and mean and median with quartile doses were obtained for the iSVZ and cSVZ and bSVZ (bilateral SVZ combined). Patients were followed at 1month and 3-monthly interval. Contrast enhanced MRI was done every 3 months. Progression-free survival (PFS) and overall survival (OS) was assessed using statistical software.

Results: Median age was 45 years (35-59 years). Overall survival rate 85.7 %. Median follow up: 14.5 months Median OS for the whole cohort is 13 months (4.6-18.35 months) and Median PFS is 6.5 months (1.44-14 months). On Univariate analysis: Initial contact to SVZ was a poor prognostic factor with median OS (10 months Vs 15 months); p value: 0.044; HR 4.04. Mean of ipsilateral SVZ median dose equal to or more than 52.14Gy had increased overall survival (p value: 0.04) in SVZ contacting tumors. Median of bilateral SVZ median dose equal to or more than 45.11Gy had an OS benefit (p value: 0.04). Initial analysis of dataset, showed statistically significant improvement in OS who received ipsilateral SVZ radiation doses (Mean of D_MED>52.14Gy) in univariate analysis when compared to those who received less dose. Our study approach involved observing only the incidental dose to the part of SVZ and not including whole ipsilateral SVZ volume in the target, potentially allowing the remaining SVZ regions to contribute to cancer stem cell (CSC) repopulation.

Conclusion: Our data from a homogeneous patient cohort suggest that contact with the subventricular zone (SVZ) may serve as an independent poor prognostic factor for progression and that irradiation of the ipsilateral SVZ will give the patient better overall survival. Inclusion of ipsilateral SVZ for SVZ contacting tumors is validated in the study, but further large prospective randomized trials are imperative.