In Vivo Test-retest Quantitative Characterization of Echo Planar Imaging Apparent Diffusion Coefficient Reproducibility for Head and Neck Cancers on a 1.5T MR-Linac Platform: Technical Validation using QIBA Metrology

Background and PurposeTo detect changes in apparent diffusion coefficient (ADC) values during radiation therapy for biological image-guided adaptive radiation therapy, the variability in ADC must be characterized. We evaluated the reproducibility of ADC values in head and neck cancers on a 1.5T MR-linac. Methods39 head and neck cancer patients (36 primary tumors, 55 lymph nodes) were imaged with echo-planar imaging diffusion-weighted MRI on a 1.5T MR-linac at two time points before the start of radiation therapy. Mean and median ADC values and volume were measured for each lesion. Absolute and percent reproducibility coefficients (RC) were calculated. Linear regression analyses and F-tests were performed to determine whether lesion volume or time between scans impacted reproducibility. ResultsFor primary tumors & lymph nodes: mean ADC, median ADC, and volume were 1.27 {+/-} 0.33 mm2/s & 1.17 {+/-} 0.34 mm2/s, 1.25 {+/-} 0.35 & 1.16 {+/-} 0.37 mm2/s, and 8.8 {+/-} 12.3 cm3 & 6.5 {+/-} 7.2 cm3, respectively. RC values of mean ADC were 0.355 mm2/s & 0.355 mm2/s for tumors & nodes, and %RC values were 29.1% & 31.1%; values were very similar for median ADC. Reproducibility was not significantly correlated with either volume or scan interval, but a trend of poorer reproducibility in smaller volumes was observed. ConclusionConsidering previous reports that the optimal %{Delta}ADC threshold for response prediction in head and neck cancers is around 15-30%, this sequence on the MR-linac has acceptable reproducibility for detecting larger ADC changes but may still miss some clinically significant changes.