Tumour ADC measurements in rectal cancer: effect of ROI methods on ADC values and interobserver variability

To assess the influence of region of interest (ROI) size and positioning on tumour ADC measurements and interobserver variability in patients with locally advanced rectal cancer (LARC). Forty-six LARC patients were retrospectively included. Patients underwent MRI including DWI (b0,500,1000) before and 6-8 weeks after chemoradiation (CRT). Two readers measured mean tumour ADCs (pre- and post-CRT) according to three ROI protocols: whole-volume, single-slice or small solid samples. The three protocols were compared for differences in ADC, SD and interobserver variability (measured as the intraclass correlation coefficient; ICC). ICC for the whole-volume ROIs was excellent (0.91) pre-CRT versus good (0.66) post-CRT. ICCs were 0.53 and 0.42 for the single-slice ROIs versus 0.60 and 0.65 for the sample ROIs. Pre-CRT ADCs for the sample ROIs were significantly lower than for the whole-volume or single-slice ROIs. Post-CRT there were no significant differences between the whole-volume ROIs and the single-slice or sample ROIs, respectively. The SDs for the whole-volume and single-slice ROIs were significantly larger than for the sample ROIs. ROI size and positioning have a considerable influence on tumour ADC values and interobserver variability. Interobserver variability is worse after CRT. ADCs obtained from the whole tumour volume provide the most reproducible results. Key Points aEuro cent ROI size and positioning influence tumour ADC measurements in rectal cancer aEuro cent ROI size and positioning influence interobserver variability of tumour ADC measurements aEuro cent ADC measurements of the whole tumour volume provide the most reproducible results aEuro cent Tumour ADC measurements are more reproducible before, rather than after, chemoradiation treatment aEuro cent Variations caused by ROI size and positioning should be taken into account when using ADC as a biomarker for tumour response.