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

Abstract

PurposeTo assess the dosimetric impact on organs-at-risk (OARs) and target coverage of a small-margin weekly adaptive treatment for glioblastoma delivered on an MR-Linac.Methods and materialsThe UNIty-Based MR-Linac Guided AdapTive Radiation thErapy for High GraDe Glioma (UNITED) phase 2 prospective trial used a 5 mm clinical target volume (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 changes using onboard MR imaging. In the present study, dosimetric data from 29 patients with UNITED plans (PUNITED) treated with 60 Gy in 30 fractions were retrospectively replanned with conventional margins (PConv) and compared. For each patient, 6 weekly adapted and 1 conventional margin nonadapted 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).ResultsA substantial and statistically significant reduction in irradiated normal brain volume was achieved with the median V60Gy lowered from 146.3-157.3 cm³ (range of median weekly values) with PConvto 84.2-93.4 cm³ for PUNITED (P ConclusionsThe UNITED small-margin weekly adaptive treatment protocol for glioblastoma improves OAR sparing and reduces the irradiated normal brain volume while maintaining consistent target coverage.