Test-retest Repeatability of Echo Planar Imaging Diffusion-weighted MRI on a 1.5T MR-linac for Head and Neck Cancers

Motivation: In order to use DWI on MR-linacs to adapt head and neck cancer radiotherapy treatment plans based on response, the variability in ADC must be characterized. Goal(s): To quantify the repeatability of ADC on a 1.5T MR-linac in a large cohort (37 head and neck cancer patients). Approach: Patients were imaged with echo planar imaging-DWI twice before the start of radiotherapy. Mean ADC values of primary tumors and lymph nodes were compared across time points using repeatability metrics. Results: Repeatability coefficients were 53.0%/35.5% for tumors/nodes, indicating that this DWI sequence is insufficient for detecting clinically significant ADC changes and must be further optimized. Impact: Our DWI test-retest results demonstrate that the current widely implemented EPI-DWI sequence for head and neck cancers on 1.5T MR-linacs has substantial ADC variability across time points and needs to be further refined.