## 4 Discussion
For the Mean EI, we selected a cut-off value of >30.5 kPa which predicts malignancy with a sensitivity of 79.3% and specificity of 95.38% compared to the Max EI, which showed a sensitivity of 86.2% and a specificity of 81.5% for a cut-off value of 40.3 kPa. The QBox ratio anticipated malignancy with a sensitivity of 68.9% and a specificity of 92.3% for a cut-off value of 2.8. A wide range of cut-off values was determined by numerous studies, with the numbers varying broadly with the equipment that was used. Other studies performed with the same technique (2D-SWE, on Aixplorer machines by SuperSonic Imagine) found comparable cut-offs for the Mean EI \(\geq\) 27.65 kPa [49], 34.5 kPa [50] and >31.2 kPa [51], with very good diagnostic quality. Given all these similar results, we suggest a rounded cut-off value of 30 kPa that is easier to use in clinical practice. Lower cut-off values for the Mean EI were established in studies that included the entire nodule in the ROI: 23.1 kPa [52]. Some studies previously undervalued the diagnostic value of SWE by showing an important overlap among benign and malignant thyroid nodules [53]. The elastography evaluation of thyroid nodules is not standardized yet, and thus, when comparing two or more studies using the same method, it should be acknowledged that the protocols that were used might slightly differ. A considerable advantage of SWE refers to its superiority when it comes to the evaluation of nodules that coexist with autoimmune thyroid disease, whereas SE is described to have lower feasibility in this setting [39; 46]. As thyroid cancers are monoclonal proliferations, finding an area with a higher risk of malignancy inside the nodule was not our goal. The aim is to find the lesions with increased stiffness which are entirely high risk, although the measurements are made from the region with the highest stiffness (excluding artifacts). The experience of the operator is essential, particularly in the case of strain elastography. SWE usually shows better reproducibility. Nevertheless, some factors such as external compression on the transducer may interfere with the measurements. The most common type of error regarding SWE evaluation refers to operator-dependent artifacts. Accordingly, although SWE is perceived as a technique that is easier to learn, both elastography techniques should invariably be performed by trained, experienced examiners [54]. Our team described that the nodules in the proximity of the trachea, particularly located on the isthmus, may be associated with more artifacts on the SWE evaluation. For this location, SE seems to perform better. Thus, conducting another head-to-head evaluation quantifying this issue would be of interest. The strength of this study regards the comparison of the two main elastography techniques in the same population, using high-end ultrasound equipment. Our findings showed strong agreement between the SE and 2D-SWE evaluation. Both techniques generated both false-negative and false-positive results. There was discordance between the two elastography assessments in a total of 13 cases. The Mean EI and strain ratio had a very good diagnostic performance individually, but also acted as an additional parameter to the B-mode evaluation. The excellent results of both elastography methods support their use in everyday clinical practice if the techniques are available; however, their use is not meant to replace, but complete the conventional US evaluation. A considerable limitation of this study was that most of the malignant nodules were PTCs; thus, the differences in SWE parameters between different thyroid malignancies could not be estimated. We intend to continue the present study on a larger group that will hopefully include a wider histopathological variation of tumors and which will also allow for a more accurate evaluation of the microcarcinoma as a separate entity. The promising results of our study underline the importance of an experienced examiner.