Diffusion Weighted Imaging for Differentiating Benign from Malignant Orbital Tumors: Diagnostic Performance of the Apparent Diffusion Coefficient Based on Region of Interest Selection Method

Objective: To evaluate the differences in the apparent diffusion coefficient (ADC) measurements based on three different region of interest (ROI) selection methods, and compare their diagnostic performance in differentiating benign from malignant orbital tumors. Materials and Methods: Diffusion-weighted imaging data of sixty-four patients with orbital tumors (33 benign and 31 malignant) were retrospectively analyzed. Two readers independently measured the ADC values using three different ROIs selection methods including whole-tumor (WT), single-slice (SS), and reader-defined small sample (RDSS). The differences of ADC values ($ADC-ROI_{WT}$, $ADC-ROI_{SS}$, and $ADC-ROI_{RDSS}$) between benign and malignant group were compared using unpaired t test. Receiver operating characteristic curve was used to determine and compare their diagnostic ability. The ADC measurement time was compared using ANOVA analysis and the measurement reproducibility was assessed using Bland-Altman method and intra-class correlation coefficient (ICC). Results: Malignant group showed significantly lower $ADC-ROI_{WT}$, $ADC-ROI_{SS}$, and $ADC-ROI_{RDSS}$ than benign group (all p < 0.05). The areas under the curve showed no significant difference when using $ADC-ROI_{WT}$, $ADC-ROI_{SS}$, and $ADC-ROI_{RDSS}$ as differentiating index, respectively (all p > 0.05). The $ROI_{SS}$ and $ROI_{RDSS}$ required comparable measurement time (p > 0.05), while significantly shorter than $ROI_{WT}$ (p < 0.05). The $ROI_{SS}$ showed the best reproducibility (mean difference ${\pm}$ limits of agreement between two readers were $0.022[-0.080-0.123]{\times}10^{-3}mm^2/s$; ICC, 0.997) among three ROI methods. Conclusion: Apparent diffusion coefficient values based on the three different ROI selection methods can help to differentiate benign from malignant orbital tumors. The results of measurement time, reproducibility and diagnostic ability suggest that the $ROI_{SS}$ method are potentially useful for clinical practice.