Background and Purpose: Obesity is often categorized by body mass index (BMI), but abdominal wall and visceral fat quantity vary with equivalent BMI. This study compares multiple fat measurements as predictors of operative complexity during robot-assisted partial nephrectomy (RAPN). Because perinephric fat around the kidney impacts the operative field, it is expected to be a superior predictor of complexity. Patients and Methods: Consecutive RAPN cases were used for multivariate analysis of the association between fat mass (BMI; measurements of abdominal wall fat in anterior, posterior, posterolateral, and lateral orientation, and perinephric fat in anterior, posterior, medial, and lateral orientation) and complexity metrics: Operative time and estimated blood loss (EBL). Adjustments were made for age, sex, tumor side, nephrometry score, comorbidities, and abdominal surgery. Results: Among 53 patients undergoing RAPN, perinephric fat measurements were independently associated with increased EBL and operative time. For each 1-mm increase in medial perinephric fat, EBL increased 24a?‰mL (95% confidence interval [CI] 13-34a?‰mL) and operative time increased 3.3 minutes (95% CI 1.0-5.7a?‰min). For each 1-mm increase in posterior perinephric fat, the EBL and operative time increases were 19a? ‰mL (95% CI 8.1-30a?‰mL) and 3.3 minutes (95% CI 1.0-5.6a?‰min). Abdominal wall fat was not associated with operative time or EBL. Conclusion: Perinephric fat thickness, particularly medial and posterior fat, is associated with increased EBL and operative time during RAPN, independent of BMI and nephrometry score. These data may be helpful for preoperative risk assessment and counseling and could be incorporated in future complexity scores.
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