## 4 Discussion
In addition, the method used in the present study allowed for a more comprehensive assessment of the nodule because the ROI covered the entire nodule in the image. Our result suggested that the tissue stiffness within malignant thyroid nodules are varied, with some areas are significantly stiffer than benign nodules (as demonstrated by significantly higher \(E_{\text{maximum}}\) and \(E_{\text{mean}}\) in malignant nodules), whereas some areas have similar stiffness as benign nodules (as demonstrated by the similar \(E_{\text{minimum}}\) between benign and malignant nodules). This may be related to the uneven distribution of tumor cells within the nodule. Results of our study demonstrated that \(E_{\text{maximum}}\) and \(E_{\text{mean}}\) are potential predictors for thyroid malignancy when using cut-off value of 67.3 kPa and 23.1 kPa, respectively. However, \(E_{\text{minimum}}\) has limited value in distinguishing benign and malignant nodules. In the present study, we evaluated the diagnostic performance of GSU alone and a combination of GSU and SWE. Results showed that the overall diagnostic accuracy of GSU alone was 58.5% and it was increased to 80.2% and 78.4% when it combined with \(E_{\text{maximum}}\) (cut-off: 67.3 kPa) and \(E_{\text{mean}}\) (cut-off: 23.1 kPa), respectively. Our results also demonstrated that when GSU combined with SWE, and the specificity increased from 46.4% to 83.3% when using \(E_{\text{maximum}}\) and to 79.8% when using \(E_{\text{mean}}\). However, the sensitivity decreased from 96.3% to 70.4% and 74.1%, respectively. The finding was different from previous reports. In [PERSON] et al. [38], they reported that the combination of GSU with SWE did not significantly improve the diagnostic accuracy of malignant thyroid nodules. In the other studies, there was no significant difference in the diagnostic accuracy of GSU alone and combination of GSU and SWE indices in distinguishing benign and malignant thyroid nodules, 86.3% and 87.2% [34]; 81% and 77.9% [39]; and, 60.3% and 60.3% [36], respectively. They also found that when combined GSU with SWE indices the specificity decreased but the sensitivity increased [34; 36; 39]. One previous study that analysed thyroid nodules in population of France and found that there was an increase in sensitivity from 77.1% to 97.1% however specificity was reduced from 58.0 to 55.3% when grey scale ultrasound was added to \(E_{\text{maximum}}\) 65 kPa [36]. Similar results were obtained by another investigation conducted on thyroid nodules in Korean population and found that sensitivity was improved from 92.9% to 95% while specificity was reduced from 60.8% to 56.7% on addition of \(E_{\text{minimum}}\) 85.2 kPa. with grey scale ultrasound [34]. Another study conducted in China found an increase in sensitivity from 76.2% to 87.1%, while specificity was reduced from 83.0 to 73.9 kPa on addition of grey scale ultrasound to \(E_{\text{minimum}}\) 39.3 kPa [39]. The different result of previous reports and the present study was due to the different criteria in determining malignant thyroid nodules when combined with GSU with SWE. In previous studies, thyroid nodules were malignant when they either had one or more malignant grey scale sonographic features or had a SWE index value greater than the cut-off. This criterion increased the sensitivity by having more true-positive findings, but it also increased the number of false-positive findings leading to decreased specificity [34; 36; 38; 39]. Sincethe extent of changes of specificity and sensitivity were similar, there was no significant improvement in the overall diagnostic accuracy when GSU combined with SWE in previous reports. However, in the present study, we considered thyroid nodules to be malignant when they had both malignant grey scale sonographic features (one or more features) and had a SWE index value greater than the cut-off.