| Summary: | Abstract Background To determine the effect of region of interest (ROI) on tumor’s apparent diffusion coefficient (ADC) and interobserver variability in thyroid nodules. Methods Thirty-three individuals with 45 pathologically-confirmed thyroid nodules were assessed by preoperative diffusion-weighted imaging (DWI) with b values of 0 and 400 s/mm2, respectively. Two readers evaluated the ADC values of lesions based on three ROI techniques: whole-volume, single-slice and small solid-sample groups. Interobserver variability was analyzed for all ROI techniques, and the mean ADCs of benign and cancerous thyroid nodules were compared. Results For the mean ADCs of non-cancerous thyroid nodules, average differences and limits of agreement (LOAs) between readers were 0.00 [− 0.17–0.17] × 10− 3 mm2/s for whole-volume ROI (ICC = 0.967), 0.00 [− 0.26–0.26] × 10− 3 mm2/s for single-slice ROI (ICC = 0.932) and − 0.02 [− 0.38–0.41] × 10− 3 mm2/s for small solid-sample ROI (ICC = 0.823). For the mean ADCs of cancerous thyroid nodules, average differences and LOAs between readers were − 0.05 [− 0.23–0.13] × 10− 3 mm2/s (ICC = 0.885), 0.01 [− 0.23–0.25] × 10− 3 mm2/s (ICC = 0.839) and − 0.07 [− 0.52–0.39] × 10− 3 mm2/s (ICC = 0.579) for the three ROI methods, respectively. The mean ADC values were more scattered in the small solid-sample ROI group in comparison with the whole-volume and single-slice groups, in noncancerous and cancerous specimens. Of all three ROI techniques, whole-volume ROI-determined ADC had the highest combined sensitivity (80.0%), specificity (88.3%) and Youden index (0.683), with a cut-off of 1.84 × 10− 3 mm2/s. Conclusions The ROI method overtly affects ADC measurements in benign and cancerous thyroid nodules. Small solid-sample ROI yielded the worst interobserver variability of average ADC measurements.
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