Increased body mass index is associated with operative difficulty during robot‐assisted radical prostatectomy

Abstract Objective This study aimed to identify factors associated with surgeon perception of robot‐assisted radical prostatectomy (RARP) difficulty. Patients and Methods This study surveyed surgeons performing RARP between 2017 and 2018 and asked them to rate operative conditions and difficulty as optimal, good, acceptable, or poor. These answers were stratified as optimal or suboptimal for this study. Associations between surgeon responses and variables hypothesized to affect surgical difficulty, including anatomic factors such as pelvic diameter and prostate volume:pelvic diameter ratio, were assessed. Results Between November 2017 and September 2018, a total of 100 patients were prospectively enrolled in the study of which 58 cases were rated as optimal and 42 were rated as suboptimal. Of the evaluated variables, only increasing clinical T stage (odds ratio [OR] 1.49, 95% confidence interval [CI] 1.03–2.15, p = 0.03) and increasing body mass index (BMI) (OR 1.14, 95% CI 1.03–1.26, p = 0.01) were associated with increased difficulty; 90‐day complication rates were similar between the optimal and suboptimal cohorts (17.3% vs. 23.8%, respectively; p = 0.5). The number of patients with previous surgery, pelvic diameter, and prostate size:pelvic diameter ratio were not significantly different between cohorts (p > 0.05 for all). Operative time (ρ = 0.23, p = 0.02) and estimated blood loss (EBL) (ρ = 0.38, p = 0.0001) were correlated with suboptimal difficulty. Conclusion The factors associated with surgeon‐reported RARP difficulty were patient BMI and clinical T stage among surgeons with significant RARP experience. These data should be incorporated into surgical decision making and patient counseling prior to performing a RARP.

Despite these technical advantages and widespread use, it is important for urologists and trainees to understand the factors that contribute to increased surgical complexity and difficulty during RARP. Technical challenges such as poor visibility, a small working space, increased intra-abdominal fat, and obscure tissue planes may result in worse perioperative and postoperative outcomes; however, current studies have demonstrated conflicting results. 3 Few studies have directly surveyed surgeons to identify challenging cases and what factors may influence the surgical complexity. Previous studies frequently use surrogate measures of surgical complexity such as estimated blood loss (EBL) or operative time, without directly evaluating surgeon feedback on the case complexity. [4][5][6][7][8][9] This study aimed to identify factors that are associated with a surgeon's perception of increased RARP difficulty. We hypothesized that anatomic factors such as body mass index (BMI) and the pelvic diameter would impact surgeon perception of difficulty. To conduct this study, we used standardized surveys administered during a randomized clinical trial to evaluate surgeon-reported RARP difficulty.

| PATIENTS AND METHODS
Patients enrolled in this study were part of an institutional review board-approved randomized, double-blind clinical trial to evaluate the effect of deep neuromuscular blockade with sugammadex reversal on shoulder pain of patients undergoing RARP at a single institution. The study is registered at https://www.clinicaltrials.gov/ (NCT03210376).
The trial enrolled 100 patients, and as part of the trial, surgeons were asked to reduce insufflation pressure to the minimum level that allows for adequate visibility, which is consistent with how all surgeons routinely perform RARPs at this institution. At the end of each case, surgeons were given questionnaires and asked to evaluate the difficulty of the operation on an ordinal scale, selecting "optimal," "good," "acceptable," or "poor" (Table S1). 10  Most commonly, patients received preoperative systemic therapy for high-risk or clinically node positive disease. Nerve sparing information was collected on the basis of operative notes and rated as "bilateral," "partial or unilateral," or "none." Additionally, based on our hypothesis that pelvic diameter may be associated with perceived difficulty, the transverse pelvic diameter was calculated by using preoperative magnetic resonance imaging (MRI) imaging (which is routinely performed preoperatively at this institution) and measuring the transverse pelvic brim distance as previously described. This measurement was selected on the basis of previous studies looking at pelvic measurements to assess operative difficulty. 11 We additionally created a prostate volume:pelvic diameter ratio as prostate size compared with diameter of the pelvis may impact the space available for robotic instruments and surgical difficulty. Prostate volume was calculated from preoperative MRI imaging measurements (prostate volume [cm 3 ] = 0.52 Â length Â width Â height).
A standard transabdominal RARP was performed by all surgeons in a similar fashion using carbon dioxide insufflation generally set at pressures of 12 mmHg or less. The insufflation pressure was recorded and averaged after each surgery. The performance of a lymph node dissection was at the discretion of each surgeon and was routinely performed for Grade Group 2 (GG2) or higher disease. An extended pelvic lymph node dissection template was most often utilized, which Given the difficulty in determining the clinical significance of a surgeon rating a case as either acceptable or good and also data distribution, we chose to convert the initial rating scale into a dichotomous variable as either optimal (corresponding to the surgeon rating the case as "optimal") or suboptimal (corresponding to the surgeon rating the case as "good," "acceptable," or "poor"). Demographic and clinical variables were compared between the optimal and suboptimal cohorts using Fisher's exact test for categorical variables and the Wilcoxon rank-sum test for continuous variables. Univariable and multivariable logistic regression was used to determine predictors of difficulty, and Spearman correlation was used to determine correlations between difficulty and EBL or operative time surgical duration. Linear regression was used to evaluate the association between EBL and BMI and used to predict EBL across BMI. Statistical significance was considered if two-tailed p value of <0.05. Statistical analysis was performed using Stata/SE Version 16.1 (StataCorp LP, College Station, TX).

| RESULTS
A total of 100 patients were enrolled in the randomized clinical trial between November 2017 and September 2018. Table 1 demonstrates the clinical and pathologic characteristics of the patients divided into two cohorts based on the surgeon-assessed operative difficulty. There were 58 surgeries rated as "optimal" compared with 42 surgeries rated as "suboptimal." The variables noted to be significantly different between the optimal and suboptimal ratings included BMI and clinical T stage. The median BMI was higher in the suboptimal cohort compared with the optimal cohort (30.6 vs. 27.3 kg/m 2 , respectively, p = 0.004). Within the suboptimal cohort, more patients were noted to have cTstage > T1 compared with the optimal cohort (p = 0.03).
No difference was noted in the rate of prior surgery, radiation, prostate volume, prostate volume:pelvic diameter ratio, or preoperative systemic therapy. Type of previous surgery was evaluated and compared between cohorts, and no significant difference was noted between cohorts based on surgery type (Table S2). Overall, there was no statistical difference in pathologic stage over all stages; however, there was a higher rate of stage pT3b in the suboptimal cohort compared with the optimal cohort (28.6% vs. 13.8%). The transverse pelvic brim distance was similar between the two cohorts. No difference was noted in predictors of recurrence such as the extracapsular extension rate (p = 0.7) or positive margin rate (p = 0.99) between the two cohorts.
Surgical variables are listed in Table 2. The median length of surgery was longer in the cases rated as suboptimal versus optimal (median 207 vs. 172.5 min, respectively, p = 0.02). No difference was noted in the median insufflation pressure in the two cohorts. We evaluated the average insufflation pressure by BMI, and no significant association was demonstrated (R 2 = 0.4%, p = 0.5). The EBL was also higher in the suboptimal cohort (median 150 vs. 100 ml, p = 0.0002).
Only two intraoperative complications were identified in the 100 patients, one occurring in each cohort.  (Table 4). Figure 1 demonstrates the distribution violin plot of the BMI stratified by difficulty cohort, demonstrating the higher range of BMI in the suboptimal cohort. Increased BMI was also associated with increased predicted EBL ( Figure S1) (R 2 = 7.5%, p = 0.006).
Lastly, we sought to determine the correlation between EBL and operative time and surgeon-assessed difficulty. Both EBL (ρ = 0.38, p = 0.0001) and operative time (ρ = 0.23, p = 0.02) were significantly associated with increased difficulty (Table S3). operative time to be indicative of surgical difficulty. 17 Several factors have been attributed to the technical challenges when performing RARP on obese patients. These include limited working space leading to poor visualization, increased distance from the skin to the working site, positioning, and ventilation issues when patients are placed in steep Trendelenburg. 17,18 Understanding the difficulties associated with operating on obese patients is important as obesity has been previously associated with not only increasing postoperative complications but also worse measures of postoperative quality of life. 15,18,19 In the current literature, operative time and EBL are often used as surrogate markers for surgical difficulty without directly assessing surgeon-reported difficulty. [4][5][6][7][8][9] We demonstrate that EBL and opera- Within this study, we found a high-grade (Clavien Grade IV-V) postoperative complication rate of 1%. This is consistent with previous studies that report the majority of complications being low-grade (Clavien ≤III) and high-grade complication rates of 1-3%. [26][27][28] The presence of high-grade complications is often influenced by the performance of a lymph node dissection. At our institution, we routinely perform lymph node dissections for all patients with GG2 or higher prostate cancer, and all patients within this study were GG ≥ 2. Given that all patients within this study had a lymph node dissection, this could not be included as a variable associated with surgeon-reported difficulty. We did not find that surgeon-reported difficulty to be strongly associated with rates of overall complications. Notably, we found that readmissions were more common among the optimal cohort. These data suggest that other patient-related factors such as comorbidities are likely more important factors affecting postoperative complication and readmissions than the technical difficulty of a surgery.

| DISCUSSION
This study is strengthened by prospectively assessing surgeonreported case difficulty using a standardized questionnaire. This study, however, has several limitations. The study sample size is relatively small. All surgeons assessed using the questionnaire have significant experience performing RARPs at a single high-volume tertiary care center, which may limit the generalizability of the findings to other populations. This may also explain the lack of a significant difference in complications between the optimal and suboptimal cohorts, even among patients with an elevated BMI. Previous reports demonstrate that patients undergoing RARP at high-volume centers have improved postoperative outcomes and reduced complications. 29 The surgeons in this study have all been in practice for ≥8 years, and the factors influencing perceived surgical difficulty may be different for surgeons assessed earlier in their learning curve. Additional, unmeasured factors that impact surgical difficulty may be present that were not addressed in this study. Lastly, the specific reasons why surgeons rated a RARP as less than optimal were not assessed in this series, and the survey instrument used, although intuitive, has not been previously validated.