Purpose: We investigated the extent of changes in the anatomical position, shape and volume of lateral ventricles (LVs) and subventricular zones (SVZs). We included other critical organs at risk (OARs) to examine their contribution to the dose delivered to these regions. Additionally, the correlation between the SVZ radiation dose and clinical outcome was analyzed using the median SVZ dose as a cut-off value for both of the structures defined on the first planning CT and the data on the changed ipsi- and contralateral SVZs on the repeated CT during the course of irradiation.
Methods: We examined changes in the ipsilateral/contralateral LV and SVZ, as well as in the relevant OARs. We evaluated the volumetric and dosimetric changes on both planning CT scans (primary CT1 and secondary CT2). The survival of the GBM patients was analyzed using the Kaplan–Meier method; the multivariate Cox regression was also performed.
Results: LV and SVZ structures exhibited significant volumetric changes on CT2, resulting in an increase of dose coverage. At a cut-off point of 58 Gy, a significant correlation was detected between the iSVZ2 mean dose and OS (27.8 vs 15.6 months, p=0.048). In a multivariate analysis, glioblastoma multiforme (GBM) patients with a shorter time to postoperative chemoradiotherapy (<3.8 weeks), with good performance status (≥70%) and higher mean dose (≥58 Gy) to the iSVZ2 had significantly better overall survival (OS). We observed that the average of all investigated dose parameters to other OARs was lower at each volume dose level than on CT1 and replanning caused significant differences on most of them.
Conclusions: Significant anatomical and dose distribution changes to the brain structures were observed, which have a relevant impact on the dose-effect relationship for GBM; therefore, involving the iSVZ in the target volume should be considered and adapted to the changes
