## 1 Introduction  
In recent decades, the proliferation of abdominal imaging studies has led to an increase in the detection of incidental renal masses, which are now estimated to occur in over 50% of patients over 50 years of age undergoing abdominal imaging with US, CT, or MRI [1]. These findings represent a diagnostic challenge, as renal tumors may remain asymptomatic for a long time, and late diagnosis is associated with a poor prognosis [2].  
US is the ideal method for early diagnosis due to its safety and wide availability [3]; however, it presents several limitations [4]. For example, a B-mode ultrasound can easily diagnose simple cysts, but it is not suitable for the study of complex cysts and solid masses [5]. This constitutes a crucial issue, as over 80% of solid renal masses are malignant, whereas benign lesions are mostly represented by oncocytomas (OCY) and angiomyolipomas (AML) [6]. The latter is the most frequent benign renal tumor and typically presents as a well-defined, homogeneous hyperechoic renal lesion; however, both fat-poor AML and hyperechoic renal cell carcinoma (RCC) have been described [7; 8], further complicating the work-up of incidental renal masses [6; 9].  
In routine practice, after detection in US, almost all renal masses undergo further examination (MRI, CT, contrast-enhanced US), although some authors suggest that small, hyperechoic lesions (<1 cm) in patients without risk factors may be suitable for a short-term follow-up with US [8]. Therefore, a complementary US-based technique that is able to distinguish between benign and malignant lesions could be particularly helpful, especially in those situations in which contrast-enhanced imaging modalities are contraindicated, not immediately available, or inconclusive due to the small size of the lesion [5].  
One option is represented by US elastography: a non-invasive imaging technique for assessing the elasticity of biological tissues. Excluding Fibroscan(r)--which is used exclusively in the hepatological field--there are two US elastography techniques [10]. Strain elastography (SE) requires the active compression of the tissue by the operator and expresses elasticity in terms of the dimensionless strain ratio between two ROIs [11]. Shear wave elastography (SWE)--the most recent method and nowadays the most prevalent one in clinical practice [11]--expresses elasticity in terms of the shear wave velocity [m/s] or Young's modulus [kPa].  
However, while US elastography is widely employed in current practice, its application in nephrology presents several challenges due to technical and methodological issues [12]. Most studies have been conducted in patients with chronic kidney disease (CKD), assessing the relation between kidney elasticity, fibrotic changes, and the glomerular filtration rate, whereas the role of US elastography in the diagnosis of focal lesions to guide a referral to further imaging or follow-up has been less extensively investigated.  
We, therefore, aimed to systematically review the applications of US elastography in the characterization of solid renal masses to derive and compare the summary estimates of different stiffness values among different subtypes of these lesions.