Expanding the Role of Ultrasound for the Characterization of Renal Masses

Abstract

The incidental detection of renal masses has been steadily rising. As a significant proportion of renal masses that are surgically treated are benign or indolent in nature, there is a clear need for better presurgical characterization of renal masses to minimize unnecessary harm. Ultrasound is a widely available and relatively inexpensive real-time imaging technique, and novel ultrasound-based applications can potentially aid in the non-invasive characterization of renal masses. Evidence acquisition: We performed a narrative review on novel ultrasound-based techniques that can aid in the non-invasive characterization of renal masses. Evidence synthesis: Contrast-enhanced ultrasound (CEUS) adds significant diagnostic value, particularly for cystic renal masses, by improving the characterization of fine septations and small nodules, with a sensitivity and specificity comparable to magnetic resonance imaging (MRI). Additionally, the performance of CEUS for the classification of benign versus malignant renal masses is comparable to that of computed tomography (CT) and MRI, although the imaging features of different tumor subtypes overlap significantly. Ultrasound molecular imaging with targeted contrast agents is being investigated in preclinical research as an addition to CEUS. Elastography for the assessment of tissue stiffness and micro-Doppler imaging for the improved detection of intratumoral blood flow without the need for contrast are both being investigated for the characterization of renal masses, though few studies have been conducted and validation is lacking. Conclusions: Several novel ultrasound-based techniques have been investigated for the non-invasive characterization of renal masses. CEUS has several advantages over traditional grayscale ultrasound, including the improved characterization of cystic renal masses and the potential to differentiate benign from malignant renal masses to some extent. Ultrasound molecular imaging offers promise for serial disease monitoring and the longitudinal assessment of treatment response, though this remains in the preclinical stages of development. While elastography and emerging micro-Doppler techniques have shown some encouraging applications, they are currently not ready for widespread clinical use.

Keywords: contrast-enhanced ultrasound; micro-Doppler; molecular ultrasound; renal cell carcinoma; renal mass; renal tumor; ultrasonography; ultrasound.

Figure 1

A 79-year-old man with a history of bladder cancer undergoing evaluation for hydronephrosis. Grayscale ultrasound image of the left kidney in the longitudinal orientation (a) shows an exophytic hypoechoic mass containing internal low-level echos. Following an intravenous injection of 1.8 cc Lumason ultrasound contrast, a contrast-enhanced ultrasound image focused at the upper pole (b) revealed the mass was completely non-enhancing (devoid of signal), which is diagnostic for a simple cyst. No further follow-up was necessary.

Figure 2

A 67-year-old man with multiple renal lesions, status post SBRT one year prior for contra-lateral RCC. Grayscale ultrasound image of the left kidney in longitudinal orientation (a) shows an isoechoic exophytic nodule. Following the intravenous administration of 1.0 cc Lumason ultrasound contrast, a contrast-enhanced ultrasound image (b) shows the nodule demonstrating predominantly solid avid enhancement relative to the adjacent renal cortex. A partial nephrectomy revealed clear cell renal cell carcinoma, grade 2.