Optimizing early response assessment in glioblastoma using diffusion imaging on a 1.5T MR-Linac

Motivation: Early response assessment for glioblastoma may be possible using regions of low apparent diffusion coefficient (low-ADC) during MRI-linear accelerator (MR-Linac) treatment, but low-ADC definition has not been optimized. Goal(s): Optimize definition of low-ADC for correlation with progression-free survival. Approach: We defined low-ADC regions from near-daily diffusion-weighted imaging and weekly contrast-enhanced T1-weighted imaging for 41 glioblastoma patients during MR-Linac treatment (3-6 weeks). We compared correlation strength across b-values (800 versus 2000s/mm2) and ADC thresholds (0.7 to 2.0μm2/ms). Results: The optimal b-value/threshold combination was b=800s/mm2 and 1.2μm2/ms (correlation for weeks 2-6). Impact: We showed that early response assessment in glioblastoma during radiotherapy is possible with weekly acquisition of ADC maps and contrast-enhanced T1-weighted imaging on MR-Linacs. Low-ADC regions could serve as targets for radiotherapy dose escalation to potentially extend patient survival.