Inclusion of the Ipsilateral Subventricular Zone (SVZ) in the irradiated field for Glioblastoma Multiforme (GBM) : a preliminary study

D. Tuano. M.D, R. Ramos. M.D, R. Torcuator. M.D, M. Calaguas. M.D

Background: The neural stem cells of the SVZ of the brain have are capable of postnatal neurogenesis. GBM tumors also possess these stem cells, which may be derived from the SVZ. GBM confers a poor prognosis due to the possible reason that therapies fail to eradicate the resistant stem cells in the SVZ.Results vary from studies that investigate irradiation of the SVZ. It was the aim of this study to determine if higher radiation doses to the ipsilateral SVZ and if the volume of the ispilateral SVZ receiving 58 Gy figure in the improvement of the progression free survival (PFS) and overall survival (OS) of GBM patients to conclude if the ipsilateral SVZ should be consequently included in the irradiated field for GBM patients.

Methods: This retrospective, cohort study began with data review of previously treated GBM patients. The ipsilateral and contralateral subventricular zones were contoured on CT planning scans. The mean doses were determined and statistically analyzed.

Results: A total of 15 patients were eligible for preliminary data analysis. The mean ipsilateral SVZ dose for patients who died and who were alive were 58.55 Gy and 59.5 Gy, respectively. Statistical analysis showed that there was no significant difference between the 2 groups compared (p=0.79); however, it can be noticed that more patients died for a lower mean dose to the ipsilateral SVZ. Comparison of the mean ipsilateral SVZ volume receiving at least 58Gy among patients who died (mean: 8.31cc) and those who were alive (mean: 9.37cc) revealed statistically insignificant results (p=0.75); however, it can be noticed that the volume of the ipsilateral SVZ receiving at least 58 Gy was smaller for those patients who died.The 5-year OS for patients who received ≤ 58 Gy and > 58 Gy is 33% and 22% respectively. TheOS of 67% was observed at 6 months for patients who received SVZ dose ≤ 58 Gy and at 12 months for patients who received SVZ dose > 58 Gy. The 5-year PFS for patients who received ≤ 58 Gy and > 58 Gywas 33% and 22% respectively. At 13 months, the PFS for ≤ 58 Gy group was 33% while the same was observed in > 58 Gy group at 24 months. The patient population for this study is a limitation and is the main consideration for the statistically insignificant results.

Conclusion: A conclusion to include the ipsilateral SVZ during GBM irradiation cannot yet be made due to the preliminary study’s small sample size. A trend towards more patients whose ipsilateral SVZs were irradiated with lower doses and had mean doses specifically lower than 58 Gy succumbing to death earlier was seen and will be further investigated.