Study Need and Importance

Cystic renal mass management is guided by the Bosniak classification system.¹ This system risk stratifies cysts for malignant potential based on radiographic features on CT or MRI. Recent population-based studies have established that cystic renal cell carcinoma (RCC) has better survival outcomes than solid clear cell RCC and that using the Bosniak system solely to risk stratify patients leads to overtreatment.^2–5 Contrast-enhanced ultrasound (CEUS) has emerged as a new tool for characterizing renal lesions, as it can distinguish between solid and cystic components, identify tumor vascularity, and further characterize indeterminate renal lesions seen on cross-sectional imaging.⁶ We conducted a prospective pilot study evaluating volumetric CEUS to calculate fractional vascularity (FV) to further risk stratify patients.

What We Found

Twenty patients undergoing surgery for a complex cystic renal mass were included in our final analysis. CEUS was performed at the time of planned surgery in selected patients (typically in the surgical holding area). Under the new classification, 2 (10%), 1 (5%), and 17 (85%) had Bosniak IIF, III, and IV lesions, respectively. On final surgical pathology, 3 lesions were benign and 17 were malignant. Of the 17 malignant lesions, 13 (76%) were clear cell RCC, 3 (18%) were clear cell papillary RCC, and 1 (6%) was papillary RCC. Four (24%) of the 17 malignant lesions were ISUP (International Society of Urologic Pathologists) Grade 3-4. Four (24%) of the 17 malignant lesions were upstaged to pT3+ disease on final pathology. We assessed the ability of P2019B (pre-2019 Bosniak classification), B2019 (current Bosniak classification), 2DFV, and 3DFV to predict malignancy. On ROC analysis, the AUC was 0.980, 0.824, 0.863, and 0.824 for P2019B, B2019, 2DFV, and 3DFV, respectively (Fig. 1). We next assessed the additive effect of combining Bosniak classification with FV (Fig. 2). Three models had an AUC of 1, and the combination of 2DFV and B2019 an AUC of 0.980 (p value <0.001). We also performed secondary analysis on the ability of FV to predict upstaging and aggressive pathology, but these were not found to be significant either in isolation or in combination.

Figure 1. ROC curve analyses of ability to predict malignancy individually.

Figure 2. ROC curve analyses of ability to predict malignancy, combined.

Limitations

First, calculation of FV required manually mapping ROI, subject to inter-reader variability, and currently limits the ability to scale up to larger cohorts. Second, the study was limited to patients undergoing surgical intervention, which represents a highly select group of patients with primarily Bosniak IV cystic lesions. In a broader study of all Bosniak IIF to IV cystic lesions, FV may provide greater discriminatory ability and needs further evaluation. Lastly, as a pilot study, the cohort size is small, limiting the statistical analysis.

Interpretation for Patient Care

FV is a novel metric in the evaluation of complex cystic renal masses that improves upon the Bosniak classification system’s ability to predict malignancy on final surgical pathology.

It may serve as an important adjunct for risk stratification for surgical intervention.