BEST IN PHYSICS (MULTI-DISCIPLINARY): Treatment Response Monitoring during MR-Linac Pancreatic SBRT with Online Diffusion MRI
Abstract
Purpose
To quantify longitudinal changes in Diffusion Weighted MRI (DWI) acquired during MR-LINAC adaptive radiotherapy for pancreatic cancer and correlate with clinical outcomes.
Methods
Multi-b-value DWI (b=0,30,150,550s/mm2) were acquired during each fraction of stereotactic body radiation therapy (SBRT, 6.6/10Gy x 5 fractions) on a 1.5T MR-LINAC in N=45 consecutive pancreatic cancer patients. DWI were repeated 2-3x prior to fraction-one (Pre-RT) and once after delivery of each fraction (Fx1,2, etc.). Locoregional failure (LF) following treatment was assessed when clinical follow-up was available (N=27). DWI were reconstructed using monoexponential apparent diffusion coefficient (ADC) fitting and AI-based intravoxel incoherent motion modeling (IVIMnet) which reconstructed true diffusivity (D). At each timepoint, mean ADC and D were computed within clinical gross tumor volumes (GTV) and in low-ADC (<1.15x10-3mm2/s) subvolumes to assess the tumor core (ADCsub, Dsub). Repeatability was assessed for each parameter using the within-subject coefficient-of-variation (wCV) computed across Pre-RT repetitions. Significant longitudinal trends were assessed across the population (paired t-test against Pre-RT values). Correlations with LF were assessed using log-rank tests.
Results
Longitudinal increases in both ADCmean and Dmean were observed across fractions. Significant (p<0.05) ADCmean increases were observed at Fx≥3 whereas significant Dmean increases were observed at Fx≥1. Increases from baseline (ΔADC, ΔD) exceeded repeatability (wCVADC=2.8±2.9%, wCVD=3.1±2.6%) in 48% and 66% of fractions, respectively. LF was observed in 5/27 patients (18.5%) with available outcome data. The median time to LF was 425 days (range 214-626). Smaller changes in diffusivity were correlated with LF for ΔDmean at Fx2 and ΔDsub at Fx1-3 (all p<0.05).
Conclusion
Significant increases in GTV diffusivity were observed during pancreatic SBRT as early as one fraction into treatment. Smaller early-treatment increases were associated with LF, particularly when using IVIM reconstruction and tumor subvolume analysis. The ability to predict LF early in treatment could provide the basis for response-based adaptive therapy.