Evaluation of Cystic and Solid Renal Lesions with Contrast-Enhanced Ultrasound: A Retrospective Study

Abstract

Purpose Renal lesions are frequent random findings on CT, MRI, and conventional ultrasound. Since they are usually found accidentally, the respective examinations have not been performed optimally to provide a conclusive diagnosis, making additional multiphase contrast-enhanced examinations necessary. The aim of the study is to correlate CEUS findings with the final diagnosis and to determine whether it is a suitable method for the conclusive characterization of undetermined renal lesions. Materials and Methods All CEUS examinations of focal renal lesions performed at our institute between 2007 and 2014 were retrospectively examined. 437 patients with a total of 491 lesions and 543 examinations were included. 54 patients had bilateral lesions. One patient had three lesions in one kidney. Histology was available in 49 cases and follow-ups in 124 cases. The sensitivity, specificity, positive and negative predictive value as well as positive and negative likelihood ratios were calculated. Results There were 54 malignant and 437 benign lesions. The sensitivity and specificity were 0.981/0.954 overall, 1.000/0.956 for cystic lesions, 0.977/0.906 for solid lesions, and 0.971/0.071 for the histologically confirmed lesions. Bosniak classification was consistent in 289 of 301 lesions (96%). Only 12 lesions (3.9%) were falsely assessed as malignant. Conclusion CEUS is an appropriate method for the clarification of undetermined renal lesions. The characterization of cystic lesions according to Bosniak is adequately possible, especially for potentially malignant lesions (types III and IV).

Keywords: Bosniak; CEUS; Contrast-Enhanced Ultrasound; RCC; Renal.

Fig. 1

Algorithm for the management of cystic kidney lesions. After CEUS and taking into account the initial imaging, the lesion is classified according to Bosniak. In case of > Bosniak II, further imaging with a dedicated CT or MR renal protocol is performed, depending on patient age and possible contraindications. The further procedure depends on the result of all examinations, with the highest classification being decisive. In the algorithm, Bosniak 0 is referred to as a pseudolesion, an inflammatory lesion, or an ischemic lesion. yrs = years, FU = follow-up , MRI = magnetic resonance imaging, CEUS = contrast-enhanced ultrasound , CT = computed tomography..

Fig. 2

Modified CONSORT flowchart for patient recruitment of the study, including number of patients, number of lesion, and CEUS performed (including any follow-ups). The study included all patients who received a renal CEUS examination with an initially unclear lesion on another imaging modality (CT/US/MRI). In six outpatients the initial examination modality could not be determined. Patients with incomplete data (n=14) and those with lesions that remained unclear despite different imaging modalities and/or follow-ups had to be excluded (n=12). The remaining patients were classified according to the morphological lesion criterion into solid and cystic lesions, as is often done clinically. The further introduced subgroup of histologically confirmed lesions is not listed in the flowchart, as it contains both solid and cystic lesions. CONSORT = Consolidated Standards of Reporting Trials, CEUS = contrast-enhanced ultrasound, CT = computed tomography, US = ultrasound, MRI = magnetic resonance imaging.

Fig. 3

Uncomplicated cyst; Bosniak I. a Hypodense, smoothly bordered, and thin-walled lesion with a diameter of 2.5 cm and water equivalent density values on contrast-enhanced CT. b On the native image an interface echo and dorsal acoustic enhancement are visible. c On CEUS the lesion shows a tender wall, no septation, and no enhancement, consistent with an uncomplicated benign renal cyst.

Fig. 4

Minimally complicated homogeneous hyperdense cyst; Bosniak II a Homogeneous hyperdense lesion in the pars intermedia of the left kidney on native CT with density values of 75 Hounsfield units. b On the native image, no septae or solid parts are detected. c On CEUS there is no contrast enhancement, consistent with an exsanguinated/protein-rich cyst.

Fig. 5

Complicated, septated cyst; Bosniak IIF. a 2.5 cm hypodense lesion at the lower pole of the left kidney with fine septation on contrast-enhanced CT (red arrow). b and c Native and contrast-enhanced ultrasound show a 1.5 mm septum with discrete contrast enhancement (green arrow). However, there is no nodular component.

Fig. 6

Highly suspicious lesion; Bosniak IV. a 6 cm hypodense lesion in the pars intermedia of the left kidney with a density between 20 and 30 Hounsfield units on contrast-enhanced CT (arterial phase). b In the portal venous phase, the lesion shows density values up to 40 Hounsfield units, but with enhancement of the septae. c On the B-mode image, low echoes with solid, echo-rich parts are displayed (arrows). d CEUS shows strong partial enhancement (circle) extending to the center, matching the vessels at the edges of the lesion. The lesion was classified as a partially cystic, partially solid tumor (Bosniak IV). Histologically it was a papillary renal cell carcinoma.

Fig. 7

Final classification of the cystic lesions. A total of 301 lesions were classified as cystic, 274 as benign, and 27 as malignant. The majority of the benign lesions were uncomplicated renal cysts Bosniak types I and II (241 of 274, 89%) which do not require further evaluation. RCC was histologically diagnosed in 17 of the cystic lesions (6%). (orange/red = (potentially) malignant, blue = benign).

Fig. 8

Solid malignant lesion, histologically papillary renal cell carcinoma. a-d Even on multiphase contrast-enhanced CT, no clear contrast agent uptake (i.e., HU > 10) could be detected. The density values varied between a native: 29 HU, b arterial: 35 HU, c portal venous: 38 HU, and d late: 36 HU. e Native ultrasound shows a hypoechogenic (arrow) lesion. f-h On CEUS, there is rapid homogeneous arterial contrast enhancement (arrow) as well as g venous and h late washout. HU = Hounsfield units.

Fig. 9

Final classification of solid lesions. A total of 215 lesions were classified as solid, with 171 of them classified as benign and 44 as malignant. The benign lesions included 98 pseudotumors, 29 inflammatory lesions, and 24 angiomyolipomas. The remaining lesions were significantly less frequent (<6). 42 of the malignant lesions were RCCs (95%) (red = malignant, blue = benign). RCC = renal cell carcinoma.

Fig. 10

Histological findings of the biopsied lesions. A total of 49 of the 491 statistically evaluated lesions were biopsied. The histological findings revealed 35 malignant lesions, 34 of which were RCCs and one was a metastasis. A total of 14 benign lesions were found, whereby the two oncocytomas were evaluated as benign in this study (red = malignant, blue = benign).

Fig. 11

Malignant and benign histology of suspicious lesions. Subdivision of the biopsied Bosniak III/IV lesions and the solid lesions according to classification on CEUS and final histological findings. Out of 10 potentially malignant Bosniak III cysts, 5 were malignant. Of the Bosniak IV cysts clearly classifiable as malignant, 12 of 14 cysts were malignant and 18 of the 25 biopsied solid lesions were malignant. Orange = malignant, blue = benign.