The simplified padua renal (SPARE) nephrometry system: A novel classification to measure the complexity of parenchymal renal tumours before partial nephrectomy

Aim of the study: Nephrometry systems provide important informa- tion for treatment planning, patient counselling and comparison between different partial nephrectomy (PN) series. The Preoperative Aspects and Dimensions Used for an Anatomical (PADUA) classifica- tion of renal tumours is one of the most popular and widely used nephrometry systems. The objective of the study are: 1) to simplify the original PADUA classification generating a new system able to predict equally or even better the risk of overall complications in patients undergoing PN; and 2) to test whether the addition of tumour contact surface area (CSA) improves the accuracy of the original and new simplified PADUA classification. Materials and methods: We analysed the clinical records of 531 patients who underwent PN for renal tumours at five tertiary academic referral centres from January 2014 to December 2016 treated with open, laparoscopic or robot-assisted approach. The ability of each variable included in the PADUA classification to predict overall complications was tested using binary logistic regression analysis. The variables that were not statistically significant were excluded from the simplified classification. Starting from original and simplified PADUA systems, other two models were generated adding tumour CSA. Receiver operating characteristic curve analysis was used to compare the ability of the four different models to predict overall complications. Binary logistic regression was used to perform both univariable and multivariable analyses looking for predictors of postoperative complications. Linear regression analysis was used to identify independent predictors of absolute change in estimated glomerular filtration rate (eGFR). Results: The Simplified PAdua REnal (SPARE) nephrometry system including 1) rim location, 2) renal sinus involvement, 3) exophytic rate and 4) tumour size showed similar performance compared to the original PADUA classification (area under the curve [AUC] 0.657 vs 0.664). Adding tumour CSA to the original (AUC 0.661) or simplified (AUC 0.658) PADUA classification did not increase the accuracy of either system to predict overall complications. The SPARE nephrome- try system was an independent predictor of postoperative overall complications (odds ratio 1.2, 95% confidence intervals 1.1–1.3). Age (p<0.001), BMI (p<0.001), Charlson index (p=0.02), preoperative eGFR (p<0.001) and tumour CSA (p=0.005) were independent predictors of absolute change in eGFR. Limitations include the retrospective design and the lack of central imaging review. Discussion: The SPARE nephrometry system is composed by only four, instead of the original six, variables, and its accuracy to predict overall complications is similar to that of the original PADUA classification. Addition of tumour CSA was not associated with an increase in prognostic accuracy. The SPARE nephrometry system should replace the original PADUA classification to evaluate the complexity of tumours suitable for PN.