In the present study, we confirmed that SR measurement through EUS-EG was useful for differential diagnosis in SPL. The SR value of PDAC patients was significantly higher than that of MFP and PNEN patients. Even after propensity-score matching, which corrects demographic difference, the SR values remained significantly different between MFP and PDAC patients. Moreover, we presented an optimal cut-off SR value that can be used for differential diagnosis of MFP and PDAC. The cut-off value presented in this study has the advantage that it is independent of demographics and shows improved accuracy compared to measures reported in previous studies.10,11
EUS-EG has been used for the diagnosis of various pancreatic diseases and prediction of clinical course. EUS-EG provides more objective information than Rosemont classification for the endoscopic diagnosis of chronic pancreatitis.12 Confirming the degree of pancreatic fibrosis through the SR value helped determine the probability of exocrine insufficiency in patients with CP.13 Additionally, measuring tissue hardness can be helpful in the differential diagnosis between hypervascular lesions, pancreatic neuroendocrine tumors, and metastatic malignant lesions.14 Strain elastography has also been used to estimate the risk of fistula formation after pancreatic surgery.15–17
As mentioned above, various efforts continue to improve the diagnostic accuracy of EUS for the differential diagnosis of SPL through elastography. Qualitative elastography, based on the color pattern of SPL, was mainly used for differential diagnosis in the early period. In the qualitative method, the stiffness of SPL was assessed by color predominance and distribution (homo- or heterogeneity).18,19 Qualitative EUS showed a relatively high sensitivity for the diagnosis of malignant SPLs but is a subjective evaluation method. Therefore, there may be inter-observer disagreement.20,21 To compensate for the shortcomings of the qualitative method, more objective semi-quantitative methods, such as SR and strain histogram, have been used.6,11,22,23
Semi-quantitative methods express SPL stiffness as an SR value or a mean histogram value. Previous studies using semi-quantitative EUS-EG showed that the SR value of a malignant mass was higher than that of an MFP and could be helpful in the differential diagnosis.10,11 In these studies, an optimal cut-off value for differential diagnosis had also been proposed, but there was a limitation in that external validation was not performed in a population other than patients who were investigated. We investigated whether the cut-off values presented in other studies showed high accuracy as well in our patients and noted that sensitivity, specificity, and accuracy were inferior compared to results from our cut-off value. In one study, 6.04 was presented as a cut-off value and showed 96.0% of accuracy for the differential diagnosis between PDAC and inflammatory mass.10 Another study presented 4.65 as their value, with an 86.5% accuracy.11 However, in this study population, their accuracy was only 52.27% each when their cut-off points were used (Table 4). In previous studies, it is thought that it was difficult to compare MFP and PDAC effectively because a relatively small number of patients with benign disease was included. To find an optimal cut-off value, additional research is needed to confirm whether the cut-off values presented in various studies have utility in other population groups as well.
However, there are still limitations when using EUS-EG for differential diagnosis. It is not easy to obtain a stable image lasting more than 5 seconds due to persistent patient breathing and normal bowel movements. The SR value, calculated by B/A, can be changed greatly even a small change in A, a strain value of ROI. If the degree of fibrosis inside the SPL is heterogeneous, it may be difficult to obtain a consistent SR value even through repeated measurements. In the present study, to overcome these limitations, the ROI was selected as a section that was large as possible in order to reflect the characteristics of the entire mass, sufficient time was spent for stable measurement of strain values, and the average of the more than three strain values was used.
Recently, a new method of elastography, which provides absolute tissue hardness through shear-wave velocity rather than SR, was introduced. However, this also has limitations, with further requirement for validation.24 Finally, EUS-EG is still an adjunctive method and is not a confirmatory diagnostic modality. There is thus, a limit to merely providing auxiliary information.
This study had several limitations. First, when compared to MFP and PDAC patients, relatively small numbers of PNEN patients were investigated. Due to a lack of PNEN patients, the optimal cut-off value for differential diagnosis between the three groups could not be suggested. The incidence of PNEN is relatively low, and the patients suspected of having a small-sized PNEN are often followed up using only imaging tests without a pathological diagnosis. Therefore, we subsequently investigated limited numbers of PNEN patients. Second, this study had a retrospective nature and was conducted by a small number of endosonographers. Therefore, a large-scale study conducted by a large number of endosonographers in a multicenter is needed to suggest universal findings. It is necessary to check whether there are differences depending on the echoendoscope and processor and whether our results were not influenced by the interference of the adjacent organs according to the location of the SPL. Despite these limitations, a strength of this study was that it was a well-organized study that investigated the cut-off SR value for differential diagnosis of SPL. This study presented relatively reliable data that suggested a cut-off score after propensity score matching in a large number of PDAC and MFP patients.
In conclusion, we provided the SR values of various SPLs through a large, retrospective study investigating quantitative EUS-EG. Moreover, we suggested optimal cut-off values for differential diagnosis between MFP and PDAC. Although tissue acquisition is essential for confirmative diagnosis, EUS-EG can provide helpful information in these cases.