Repeatability of rectal cancer apparent diffusion coefficient measurements on a 1.5 T MR-linac

The repeatability of the apparent diffusion coefficient (ADC) during radiotherapy for rectal cancer on a 1.5 T MR-linac was investigated by acquiring two sequential diffusion-weighted imaging (DWI) sequences at each fraction. In 109 treatment sessions involving 22 patients, tumors were separately delineated on the b500 images. ADC maps were generated with all b-values (0, 30, 150, and 500 s/mm²) on the MR-linac, and the median ADC values were used in Bland-Altman analyses. A relative repeatability coefficient of 17.0 % was determined, providing a threshold to differentiate between measurement variability and true treatment response. This threshold can be used for potential response monitoring and personalized treatment adjustments.