External Validation of Contact Surface Area as a Predictor of Postoperative Renal Function in Patients Undergoing Partial Nephrectomy

Purpose: We sought to externally validate a mathematical formula for tumor contact surface area as a predictor of postoperative renal function in patients undergoing partial nephrectomy for renal cell carcinoma. Materials and Methods: We queried a prospectively maintained kidney cancer database for patients who underwent partial nephrectomy between 2014 and 2016. Contact surface area was calculated using data obtained from preoperative cross-sectional imaging. The correlation between contact surface area and perioperative variables was examined. The correlation between postoperative renal functional outcomes, contact surface area and the R.E.N.A.L. (radius, exophytic/endophytic properties, nearness of tumor to collecting system or sinus, anterior/posterior, location relative to polar lines and tumor touches main renal artery or vein) nephrometry score was also assessed. Results: A total of 257 patients who underwent partial nephrectomy had sufficient data to enter the study. Median contact surface area was 14.5 cm ² (IQR 6.2–36) and the median nephrometry score was 9 (IQR 7–10). Spearman correlation analysis showed that contact surface area correlated with estimated blood loss (r _s = 0.42, p <0.001), length of stay (r _s = 0.18, p = 0.005), and percent and absolute change in the estimated glomerular filtration rate (r _s = –0.77 and –0.78, respectively, each p <0.001). On multivariable analysis contact surface area and nephrometry score were independent predictors of the absolute change in the estimated glomerular filtration rate (each p <0.001). ROC curve analysis revealed that contact surface area was a better predictor of a greater than 20% postoperative decline in the estimated glomerular filtration rate compared with the nephrometry score (AUC 0.94 vs 0.80). Conclusions: Contact surface area correlated with the change in postoperative renal function after partial nephrectomy. It can be used in conjunction with the nephrometry score to counsel patients about the risk of renal functional decline after partial nephrectomy.