Monitoring glioblastoma dynamics during chemoradiation on the 1.5 T magnetic resonance imaging-linear accelerator

Magnetic resonance-guided radiotherapy enables repeated on-treatment imaging during chemoradiotherapy for glioblastoma (GBM), providing an opportunity to study dynamic tumor changes. We evaluated longitudinal volumetric change and spatial migration of T2/FLAIR signal abnormalities during treatment using a 1.5 T MR-Linac and examined their association with early disease progression.

GBM patients suitable for chemoradiation on a 1.5 T MR-Linac were prospectively enrolled. Regions of T2/FLAIR signal abnormalities were contoured at baseline (F0), during selected treatment fractions, and at one-month post-radiotherapy (PM1). Tumor dynamics were quantified using percentage change in volume relative to F0 (V_rel) and migration distance (d_migrate), defined as the maximum linear displacement relative to F0. Patients were classified as early progressors (disease progression within 6 months after chemoradiation) or non-early progressors. Linear mixed-effects models evaluated longitudinal differences by early progression status, methylation, and extent of resection.

Thirty-three patients were included; 28 had ≥ 6 months follow-up and 12 were classified as early progressors. In adjusted linear mixed-effects models, V_rel was higher in early progressors than in non-early progressors from F7 onward (all p < 0.025), with larger differences later in treatment (F15: 38%, p = 0.004; F30: 104%, p = 0.019). Median V_rel was 2% at F1, 2% at F15, and 18% at F30. Median d_migrate increased from 5.0 mm at F1 to 12.6 mm at F30 and was higher in early progressors from F19 onward (difference 5.9 mm, p = 0.02) and at F30 (8.5 mm, p = 0.01). At end of treatment, V_rel ≥ 10% occurred in 12/28 patients (43%) and d_migrate ≥ 10 mm in 13/28 (46%). Among patients with V_rel ≥ 10%, 10/12 (83%) were early progressors; among those with d_migrate ≥ 10 mm, 9/13 (69%) were early progressors.

Serial 1.5 T MR-Linac imaging enables longitudinal assessment of T2/FLAIR abnormality volume (V_rel) and spatial migration (d_migrate) during chemoradiotherapy for glioblastoma. Greater increases in V_rel and d_migrate were associated with early progression, supporting the potential role of on-treatment imaging in informing risk-adapted MR-guided radiotherapy strategies.