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Objective: This study evaluated the correlation between intratumoural stroma proportion, expressed as tumour-stroma ratio (TSR), and apparent diffusion coefficient (ADC) values in patients with rectal cancer., Methods: This multicentre retrospective study included all consecutive patients with rectal cancer, diagnostically confirmed by biopsy and MRI. The training cohort (LUMC, Netherlands) included 33 patients and the validation cohort (VHIO, Spain) 69 patients. Two observers measured the mean and minimum ADCs based on single-slice and whole-volume segmentations. The TSR was determined on diagnostic haematoxylin & eosin stained slides of rectal tumour biopsies. The correlation between TSR and ADC was assessed by Spearman correlation (r s )., Results: The ADC values between stroma-low and stroma-high tumours were not significantly different. Intra-class correlation (ICC) demonstrated a good level of agreement for the ADC measurements, ranging from 0.84-0.86 for single slice and 0.86-0.90 for the whole-volume protocol. No correlation was observed between the TSR and ADC values, with ADC mean r s = -0.162 (p= 0.38) and ADC min r s = 0.041 (p= 0.82) for the single-slice and r s = -0.108 (p= 0.55) and r s = 0.019 (p= 0.92) for the whole-volume measurements in the training cohort, respectively. Results from the validation cohort were consistent; ADC mean r s = -0.022 (p= 0.86) and ADC min r s = 0.049 (p= 0.69) for the single-slice and r s = -0.064 (p= 0.59) and r s = -0.063 (p= 0.61) for the whole-volume measurements., Conclusions: Reproducibility of ADC values is good. Despite positive reports on the correlation between TSR and ADC values in other tumours, this could not be confirmed for rectal cancer., (Copyright © 2020 The Author(s). Published by Elsevier B.V. All rights reserved.)