Predicting ease of perinephric fat dissection at time of open partial nephrectomy using preoperative fat density characteristics

Abstract

Objective: To predict the ease of perinephric fat surgical dissection at the time of open partial nephrectomy (OPN) using perinepheric fat density characteristics as measured on preoperative computed tomography (CT).

Patients and methods: In all, 41 consecutive OPN patients with available preoperative imaging and prospectively collected dissection difficulty assessment were identified. Using a scoring system that was adopted for the purposes of this study, the genitourinary surgeon quantified the difficulty of the perinephric fat dissection on the surface of the renal capsule at the time of surgery. On axial CT slice centred on the renal hilum, we measured the quantity and density of perinephric fat whose absorption coefficient was between -190 to -30 Hounsfield units. Correlation between perinephric fat surface density (PnFSD) as noted on preoperative imaging and as observed by the surgeon at time of surgery were correlated in a completely 'double-blinded' fashion. Density comparisons between fat dissection difficulties were made using an anova. Associations between covariates and perinephric fat density were evaluated by univariate and multivariate logistic regression analyses. Receiver-operating characteristic (ROC) curves for six different predictive models were created to visualise the predictive enhancement of PnFSD.

Results: PnFSD was positively correlated with total surgical duration (Pearson's correlation coefficient 0.314, P = 0.04). PnFSD significantly correlated with gender (P = 0.001) and difficulty of perinephric fat surgical dissection (P < 0.001) scores. The mean (sd) PnFSD for a dissection that was not difficult (n = 19) was 5598.32 (1367.77) surface density pixel unit (SDPU), and for a difficult dissection (n = 22) was 10272.23 (3804.67) SDPU. Univariate analysis showed gender (P = 0.002), and PnFSD were predictive of the presence of 'sticky' perinephric fat. A multivariate analysis model showed that PnFSD was the only variable that remained an independent predictor of perinephric fat dissection difficulty (P = 0.01). Of the six ROC models assessed, only PnFSD had a significant capability to predict the difficulty of the perinephric fat dissection due to the presence of highly adherent 'sticky' fat, with an area under the curve of 0.87 (P < 0.001).

Conclusion: Accurate preoperative assessment of perinephric fat density constitutes a strong indicator of perioperative fat dissection difficulty. Perinephric fat densities can be practically obtained from preoperative CT to identify 'sticky' fat, which may help determine the anticipated ease of surgical dissection, which can guide education, preoperative surgical planning, and potentially the surgical approach offered to patients.

Keywords: fat density; partial nephrectomy; perinephric fat; renal cell carcinoma (RCC); sticky fat.