Application of Transthoracic Shear Wave Elastography in Evaluating Subpleural Pulmonary Lesions.

AIM: The objective of this research was to investigate the feasibility of transthoracic shear wave elastography in the differentiation of subpleural masses.
METHODS: Between December 2019 and November 2020,82 consecutive patients with radiographic evidence (including chest X ray and thoracic computed tomography CT) of single subpleural lesion enrolled in this research. The Young's modulus E (including Emean and Emax) of each lesion was detected, and the Young's modulus E of malignant lesions were compared with those of benign ones. We made diagnoses according to the results of pathology or standard clinical course for at least 3 months. Receiver operating characteristic (ROC) analysis was plotted to determine the cut-off point by maximizing the Youden index.
RESULTS: The Emean and Emax of the benign and malignant group was 34.68 ± 12.12 kPa vs. 53.82 ± 11.95 kPa (p < 0.001), 57.77 ± 14.45 kPa vs. 76.62 ± 17.04 kPa (p < 0.001). The ROC of Emean showed that when the cut-off point was 43.8 kPa, the Youden index (0.53) for distinguishing benign and malignant tumors was the largest (sensitivity 80.4 %, specificity 72.2 %, AUC = 0.848, p < 0.0001). When the cut-off point recommended by Emax ROC was 73.5 kPa, the Youden index (0.44) for distinguishing benign and malignant tumors was the largest (sensitivity 76.1 %, specificity 66.7 %, AUC = 0.780, p < 0.0001).
CONCLUSIONS: This study demonstrated that we can employ transthoracic shear wave elastography as a valuable instrument in differentiating benign subpleural lesions from malign ones.

Introduction

Despite achievements has been made so far, lung cancer remained to be the leading reason of cancer death globally [1]. Early diagnosis of lung carcinoma provided better prognosis and good reference for therapeutic plan. Except for the inherent advantages of ultrasound [2], transthoracic ultrasonography has become a widely used radiographic tool for the diagnosis of peripheral lung lesions [3], benefiting from the improvement in imaging capacities and penetrating power [4].

B-mode ultrasound could provide morphologic imaging of lesions, including the shape, echogenicity, margin and blood flow [5]. However, no specific B-mode characteristic could distinguish benign lesions from the lung carcinoma masses precisely [6].

Compared with B mode ultrasound, ultrasound elastography (UE)which was firstly developed by Ophir in the 1990s [7] could assess the elasticity and stiffness of tissue that could be changed by pathological or physiological processes [8]. In the conventional Strain Elastography [9], the radiologist compresses the target organ manually using ultrasound transducer, so that the induced tissue deformation could be measured. While, in SWE [9], we apply ARFI (acoustic radiation force impulse) to deform tissue. Unlike the single focal location in ARFI strain imaging and PSWE, multiple lesion regions are questioned in fast frequency which creates a near-cylindrical shear wave cone by which can we measure the speed of the shear waves and converted it to Young’s modulus E to provide quantitative evaluation of tissue stiffness. What’s more, strain elastography was more operator dependent and less reproducible in comparison with SWE. At present, SWE is broadly applied in various apparatus [[10], [11], [12], [13], [14]] with great feedback. However, there has been limited research in the usability of SWE in peripheral pulmonary lesions [15]. Therefore, we launched this research to investigate the value of SWE in subpleural disease.

Material and methods

In this study, 82 patients with radiographic evidence (including chest X ray and pectoral computed tomography CT) of single subpleural solid lesions between December 2019 and November 2020 participated in the investigation. Patients with a poor image quality, great influence of heart or great vessels, or multiple pleural effusion [16,17]causing unacceptable shear-wave propagations [18], and those who were unable to hold their breath for at least 5 s were excluded [19]. All the patients included in this study underwent a transthoracic B mode ultrasound examination and subsequently a SWE examination in the target pulmonary lesion by the same radiologist with 10 years’ experiences of pulmonary ultrasound who was blind to CT or pathology results. This study got approval from the research ethic committee of the hospital and all patients enrolled in this research signed informed consent before examination.

We used the Super Sonic Aixplorer Ultrasound Machine with a 1–6 MHz convex transducer [20] which could provide adequate penetration into chest wall for chest ultrasound examinations. According to the location of the lung mass shown on the CT images of the patient, we asked the patients to take appropriate posture (such as sitting, supine, prone etc.) to extend the intercostal space covering the peripheral pulmonary lesions as much as possible. And then the convex probe was gradually moved to the intercostal space and kept parallel to the ribs as far as possible to get sonographic image of lesion of great quality.

Afterwards, the SWE examination was performed as plan. Patients were told to hold their breath and keep steady for 5 s for stabilized images. We adjusted the diameter of ROIs (region of interest) according to the size of lesion. ROIs were placed in the solid and homogenous component of the identified lesion to improve the precision of this examination [21]. We selected 5 ROIs with different location, recording their Emax and Emean. Take the average of 5 measurements respectively, and select clear and stable image to store.

Cases whose CT findings suggested a benign trend were given standard medical treatment [22]. About 3 months later, those whose lesions vanished in the following thoracic CT or US were finally diagnosed as benign. While the final diagnosis was made referring to results of US or CT biopsy or surgical operation for patients without radiographic remittence and who were initially thought to be malign in CT.

Data used in this study were presented as mean ± SD. We routinely used Kolmogorov-Smirnov Test and Levene 's Test to determine whether to use independent student’s t test (normal distribution and equal variances assumed) or Wilcoxon test (normal distribution and equal variances not assumed) to statistically compare the baseline characteristics and Young’s modulus E (Emax and Emean) of benign and malign cohorts. While, for the comparison of Young’s modulus E in various pathological cohorts in the malignant cases, we used One-way ANOVA (analysis of variance) instead. We plotted a ROC curve and obtained the cut-off point. Z test was used in the AUC comparison between Emax and Emean. Furthermore, we used sensitivity, specificity, positive predictive value, negative predictive value, accuracy, positive likelihood ratio, and negative likelihood ratio to evaluate the diagnostic performance of SWE. With the data collected, we conducted a multi-factor logistic regression analysis to find the probable predictor of pulmonary cancer. All statistical analyses were carried out using SPSS 20.0 software. A p value of 0.05 was considered to be statistically significant.

Results

52 males and 30 females were involved in this research, mean age 62.88 ± 11.57 years (range, 37–83 years). Forty-nine (59.7 %) of the cases had smoking habit, while 33 (40.3 %) didn’t smoke at all. Thirty-six lesions were diagnosed as benign (32 pneumonia, 4 tuberculosis) and forty-six lesions were diagnosed as malignant, including 43 primary lung cancer (25 adenocarcinomas, 11 squamous cell carcinomas, 3 large cell lung cancer,4 small cell lung cancer) and 3 metastatic lung cancer (one breast carcinoma, two colon adenocarcinoma). Among the 46 patients finally diagnosed of pulmonary cancer, 37 cases underwent ultrasound-guided biopsy, 6 patients received computed tomography-guided lung biopsy, 3patients underwent surgical biopsy. Of all the cases whose radiographic results suggested a benign trend getting standard clinical course, 27 cases were diagnosed as benign for their followed chest radiography showing complete remission, while 9 cases who didn’t get remission underwent UGNAB (ultrasound guided needle aspiration biopsy) for precise diagnosis.

The clinical characteristic of the cancerous and benign group was presented in Table 1. The mean age of patients in malignant and benign group was 60.56 ± 10.58 years vs. 62.7 ± 12.08 years (p = 0.108). The proportion of man was significantly different in the two groups (73.9 % vs. 50 %, p = 0.026). We found difference between the two groups in the proportion of patients who had smoking history (69.6 % vs. 47.2 %, p = 0.041).

Table 1

Clinical characteristics of the 82 patients in the study.

Characteristics	Data
Age, y, mean ± SD	62.88 ± 11.57
Male, n(%)	52 (63.4)
Smoking, n(%)	49(59.8)
Benign Lesions, n(%)	36(43.9)
Pneumonia	32(39.0)
Tuberculosis	4(4.9)
Malignant Lesions, n (%)	46(56.1)
Adenocarcinoma	25 (30.4)
Squamous cellcarcinoma	11(13.4)
Small cell lung cancer	4(4.9)
Large cell lung cancer	3(3.7)
Metastatic lung cancer	3(3.7)

SD = standard deviation.

Table 2 listed the comparisons of the Young’s modulus E between the cancerous and benign group. Compared with benign lesions, cancerous nodules had greater Emean (53.82 ± 11.95 kPa vs. 34.68 ± 12.12 kPa; p < 0.0001). Similarly, compared with benign lesions, cancerous nodules had greater Emax (76.62 ± 17.04 kPa vs. 57.77 ± 14.45 kPa; p < 0.0001).

Table 2

Young’s modulus E of benign and malignant peripheral pulmonary lesions.

Young’s modulus E	Malignant	Benign	p-value
Emean,(kPa)	53.82 ± 11.95	34.68 ± 12.12	<0.001
Emax, (kPa)	76.62 ± 17.04	57.77 ± 14.45	<0.001

The Emean and Emax were compared between the five different pathological group in malignant cases in Table 3. The Emean of squamous cell carcinoma was significantly different from that of adenocarcinoma (55.68 ± 12.33 vs. 51.18 ± 11.9, p = 0.023), large cell lung cancer (55.68 ± 12.33 vs. 58.38 ± 6.27, p = 0.018), metastatic lung cancer (55.68 ± 12.33 vs. 52.44 ± 5.95, p = 0.000). The Emax of squamous cell carcinoma was significantly different from that of adenocarcinoma (81.65 ± 13.81vs. 73.93 ± 8.94, p = 0.000), small cell lung cancer (81.65 ± 13.81 vs. 73.08 ± 12.18, p = 0.025), metastatic lung cancer (81.65 ± 13.81 vs. 83.40 ± 4.55, p = 0.0038). There was no significant difference in Emean (53.22 ± 11.94 vs. 52.44 ± 5.95, p = 0.730) and Emax (75.66 ± 14.58 vs. 83.40 ± 4.45, p = 0.371) between primary lung cancer and metastasis carcinoma.

Table 3

Comparison of the Young’s modulus E of the malignant lesions with different pathology pattern.

Pathology	p-value (Emean)	p-value (Emax)
Adenocarcinoma
versus squamous cell carcinoma	0.023	0.000
versus small cell lung cancer	0.533	0.285
versus large cell lung cancer	0.418	0.518
versus metastatic lung cancer	0.216	1.000
Squamous cell carcinoma
versus small cell lung cancer	0.085	0.025
versus large cell lung cancer	0.018	0.271
versus metastatic lung cancer	0.000	0.038
Small cell lung cancer
versus large cell lung cancer	0.267	0.492
versus metastatic lungcancer	0.419	0.760
Large cell lung cancer
versus metastatic lung cancer	0.152	0.486
Primary lung cancer
versus metastatic lung cancer	0.730	0.371

We plotted ROC curves and obtained the cut-off point by maximizing the Youden index in Fig. 1. The ROC of Emean showed that when the cut-off point was 43.8 kPa, the Youden index (0.53) for distinguishing benign and malignant tumors was the largest (sensitivity 80.4 %, specificity 72.2 %, AUC = 0.848, p < 0.0001). When the cut-off point recommended by Emax ROC was 73.5 kPa, the Youden index (0.44) for distinguishing benign and malignant tumors was the largest (sensitivity 76.1 %, specificity 66.7 %, AUC = 0.780, p < 0.0001) Table 4. While, we didn’t find statistic difference between AUC of Emean and Emax (p = 0.1586) in Table5.

Fig. 1

Receiver operating characteristic analysis of Emean and Emax.

Table 4

Diagnostic performance of cut-off point of Emean and Emax in predicting malignant lesions.

	SEN (%)	SPE (%)	PPV (%)	NPV (%)	Accuracy (%)	LR+	LR-
Emean> 43.8kPa	80.4	72.2	78.7	74.3	76.8	2.89	0.27
Emax> 73.5 kPa	76.1	66.7	74.5	68.6	71.9	2.29	0.36

SEN, sensitivity; SPE, specificity; PPV, positive predictive value; NPV, negative predictive value; LR+, positive likelihood ratio; LR- negative likelihood ratio.

Table 5

AUC comparison between Emax and Emean in diagnosing pulmonary carcinoma.

	Emean	Emax
AUC	0.848	0.780
95 % ConfidenceInterval	0.0262 – 0.160
Z-value	1.410
p-value	0.1586

In logistic regression, age, sex, smoking habit, Emean and Emax were included in the regression model (Table 6), which indicated that sex(male), Emean and Emax were the independent predictors of lung cancer, with odds ratios of 18.66 (p = 0.011), 1.88(p = 0.008) and 1.53 (p = 0.032), respectively.

Table 6

Binary logistic regression analysis of factors associated with malignancy.

Variable	B	SE	p-value	OR	95 % Cl for OR
Age	0.064	3.72	0.054	1.066	0.99−1.137
Emean	0.633	7.052	0.008	1.884	1.180−3.007
Emax	0.426	4.610	0.032	1.532	1.038−2.206
Sex(male)	2.927	6.390	0.011	18.663	1.930−180.468
Smoking	0.458	0.212	0.645	1.581	0.225−11.297

OR = odds ratio.

Discussion

This research aimed to verify the feasibility of transthoracic SWE in distinguishing between malignant and benign peripheral pulmonary masses. The cut-off point of Emean (43.8 kPa) and Emax (73.5 kPa) suggested in this study had great performance in distinguishing benign from malignant lesions. For different pathological group in malignant cases, we could only find differences of SWE between squamous cell carcinoma and other malignant cohort. While, we can hardly tell squamous cell carcinoma from other pulmonary cancer using this result for the reason that we didn’t find valid cut off point of squamous cell carcinoma using SWE. There were not differences between primary lung cancer and metastatic cancer in this research, which indicated that SWE had limited value in differential diagnosis between various pathological cohort. The Regression model conducted by us showed that sex(male), Emean and Emax were the independent predictors of lung cancer.

It’s apparent that malignant masses are stiffer than the benign ones (Fig. 2A, 3A) owing to excessive fibrillar collagen accumulations and abnormal blood vessel formation resulting from disorder of the homeostasis that governs extracellular matrix synthesis and turnover [23]. Chemokines, cytokines, paracrine signaling of growth factors and autocrine all impact the process of tumor desmoplasia. In a word, extracellular matrix reorganization and remodeling are dominant factors that influence the stiffness of tumor tissue. Until now, integrin α11β1, IGF-2, CLCF1 are the molecules founded to participate in the procession of fibrosis in Non-small cell lung cancer, which explained our results molecularly [24].

Fig. 2

Conventional B mode along with SWE US images (2A) and thoracic CT images (2B) of subpleural lung lesion in a 53-years-old male who was diagnosed as pneumonia for the remission after 3 months’ standard clinical course. We learned that the mean Emean of this lesion was 30.7 kPa, while the Emax of the 5 ROIs was noted respectively to calculate the mean Emax 38.9 kPa.

Conventional B-mode ultrasound could provide morphologic imaging of lesions. However, it was hard for B-mode ultrasound to differentiate between pneumonia which was heterogeneous and necrosis caused by lung cancer. SWE is the newest radiological tool applied to evaluate tissue stiffness quantitatively and objectively by which can we can distinguish target lesion from surrounding tissue easily. Therefore, we can assume that these two ultrasound modes are complementary in the diagnosis procedure. For clinicians, SWE could help them to differentiate benign lesions from malignant ones preliminarily and determine which lesion to be biopsied [25]. Besides, we could perform procedures such as biopsy guided by ultrasound more precisely and safely because radiologist can accurately locate the target region using SWE in the premise that the region of greater malignancy is more likely to be stiffer, which presented as a higher Young’s modulus E (Fig. 3).

Fig. 3

Conventional B mode along with SWE US images (3A) and thoracic CT images (3B) of subpleural lung lesion in a 71-year-old female who received left pneumonectomy after diagnosis of left squamous cell carcinoma. We found subpleural lesion in right lung in the following thoracic CT and she was diagnosed as squamous cell carcinoma of right lung with the pathological results of US biopsy. We learned that the mean Emean of this lesion was 87.5 kPa, while the Emax of the 5 ROIs was noted respectively to calculate the mean Emax 105.8 kPa.

We noticed that two patients who were finally diagnosed as tuberculosis were misdiagnosed as pulmonary cancer with SWE. Their Emean and Emax were 106.8 kPa, 67.4 kPa and 120.6kpa, 95.5 kPa, respectively. On account that lesion fibrosis is the usual pathological reaction of tuberculosis, we found that it was hard for us to distinguish malignant lesions from tuberculosis ones, especially for chronic fibrocavernous pulmonary tuberculosis patients (Fig. 4).

Fig. 4

Conventional B mode along with SWE US images (4A) and thoracic CT images (4B) of subpleural lung lesion in a 66-year-old female who was diagnosed as tuberculosis with the pathological results of US biopsy. We learned from the figure that the portion which was proved to be necrosis and fibrosis of tuberculosis was stiff with mean Emean of 106.8 kPa and Emax of 120.6 kPa, which indicated our misdiagnose with SWE.

Mesut Ozgokce et al. [26] indicated that metastatic lesions (SWV = 4.12 m/s) was harder than primary lung cancers (SWV = 3.43 m/s). While, we didn’t find differences in Young’s modulus E between them using SWE. Cédric Zeltz et al. [23] pointed out that different carcinoma has different molecules that participate in tumor fibrosis which could cause diverse Young’s modulus E in different carcinoma. Because of the small sample size of metastatic cancer patients in this research, we all agreed that large sample size of metastatic cancer patients was necessary to verify our results. Sperandeo et al. [5] found that malignant masses showed less elasticity compared with benign ones . Obviously, we got our results in common. However, he used conventional UE and score from 1 to 5 to evaluate the stiffness of nodules which is more complicated and less objective compared with SWE. Yao-Wen Kuo et al. [18]. found that transthoracic SWE had predictive value in differential diagnosis in peripheral pulmonary lesions and 65 kPa to be the appropriate cut off point. While, 65 kPa is higher than 43.8 kPa gained from our research. As the diameter differed significantly among the target lesions, we selected appropriate diameter of ROIs to cover the major homogeneity of lesion which was different from theirs. So far as we acknowledged, diameter as a parameter of SWE for subpleural lung lesions was not standardized. Generally speaking, there has been limited research in the literature, and future research will significantly contribute to verify our results.

There were several limitations in this study. First, we measured the speed of shear waves which was transformed to Young’s modulus E to objectively evaluate the stiffness of lesions. However, lung is a well aerated organ making it easy for shear wave to decay which could decrease the accuracy of Young’s modulus E measurement. Hence, acceptable depth of lung tissue for radiologist to select as ROIs has maintained to be investigated. Second, many patients were excluded from this examination for reasons such as being not able to hold their breath for at least 5 s, great influence of heart or great vessels, or multiple pleural effusion causing unacceptable shear-wave propagations. These limited the clinical use of SWE in evaluation of subpleural lesions. Finally, the sample size was small on account of limited time.

Conclusion

To sum up, SWE can distinguish between malignant and benign subpleural lung lesions with great diagnostic performance and could be a potential option for the clinician to choose, assessing the malignancy potentials of peripheral lung lesions noninvasively. For the patients whose lesions had high Young’s modulus E(Emean﹥43.8 kPa, Emax﹥73.5 kPa), pathological examination should be strongly recommended because of the high malignant possibility.

Ethic statement

This study got approval from the research ethic committee of this hospital and all patients participating in this study signed informed forms before examination.

Funding statement

There is no funding available.

Credit author statement

The manuscript has not been published before and is not being considered for publication elsewhere. All authors have contributed to the creation of this manuscript for important intellectual content and read and approved the final manuscript.

Declaration of Competing Interest

The authors report no declarations of interest.