The current study demonstrated that tumor volume (cutoff value ≥ 14.11 cm3, indicating a sphere with a diameter ≥ approximately 3 cm) and 3 points for the P factor of the DAP system, indicating a tumor crossing the axial renal midline, were significant factors for predicting eGFR reduction after RPN. The simple classification system using these two factors had the best accuracy for predicting eGFR reduction after RPN compared with existing nephrometry scoring systems such as the R.E.N.A.L., DAP, and PADUA scores. Our classification system can provide prognostic information for counseling patients about renal function after RPN and assist in preoperative decision making.
To assess overall survival benefits in patients with renal cell carcinoma (RCC) after partial or radical nephrectomy, predicting both oncologic outcomes and decreased renal function to avoid chronic kidney disease is essential [12,13]. Some studies have determined that an SCr level > 2.0 mg/dl or the occurrence of stage-5 CKD is the endpoint of postoperative renal function [14,15]. However, a limitation of these endpoints is that the outcome can depend on the preoperative state. To circumvent this limitation, analysis of eGFR reduction from baseline to a point after surgery is crucial for predicting accurate postoperative renal function. General functional reduction after PN averages approximately 10% in the two-kidney and 20% in the one-kidney model . Thus, we set the cutoff point for reduced eGFR to 20% with reference to a previous report .
Recent reports have shown that a nomogram with the sum of the R.E.N.A.L. score incorporated accurately predicts significant eGFR reduction after PN [17,18]. Therefore, the nephrometry scoring systems might be able to predict a decline in renal function after PN. Simmons et al. reported that the DAP score, which is a modified version of the R.E.N.A.L. classification and c-index, had simplified methodology and was associated with volume loss and renal function after PN. Although the cutoff value for each factor was different, similar to the DAP score including tumor diameter, axial distance from the center point, and polar distance from the midline, tumor volume, nearness to the collecting system (distance from the tumor to the collecting system), and location relative to polar lines were significantly associated with eGFR decline after RPN in this study. Among these three factors, nearness to the collecting system was not an independent factor associated with the outcome of interest. However, the distance from the tumor to the collecting system tended to be shorter as the P factor score of the DAP system increased, and it was negatively correlated with tumor volume (Supplementary Fig. 2, R = 0.481, p < 0.001). Thus, the two independent factors used in our classification system may also reflect proximity to the collecting system.
In complex cases, long ischemia times are required for complete tumor resection . For renal function preservation, various techniques during PN have been described (e.g., off-clamp, selective/super-selective clamp, and early unclamp, or cooling techniques for hypothermia) [20,21]. We showed that our classification system was significantly correlated with the WIT (Fig. 2B); thus, our classification system might be related to the complexity of the surgery. Therefore, our classification system might help to select patients who need various surgical techniques to avoid renal insufficiency.
Our results should be interpreted with caution because of several limitations. First, this study was based on data from patients who were treated at a single center, and external validation is needed before applying the classification system to select patients. Second, the study was retrospective in design with, and the follow-up period was relatively short. Third, while perioperative variables such as the WIT and estimated blood loss were not considered, these variables are likely important influencers of postoperative renal function. However, the purpose of this study was to determine which combination of preoperative factors such as nephrometry scoring systems were best for predicting eGFR reduction. We also calculated the tumor volume assuming that each tumor was an ellipsoid. This was not a true volume, but the calculation of tumor volume is easily obtained from preoperative 3D-CT scans.