the incidence of renal cell carcinoma is consistently increasing , and the increase in use of imaging technologies has resulted in an increase in the incidental detection of renal cell carcinoma , especially of small renal masses . nephron - sparing surgery has become the standard treatment for small renal masses , demonstrating improved overall survival and superior preservation of renal function compared with traditional radical nephrectomy . in addition , laparoscopic partial nephrectomy ( lpn ) and robot - assisted partial nephrectomy ( rpn ) have been accepted as treatment options for small renal masses with adequate oncologic outcomes . several factors predict renal functional outcome after partial nephrectomy , including older age , sex , lower preoperative glomerular filtration rate ( gfr ) , single kidney , tumor size , and longer ischemic interval . the limit of 30 minutes in patients with normal preoperative kidney function is currently accepted as a safe warm ischemia time ( wit ) . our previous study demonstrated that patients with a wit>28 minutes had a significantly greater decrease in the gfr of the affected kidney ; in multivariate analysis , wit was an independent predictive factor of functional reduction of the affected kidney . although a wit ranging from 20 to 30 minutes was thought to be safe , some authors have suggested that every minute counts when the renal hilum is clamped . to minimize ischemic renal injury , many surgeons make an effort to decrease the wit to less than 30 minutes during partial nephrectomy under pneumoperitoneum . however , there are few studies on preoperative predictors of prolonged wit during lpn or rpn performed by a single surgeon . in this study , we evaluated patients who underwent lpn or rpn and tumor characteristics predictive of prolonged wit . with the approval of our institutional review board , we retrospectively analyzed the medical records of 317 patients who underwent partial nephrectomy . partial nephrectomy was conducted by laparoscopic or robot - assisted surgeries and was performed by the same surgeon between october 2007 and may 2013 . according to the tumor location , we performed transperitoneal approaches for anteriorly or laterally located tumors and retroperitoneal approaches for posteriorly located tumors . for arterial clamping during surgery , we used bulldog clamps ( aesculap , center valley , pa , usa ) ( fig . color doppler laparoscopic ultrasound was used to confirm whether hilar clamping was proper or not . resection of the tumor was completed with scissors , without application of electrosurgical coagulation devices ( fig . after tumor excision , surgical bed hemostasis was achieved by oversewing vessels by using 3 - 0 pds or 3 - 0 v - loc sutures with a lapra - ty clip ( ethicon , cincinnati , oh , usa ) at the terminal end ( fig . the renal parenchyma was repaired with 1 - 0 vicryl sutures placed in an contiuous fashion across the defect , and the sliding technique with hem - o - lok clip was used to tighten and secure the sutures ( fig . ischemia time was defined as the interval between placement of the first arterial clamp and removal of the last clamp . demographic and patient characteristics were recorded , including age , sex , body mass index , american society of anesthesiologists classification , preoperative estimated gfr , clinical tumor size , and pathologic outcome . clinical tumor size was recorded as the largest diameter seen on radiological images . the nephrometry score ( nephrometry score ; r.e.n.a.l . [ radius , exophytic / endophytic , nearness to collecting system or sinus , anterior / posterior and location relative to polar lines ] score , padua [ preoperative aspects and dimensions used for an anatomical ] score , and c - index ) was determined by a retrospective review of images . patients were divided into two groups : group a was defined as prolonged wit ( 30 minutes ) and group b as short wit ( < 30 minutes ) . perioperative outcomes including operative time , estimated blood loss ( ebl ) , pathologic tumor size , method of surgery ( lpn or rpn ) , and approach method ( transperitoneal or retroperitoneal ) were compared between groups . we evaluated multiple factors as predictors of prolonged ischemic time . to assess the impact of surgeon experience on wit , we categorized the patients in groups of 50 consecutive cases . 20.0 ( ibm co. , armonk , ny , usa ) and sas 9.4 ( sas institute inc . , cary , nc , usa ) . student t - tests and pearson chi - square tests were used for comparisons of means and proportions , respectively . among the 317 consecutive patients , there were 209 males ( 65.8% ) and 108 females ( 34.2% ) with a median age of 53.5 years ( range , 29 - 78 years ) . a total of 80 patients were included in group a and 237 patients in group b. mean wit was 38.68.9 minutes in group a and 19.85.8 minutes in group b , respectively ( table 1 ) . . however , the padua score ( p=0.006 ) , r.e.n.a.l . score ( p=0.003 ) , operative time ( p<0.001 ) , ebl ( p<0.001 ) , and approach method ( p<0.001 ) did differ significantly between the two groups ( table 1 ) . cases 1 - 50 ( group a : 68% vs. group b : 32% ) and 51 - 100 ( group a : 54% vs. group b : 46% ) had higher proportions of group a than group b. after these 100 cases , wit significantly decreased , ultimately demonstrating an absolute majority of patients in group b after the surgeon had accumulated more than 150 cases ( p<0.001 ) ( fig . univariable analysis identified padua score ( p=0.008 ) , approach method ( < 0.001 ) , and surgeon experience ( p<0.001 ) to be significantly associated with prolonged wit . in the multivariable analysis , padua score ( p=0.032 ) , tumor size ( odds ratio [ or ] , 2.98 ; 95% confidence interval [ ci ] , 1.48 - 5.96 ; p=0.002 ) , and surgeon experience ( p<0.001 ) were independent predictors of prolonged wit . among these , surgeon experience was the strongest independent predictor . according to cumulative surgery cases , compared with the experience of 50 cases of surgery , although the risk of prolonged wit was not reduced from cases 51 - 100 ( p=0.202 ) , as subsequent surgery experience accumulated , a significant reduction in prolonged wit was shown ( table 2 ) . in a subanalysis according to type of surgery ( rpn or lpn ) , tumor size ( or , 3.21 ; 95% ci , 1.24 - 8.28 ; p=0.016 ) and surgeon experience ( p<0.001 ) were independent predictors of prolonged wit in the rpn group , whereas surgeon experience ( p<0.001 ) was the only independent predictor of prolonged wit in the lpn group ( table 3 ) . among the variables making up the nephrometry score , the r.e.n.a.l . score in patients who underwent rpn ( 6.751.42 ) was higher than that of those who underwent lpn ( 5.961.48 , p<0.001 ) . current guidelines show that patients with clinically staged t1 renal tumors should undergo partial nephrectomy whenever technically feasible . several retrospective studies have shown acceptable oncologic outcomes of partial nephrectomy for not only small renal tumors , but also large localized renal tumors . postoperative renal function is better preserved with partial nephrectomy than with radical nephrectomy ; however , several clinical factors impact renal function after surgery , and profound renal functional loss can occur even after partial nephrectomy . since lane et al . showed that ischemia time during partial nephrectomy is the greatest modifiable predictive risk factor of postoperative functional loss , wit has been found to be associated with a significant loss of renal function of the affected kidney , mainly in patients who experienced prolonged wit . to date , however , there are few studies on factors that affect wit prolongation . in the comparison of perioperative factors between groups this finding is attributed to the fact that the transperitoneal approach was used in a large proportion of the initial cases in our series . as surgical experience accumulated , the surgeon 's preference changed to a retroperitoneal approach for posterior or laterally located renal tumors . actually , surgical approach was not a predictor of wit , as shown by the multivariable analysis . there are several reports of a higher nephrometry score being significantly associated with increased wit . .   found that surgeon experience , clinical tumor size , anatomic characteristics determined by the padua classification score , and upper collecting system repair were independent predictors of wit>20 minutes . .   showed a much stronger correlation between nephrometry score and wit , with the c - index system ( coefficient , -0.609 ) and the padua score system ( coefficient , 0.735 ) showing the strongest correlation in the overall analysis , whereas the r.e.n.a.l . in concordance with previous studies , our results indicate that each component of the nephrometry score is correlated with wit . among the variables making up the nephrometry score , our study demonstrated that only the padua score ( p=0.032 ) could predict wit>30 minutes . these findings suggest that risk group stratification using the padua score may improve patient selection for partial nephrectomy , especially for novice surgeons . however , the results of our subgroup analysis showed that using the nephrometry score to predict prolonged wit in the groups divided by surgical type ( rpn or lpn ) was not effective . previous studies have set the wit cutoff value to 20 minutes , whereas we evaluated the predictors of prolonged wit defined as longer than 30 minutes . regarding surgeon experience , mottrie et al .   demonstrated that surgeon experience significantly correlated with robotic console time ( p<0.001 ) and wit ( p<0.0001 ) . when we compared each period , the ratios of group a were 68% , 54% , 30% , 6% , 4% , and 4% , respectively . our data showed that prolonged wit was very rare when surgeon experience was greater than 150 cases of partial nephrectomy under pneumoperitoneum . in further subgroup analysis , surgeon experience was the most important predictor of wit in both the rpn and lpn groups . after 50 cases of each type of surgery were accumulated , the risk of prolonged wit was reduced dramatically compared with that in the first 50 cases ( fig . several current studies have shown shorter learning curves for rpn than for lpn . two systematic reviews and meta - analyses of rpn versus lpn reported no significant differences in perioperative outcomes between the two groups , except for a significantly shorter wit . however , type of surgery was not predictive of prolonged wit in the present study . more complex cases were treated with rpn because of its likelihood to greatly mitigate the difficulty of complex partial nephrectomy . surgeon experience , padua score , and tumor size are significant predictors of prolonged wit during partial nephrectomy under pneumoperitoneum . among these predictive factors , accumulating surgical experience is the most important and the only modifiable factor for reducing the risk of prolonged wit . an accompanying video can be found in the ' urology in motion ' section of the journal homepage ( www.kjurology.org ) . the supplementary video clips can also be accessed by scanning a qr code located on the fig . 1 of this article , or be available on youtube ( https://youtu.be/fcfhdcib1ue ) .