In patients with soft-tissue masses and a definitive histological diagnosis, malignant tumors had significantly higher elasticity and T/F values than did benign tumors. However, sensitivity was limited for both elasticity (50% with a cutoff of 40.8 kPa) and T/F (46% with a cutoff of 3.5). Specificity was higher (79 and 84%, respectively). When we performed a separate analysis of tumors with and without fat components, we found that the differences in elasticity and T/F between malignant and benign tumors were significant only for the tumors devoid of fat.
US and MRI often fail to confidently separate benign from malignant in soft-tissue masses. Malignant tumors have been reported to be harder in consistency than benign tumors, notably breast and thyroid carcinomas [20]. The abundance of stromal fibrous reaction in some carcinomas, for example in breast carcinoma, papillary thyroid carcinoma or pancreatic adenocarcinoma, or the high cell density in some malignant tumors such as prostate and lung carcinomas, combined with the strong attachment of malignant cells to surrounding tumors, may explain this finding [21]. However, in a prospective study of 50 STTs done at a sarcoma center, longitudinal SWV was 30% slower in malignant than in benign tumors, indicating greater softness of the malignancies, and did not vary significantly with age, sex, or tumor size [22]. In soft tissue tumors, cell density is not always proportional to malignancy, unlike in epithelial tumors. The considerable variability in tissue structure of STTs and heterogeneity of mesenchymal tumors in cell density and stroma (absent, fibrous, myxoid, hemorrhagic, highly or poorly vascularized) create diagnostic challenges compared to organs where tumor structure is more uniform, such as papillary thyroid carcinoma. The absence of significant differences in elasticity and T/F in our study may be ascribable to the considerable variability in tissue structure of soft-tissue masses. Thus, an evaluation of 48 superficial STTs showed that epidermoid cysts were stiffer than were ganglion cysts and lipomas, in keeping with their greater hardness to palpation [23]. Moreover, selection bias occurred in our study, as we included only patients deemed eligible for a biopsy. Thus, most patients with benign lipoma-type lesions that did not require further investigation were not studied. The lipomas in our cohort had atypical presentation due to tissular alterations, resulting in diagnostic uncertainty. An earlier study in a smaller sample (n = 32) included a high proportion of lipomas and had no hard benign masses such as myositis ossificans [24]. In a feasibility study of 32 superficial STTs, of which 12 were malignant, greater hardness correlated with a greater risk of malignancy [24]. When used with US, SWE improved the discrimination between benign and malignant masses in soft-tissue in a study of 206 tumors, including 79 malignancies, diagnosed by biopsy [17]. The diagnostic contribution of SWE varied according to patient age and to whether the tumor was superficial or deep to the superficial fascia. No elasticity or T/F cutoffs were determined. Ultrasound measurements of mean and maximum SWV for 43 benign and 37 malignant STTs found no significant differences in either parameter [21]. SWV was not associated with the malignant or benign nature of STTs in studies of 105 and 151 tumors [16]. These differences across studies may be due to differences in the structure of the STTs, with variability regarding fibrotic changes, bleeding, necrosis, myxoid changes, calcification, and cystic degeneration [25].
One of the original features of our study is the measurement of the elasticity ratio of the tumor and the surrounding fatty tissue (T/F). However, this parameter did not add meaningfully to the diagnostic performance of SWE. In addition, the border between the mass and the surrounding unaffected tissue can be difficult to assess for infiltrating tumors. Strain elastography has been reported to improve the diagnosis of soft-tissue masses, with a significantly lower strain ratio for benign tumors, although strain histograms and visual scoring showed no significant differences [26].
Maximum median diameter was greater in the malignancies in the overall cohort and the subgroup of fatty tumors. None of the other US features differed significantly between the benign and malignant soft-tissue masses. A study in which experienced musculoskeletal radiologists evaluated 823 STTs showed 81% diagnostic accuracy of US, with 93.3 and 97.9% sensitivity and specificity of US for diagnosing malignancies, respectively [27]. Diagnostic performance increased with radiologist experience. In keeping with our findings, published data indicate that factors associated with malignancy include older age, deeper location, larger size, less sharply defined margins, hypoechoic appearance, and hypervascularity [16, 17, 22, 25, 28]. However, age and lesion depth, well-established criteria for malignancy in soft tissue sarcomas, were not significant discriminators of malignant versus benign lesions in the present study. We believe that this is due to a lack of power.
Our study has several limitations. We used only ultrasonography and SWE: we did not assess the potential diagnostic improvements that might be obtained by adding MRI. Neither the size of the ROI nor its location within heterogeneous masses was standardized. All biopsies were done under US guidance, as opposed to computed tomography guidance, but this method constitutes standard practice. Indeed, CT is not an ideal technique for soft tissue lesions especially if they are superficial and small.
Another limitation concerns the technical and physical properties of the technique itself, especially when evaluating tumors of different depths (distance from the probe) and tumors surrounded by different tissues, such as subcutaneous fat versus muscle, which influence shear wave propagation differently. Moreover, some large tumors might protrude to the surface, which can significantly influence SWE examination. However, in our series, there was no such tumor. Furthermore, the larger tumor might increase the internal heterogeneity echo. When the tumors were heterogeneous, we averaged the hardness.
Finally, we studied only two soft-tissue masses subgroups, with and without fat components, although soft-tissue masses are highly heterogeneous in histological nature and structure. It might therefore be of interest to evaluate the diagnostic usefulness of SWE in subgroups determined by standard US.