In the present study, we clarified the effect of both patient-specific factors and tumor-specific factors on TTC. Unlike radical nephrectomy, partial nephrectomy is characterized by the fact that the extent of the dissection operation varies with the tumor location and influences tumor identification, and that the fat status influences tumor identification. The current study is significant in that it provides data on the degree of difficulty of the procedure for surgeons, which is useful for preoperative evaluation.
Various fat-related parameters have been reported as factors affecting surgery. Obesity has been reported to be a risk factor of laparoscopic surgery. Several investigators have reported the association between surgical difficulty and obesity (14–16). From the perspective of fat volume, visceral fat volume measured by CT is related to prolonged pneumoperitoneum time of laparoscopic radical nephrectomy (13). In addition, visceral fat area also affects operative time and is reported to have a higher impact than BMI in laparoscopic RN (17). In terms of robot-assisted urological surgery, high BMI is associated with longer operative time and blood loss in upper urinary tract and renal surgery (18). Other studies demonstrated that obesity, particularly increased visceral fat, was associated with surgical difficulty and increasing the risk of postoperative complications in PN (19, 20).
When we look at the perirenal fat environment, PNF is a frustrating surgical variable encountered during PN, limiting renal mobilization and renal tumor identification. In assessing fat status, it is important to evaluate not only its partial characteristics but also total volume. We evaluated PNFV using the VINCENT system. In this method, a manual process was used to set the ROI for the entire perinephric fat area, but the automated algorithm recognizes kidney and tumor regions by maximizing an evaluation measure consisting of the sum of the target likelihood values for each Hounsfield unit value from voxels around the long axis, the edge strength of the boundary, and the fit to the ellipsoid model (13, 21). This system has been widely adopted as a method to measure the volume of a specific target and has been reported to calculate accurate volumes with high reproducibility and inter-observer agreement (21, 22).
Adherent perinephric fat (APF) is also one of the major factors complicating partial nephrectomy in obese patients resulting in difficult dissection, higher blood loss and conversion to open surgery or radical nephrectomy (23). Other studies have indicated that APF is associated with adverse perioperative outcomes including longer operating time (24) and greater EBL (25). The MAP score, which is based on the posterior PNF thickness and stranding, was found to be significantly related to prolongation of the dissection phase time during RAPN (26) or operating time and EBL (27). The MAP score is the most widely used model to assess APF. Posterior perinephric fat thickness has been reported to be significantly related to APF and complications in RAPN (6). It should be noted that while posterior perinephric fat thickness can be measured quickly, the scoring of fat stranding is subjectively mediated and may introduce bias due to inter-observer variability (28). In addition, the pathogenesis of APF may correlate with inflammation, and cancer-related inflammation has been implicated in the development and progression of RCC (29). In contrast, it is interesting to note that histological analysis revealed that larger adipocytes, no inflammatory infiltrate, and no difference in fibrosis in APF was observed with MAP score of ≥ 2 (30). Further analysis of APF in this regard is warranted.
In our multivariable analysis, R.E.N.A.L. nephrometry score was also found to be an independent factor for prolonged TTC. R.E.N.A.L. nephrometry score is well known to be related to the surgical margin, WIT and complications (5). Although our study evaluated TTC, it was thought that the identification and marking of the tumor before clamping can influence TTC, especially in the case of the endophytic type as a tumor-specific factor. Furthermore, there was no difference in operation time and the incidence of postoperative complications regardless of fat volume or APF status in robot-assisted partial nephrectomy. The reason for the former is that many Asian patients were not large in body size and there was a only a small group of patients with MAP score of 5. Consequently, there was no difference in perinephric fat manipulation. The reason for the latter is that the number of complications in our series was lower to begin with. Several previous studies have also reported that MAP scores do not correlate with complication rates (24, 31).
Several limitations exist in the present study. First, this study of a relatively small patient cohort has possible selection bias due to the retrospective nature. Second, the RAPNs were performed by multiple surgeons during the study period. However, all RAPNs were performed by surgeons highly experienced in the da Vinci Xi system. Therefore, our conclusions may not be directly applicable to situations relevant to all surgeons and institutions. Third, the perinephric fat assessment was obtained only from Japanese patients. Therefore, they should not be applied to other races until our results are validated in a global cohort, since there are clear differences in fat composition and body size by race.