SWE is a new US diagnostic technology. It uses shear waves induced by acoustic radiation pulses to visualize and quantify tissue stiffness in a real-time, reliable, and reproducible manner [18]. Tissue stiffness is an important parameter for diagnosing potential malignancies or other diseases [19]. Our study showed that compared with benign breast lesions, malignant lesions had higher Emax, Emean, and Eratio, which was consistent with the results of several previous studies [20,21,22].
Furthermore, considering that preoperative knowledge of axillary lymph node status is crucial for the use of neoadjuvant chemotherapy and appropriate surgical treatment, it is necessary to develop imaging tools for LNM diagnosis [23, 24]. As expected from current trials, the main role of US on the breast in the near future may be to exclude the presence of axillary LNM [25, 26]. Our study showed that the best cut-off values of the SWE parameters for diagnosing lymphatic metastasis were calculated as follows: 107.5 kPa for Emax, 85.15 kPa for Emean, and 3.9 for Eratio.
According to ROC analysis, Eratio exhibited the best diagnostic performance with an AUC of 0.85 (0.75–0.94). Xin Wen et al. [27] reported that lesion stiffness was a predictor of axillary lymph node metastasis in breast cancer. Additionally, the optimal cut-off values of SWE parameters for predicting LNM were calculated as follows: 111.05 kPa for Emax, 79.80 kPa for Emean, and 6.89 for EmeanR. According to ROC analysis, Emax exhibited the best diagnostic performance with an AUC of 0.82 (0.76–0.87). This difference may have been caused by the following three factors: fewer cases were included in our study, the setting of the instrument and how Eratio was obtained (there is no relevant information in Wen's report), and operator-dependent changes.
Similarly, the present study demonstrated that the shape and arrangement of ECM collagen fibers are divided into three categories, which positively correlate with the Emax, Emean, and Eratio of breast lesion stiffness assessed by SWE. According to the results of all 94 lesions, Emax, Emean, and Eratio showed an increasing tendency from category 0 to category 2 (all P < 0.05).
In normal breast tissue, collagen fibers showed wavy or sinusoidal waves [13, 14]. Thus, this type of collagen fiber was assigned to category 0. Brabrand et al. reported that collagen in the peritumoral region was arranged in a more parallel alignment and aligned or linearized collagen correlated with increased stiffness of the breast tissue or lesions [10, 28]. We classified this type as class 1. Type 2 is characterized by bundles of straightened collagen fibers perpendicular to the tumor boundary, which can change into a honeycomb shape and show high hardness [16]. These results indicate that breast lesion stiffness increases when ECM collagen changes into taut, straightened and arranged parallel fibers. Additionally, a more complicated structure, collagen fibers connected in a honeycomb arrangement, indicated the highest stiffness. This indicates that ECM collagen fiber shape and arrangement may also contribute to the stiffness variance in malignant breast lesions.
The survival of patients with breast cancer is correlated with stromal biology, including the reorganization of the ECM to promote cancer invasion and migration [29]. Uddin et al. suggested that the expression of the stromal gene COL1A1 is associated with the progression of breast cancer and recurrence-free survival in breast cancer patients [30]. In our cohort, 30 tissue samples in the benign group and 64 tissue samples in the malignant group were consecutively included for COL1A1 immunohistochemical staining. Pearson correlation analysis showed a positive correlation between Emax, Emean, Eratio of breast lesions, and the expression level of COL1A1 (r = 0.406, 0.362, and 0.425, respectively; P < 0.001).
Furthermore, the average expression level of COL1A1 in benign breast lesions was significantly lower than that in malignant breast lesions. COL1A1 expression can be used to predict lymph node metastasis. An average optical density > 0.150 was taken as the cut-off value, the effectiveness was 0.80 (0.69–0.91), and the sensitivity and specificity were 96% and 59%, respectively. Moreover, many studies [31, 32] have also proved the correlation between collagen fiber content and SWE in breast lesions and proved that changes in collagen fiber content lead to changes in tumor tissue hardness. Li et al. [33] studied 148 triple-negative breast cancers and found that high COL1A1 expression in triple-negative breast cancer was an independent prognostic factor.
In addition, protein–protein interaction network analysis confirmed that COL1A1 is a prognostic matrix gene in breast cancer, and its expression is related to the progression of breast cancer [30]. These results suggest that COL1A1 may regulate tumor metastasis through some signaling pathways, and we have begun to investigate the possible signaling mechanisms using in vivo and in vitro methods.
This study had four limitations that need to be addressed. First, this study did not include the correlation between stiffness and other components of the ECM, such as elastic fibers, laminin, and fibronectin; however, collagen is the most important structural protein in the ECM, and its correlation with the elastic modulus may be the most important. Second, the number of included cases was small. Third, limited to the type of US application, examinations were performed by only one operator (no evaluation of inter-observer variability).