Evaluation of Organs-at-Risk Sparing for Glioblastoma Patients Using a Small-Margin Weekly Adaptive Protocol on a 1.5T MR-Linac

To assess the dosimetric impact to organs-at-risk (OARs) and target coverage of a small margin weekly adaptive treatment for glioblastoma delivered on an MR-linac. The UNIty-Based MR-Linac Guided AdapTive RadiothErapy for High GraDe Glioma (UNITED) phase 2 prospective trial utilized a 5 mm CTV margin compared to conventional 1.5 to 3 cm CTV margins. Margin reduction was supported by weekly administration of gadolinium contrast and adaptation to gross tumor volume (GTV) changes using onboard MR imaging. In the present study, dosimetric data from 29 patients with UNITED plans ( P UNITED ) treated with 60 Gy in 30 fractions were retrospectively re-planned with conventional margins ( P Conv ) and compared. For each patient, six weekly adapted and one conventional margin non-adapted plan were included in the analysis. Relevant OAR dose metrics were compared using Friedman tests followed by Nemenyi post-hoc analysis (α=0.005, corrected for multiple comparisons). A substantial and statistically significant reduction in irradiated normal brain volume was achieved with the median V60Gy lowered from 146.3-157.3 cm 3 (range of median weekly values) with P Conv to 84.2-93.4 cm 3 for P UNITED (p<0.0001). The median max dose to the brainstem and optic chiasm was reduced from 56.1 Gy to 49.0 Gy (p=0.0001) and from 41.5 Gy to 38.4 Gy (p<0.0001), respectively. In addition to OAR sparing, 10 of 29 patients experienced suboptimal target coverage when conventional margin dose distributions were applied to weekly adapted contours, with five patients showing consistent under-dosing and CTV D98% below 50 Gy, highlighting the importance of adaptation for maintaining target coverage. The UNITED small margin weekly adaptive treatment protocol for glioblastoma improves OAR sparing and reduces the irradiated normal brain volume while maintaining consistent target coverage.