 Introduction/Background Renal masses are increasingly detected in asymptomatic individuals as incidental findings. Many of these are small renal tumors that vary widely in biological aggressiveness, ranging from benign tumors to high-grade renal cell carcinomas (RCCs). An indeterminate renal mass cannot be diagnosed confidently as benign or malignant at the time it is discovered. Masses that can be definitively characterized on the first imaging test will not be discussed in this review. Special Imaging Considerations Dual-energy CT and contrast-enhanced ultrasound (CEUS) are gaining traction in the characterization of indeterminate renal masses. Several studies have demonstrated that dual-energy CT can improve the differentiation between nonenhancing cysts and low-level-enhancing tumors [8-11]. Dual-energy CT with reconstruction of virtual monochromatic images has been shown to decrease or overcome renal cyst pseudoenhancement [12]. Other studies have shown that dual-energy CT can differentiate between solid tumors and hyperdense cysts incidentally detected on a single-phase postcontrast CT [13-15] and can be useful when a comprehensive multiphase renal protocol CT is not available. CEUS with microbubble agents is a useful alternative for characterizing renal masses, especially for patients in whom iodinated CT contrast or gadolinium-based MRI contrast is contraindicated. The microbubble agents are not excreted by the kidneys and therefore do not affect renal function. CEUS allows real-time evaluation of microvasculature and has been shown to be valuable for differentiating between cystic and solid renal lesions and for characterizing complex renal cystic lesions and indeterminate renal masses [16-18]. CEUS may result in assignment of a higher Bosniak classification compared to contrast-enhanced CT [19,20]. However, a typical CEUS examination does not result in a complete evaluation of both kidneys for additional renal masses.
Indeterminate Renal Mass.  Introduction/Background Renal masses are increasingly detected in asymptomatic individuals as incidental findings. Many of these are small renal tumors that vary widely in biological aggressiveness, ranging from benign tumors to high-grade renal cell carcinomas (RCCs). An indeterminate renal mass cannot be diagnosed confidently as benign or malignant at the time it is discovered. Masses that can be definitively characterized on the first imaging test will not be discussed in this review. Special Imaging Considerations Dual-energy CT and contrast-enhanced ultrasound (CEUS) are gaining traction in the characterization of indeterminate renal masses. Several studies have demonstrated that dual-energy CT can improve the differentiation between nonenhancing cysts and low-level-enhancing tumors [8-11]. Dual-energy CT with reconstruction of virtual monochromatic images has been shown to decrease or overcome renal cyst pseudoenhancement [12]. Other studies have shown that dual-energy CT can differentiate between solid tumors and hyperdense cysts incidentally detected on a single-phase postcontrast CT [13-15] and can be useful when a comprehensive multiphase renal protocol CT is not available. CEUS with microbubble agents is a useful alternative for characterizing renal masses, especially for patients in whom iodinated CT contrast or gadolinium-based MRI contrast is contraindicated. The microbubble agents are not excreted by the kidneys and therefore do not affect renal function. CEUS allows real-time evaluation of microvasculature and has been shown to be valuable for differentiating between cystic and solid renal lesions and for characterizing complex renal cystic lesions and indeterminate renal masses [16-18]. CEUS may result in assignment of a higher Bosniak classification compared to contrast-enhanced CT [19,20]. However, a typical CEUS examination does not result in a complete evaluation of both kidneys for additional renal masses.