Partial nephrectomy (PN) is the gold-standard treatment of small renal masses, and shows similar results as radical nephrectomy (RN) in oncological outcomes.¹ RN causes renal insufficiency and cardiovascular events that lead to increased morbidity and mortality.^2,3 PN can provide excellent results in patients presenting with a renal mass in a solitary kidney, stage IV chronic kidney disease, multiple renal tumors, and hereditary disorder, thereby preventing the need for dialysis.^4,5 Therefore PN should receive further attention as the treatment-of-choice for renal masses.⁶

Laparoscopic PN (LPN) and robotic PN (RPN) were introduced as minimally-invasive PN treatments. Hurber et al. reported that the results of RPN are similar to those obtained with LPN.⁷ Long et al.⁷ reported that RPN had the merits of high definition, 3-dimensional optics, and wristed instruments. However open PN (OPN) was still preferred in more complex surgical cases. The radius exophytic/endophytic nearness anterior/posterior location (RENAL) system, Centrality-index (C-in-dex), and preoperative aspects and dimensions used for an anatomical (PADUA) classification system are well-known assessment tools of renal masses, to determine whether a given renal tumor was a suitable candidate for PN. These have validity as assessment tools for renal tumors undergoing nephron-sparing surgery (NSS). The RENAL system was developed in 2009, and considers renal tumor size, exophytic extension, distance between the renal tumor and the renal sinus and urinary collecting system (UCS), and tumor location.⁸ The RENAL system has the advantage that the parameter measurements are generally easily measured. The exception is that it is not always easy to measure the distance between the tumor mass and the renal sinus/UCS, resulting in different distance estimates among observers. Also, coronal abdominopelvic computed tomography (CT) imaging or magnetic resonance imaging (MRI) is necessary to know the correlation between renal tumor orientation and renal polarity.⁹

The C-index was developed by Simmon in 2010, and calculates tumor size and renal site using preoperative contrast-enhanced CT imaging. This approach provided a better description of the renal tumor and is related to the postoperative nadir estimated glomerular filtration rate (eGFR) and the percent decrease in eGFR following PN.¹⁰ However, the C-index is not simple and did not precisely calculate the actual renal tumor location. Therefore, Long et al.⁷ said that the C-index did not predict PN perioperative outcome better than the PADUA and RENAL score systems.

The PADUA classification system is a tool for predicting patient outcome after NSS. This system was invented by urologists that were blinded to patient information.⁹ The PADUA system differs from the RENAL system primarily in its evaluation of renal sinus/UCS involvement of the renal tumor. Coronal and/or axial CT or MRI views are used to measure the renal sinus line. Reproducibility is excellent because measuring the renal tumor placement relative to the renal sinus/UCS is easier. This classification was useful for predicting post-operative complications, warm ischemia time, and estimated blood loss in PN.^11-14 We did not locate any published reports regarding the use of the PADUA classification system in predicting intraoperative RN conversion in PN patients. Thus, we analyzed the reasons for intraoperative PN to RN conversion, and investigated the utility of the PADUA classification in pre-dicting PN conversion to RN.

PN is the gold-standard treatment of small renal tumors with clinical stage 1.¹ Van Poppel et al.¹⁵ previously contended that RN showed a better result than PN in decreasing perioperative complication, required a shorter operation time, and resulted in less morbidity and mortality. However their results were reported before more PN experience had emerged. Our team of urology surgeons has significantly more experience in PN in recent years, and we think that PN shows better results than RN for treating small renal tumors with respect to renal functional preservation. Some factors such as the volume of remaining renal parenchyma, and intact renal sinus or UCS require consideration. When minimally invasive PN encounters difficulties, we recommend conversion to OPN instead of RN.¹⁵ Thus it is important to predict the likelihood of needing to intraoperatively convert from PN to RN.

There were many factors that could affect the need for intraoperative conversion from PN to RN. We divided these factors into anatomical aspects and pathophysiological aspects. In terms of anatomical aspects, large renal tumors or tumor discrepancies made dissection of the renal tumor more difficult and reduced the amount of remaining renal parenchyma. Additionally, involvement of renal sinus and UCS made it difficult to discern clear tumor margins or the complete renal pelvis, and resulted in an increased PN to RN conversion rate. We found that tumor involvement of renal sinus was the most significant factor of intraoperative conversion to RN with odd ratio of 9.075. UCS involvement also had a significant impact on radical conversion during PN. Although they were not counted in the evaluation, we believe additional intraoperative factors such as difficulty of vessel clamping, and renal vessel injuries might have influenced on radical conversion during PN in some degree. Thus, radical conversion was in most cases caused by anatomical factors.

In a study by Galvin et al.,⁵ the conversion rate to RN was higher with larger renal tumors and when tumors were located closer to renal center. Long et al.¹⁶ observed that increased renal tumor size and higher RENAL nephrometry scores increased the intraoperative PN to RN conversion rate. However, their classification of renal tumors to the medial or peripheral kidney was ambiguous, and reproducibility was poor. Thus there remains no consensus regarding which factors, besides tumor size, increase the PN to RN conversion rate.

For evaluating the influence of any parameter on radical conversion rates in NSS, it is necessary that the reproducibility of diagnostic tools is high and assessment is easy. Because of these reasons, we investigated the effect each item of the PADUA classification scale on intraoperative conversion from PN to RN. The PADUA classification is composed of six parameters, plus anterior or posterior surface. Among these parameters, tumor size and the involvement of the renal sinus/UCS were the most important factors in predicting the need for radical conversion. Tumor location was not a predictor of conversion in our study, in contrast to findings in other studies.⁵ We showed that tumor involvement of UCS or renal sinus were the most important factors in predicting intraoperative PN to RN conversion, in both univariate and multivariate logistic regression analyses.

The pathophysiological condition of patients should be considered when selecting the operation method for NSS. If patients have advanced chronic kidney disease (stage≥3) along with a comorbidity such as uncontrolled diabetes, hypertension, coronary heart disease, or dyslipidemia, it is important that operation time and anesthesia time are minimized to enhance patient outcomes. Galvin et al.,⁵ in their survey found that the American Society of Anesthesiologists physical status classification grade, which is a 12 surrogate marker of comorbidity, did not influence the conversion rate of PN to RN. This suggests that PN conversion rates to RN were not affected by comorbidity itself, but instead by a lowered surgeon's threshold to radical conversion because it was worried to negatively affect the general condition of patients after surgery. If the operator becomes more experienced with PN and predicting the radical conversion rate before operating, unscheduled PN to RN conversions may be reduced. Patients with local symptoms like hematuria, flank pain, and fever were reported to have a higher PN to RN conversion rate.⁶ The association of local symptoms to the rate of conversion to RN from PN should be further investigated in future studies.

In our results, 17 of 593 total cases (2.9%) underwent PN to RN conversion, with 4 cases being conversion from OPN to ORN. Our conversion rate to RN was remarkably different from those of Long et al.¹⁶ (16% PN to RN conversion rate), and were also lower than the rate reported by Galvin et al.⁵ (6% conversion rate). The reasons for our lower conversion rate may be that our procedure was performed by a highly experienced staff urologic surgeon in Seoul National University Bundang Hospital.

There were some limitations to our study. First, this study was done retrospectively, but these results are viable because the PADUA classification was only introduced in 2009. Second, we did not compare the difference between the results of OPN and RPN. We investigated the impact of PADUA classification in overall NSS. Third, there may be selection bias in the more complex cases. The da Vinci Robotic Surgical System (Intuitive Surgical Inc., Sunnyvale, CA, USA) offers 3-dimensional vision and easy feasibility to renorrhaphy, and is more suitable for complex cases of NSS than LPN. OPN and RPN were successfully performed for the high-complexity cases because of our surgeon's accumulated experience and our advanced technology. Thus, cases that underwent both RPN and OPN were enrolled in this study.

The PADUA classification is a useful tool to predict the likelihood of intraoperative PN to RN conversion. Tumor involvement of UCS or renal sinus appear to be the best predictors of radical conversion in patients undergoing PN. Further surgical experience is required to differentially validate PADUA pre-dictive power of radical conversion in OPN versus RPN.