Early response prediction during radiotherapy in rectal cancer using sequential diffusion-weighted imaging at a magnetic resonance image-guided linear accelerator.

Background and purpose: Varying response to chemoradiotherapy (CRT) challenges the treatment of locally advanced rectal cancer (LARC). Our purpose was to timely stratify responders by investigating the predictive potential of sequential, in-treatment diffusion-weighted (DW) magnetic resonance (MR) images. Materials and methods: DW images were acquired with a 1.5 T MR-Linac at baseline and during MR-guided CRT in 15 patients with LARC prescribed long-course CRT, prospectively enrolled between 2018 and 2021. Tumour response was classified as good or poor based on pathologic tumour regression. Changes in mean tumour apparent diffusion coefficient (ADC) were analysed by linear mixed-effects models and compared to modelling using volume. Model coefficient analysis and receiver operating characteristics (ROCs) with area under the curve (AUC) were applied for timepoint investigation of response. Results: Six patients had good response, with significant difference in median (range) baseline ADC; ADC Conclusions: Early indication of treatment response in LARC was achieved by combining baseline and sequential ADC information. These encouraging results should be validated in a larger cohort.