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Sawada et al. Mini-invasive Surg 2021;5:6 I http://dx.doi.org/10.20517/2574-1225.2020.100 Page 3 of 11

Vinci S or Xi surgical system (Intuitive, CA, USA). In many cases, the renal artery was clamped using a
bulldog clamp. However, when the tumor was superficial and peripheral, the zero ischemia technique was
[15]
performed, in which the renal artery was not clamped . The tumor was then resected along its outline, as
confirmed by ultrasonography beforehand. The resection margin was 3-5 mm. If the renal pelvis was open,
a central suture was performed to ensure that there was no urine leak before renorrhaphy. The renal artery
was declamped after renorrhaphy to check for bleeding from the cut surface.

OPN was performed using the subcostal or flank approach. In most cases, the retroperitoneal approach was
used, and OPN under cold ischemia was performed. The renal artery was clamped, and the entire kidney
[16]
was surrounded by ice slush for 5-10 min before tumor resection . Open calyces and bleeding sites were
carefully repaired and renorrhaphy was performed. The renal artery was declamped after renorrhaphy.

Outcomes of interest
The primary and secondary outcomes were examined and compared as evaluation points between RAPN
and OPN.

The primary outcomes were perioperative outcomes, namely estimated blood loss (EBL), operative time,
ischemia time, and hospital stay. All intraoperative and postoperative complications were also evaluated
[17]
based on the Clavien-Dindo (CD) classification .
The secondary outcomes were pathological and functional outcomes, namely the rates of malignancy,
positive surgical margins in malignancy, and pathological stage. Renal function was measured at baseline
and at 1 day and 3 months postoperatively based on the estimated glomerular filtration rate (eGFR). The
ratio of eGFR at both 1 day and 3 months postoperatively to the baseline eGFR (% preservation of eGFR)
was used as an index to evaluate the postoperative residual renal function.

Covariates
Patients’ preoperative variables were analyzed as covariates, including age at treatment, sex, body mass
[18]
index (BMI), Charlson comorbidity index (CCI) , preoperative eGFR, clinical stage, clinical tumor
size (the maximum diameter at preoperative imaging), and tumor side (left or right). Tumor complexity
and anatomical characteristics were determined by the urologist and defined using the total “RENAL”
[19]
nephrometry score , namely Radius (tumor size as maximal diameter), Exophytic/endophytic properties
of the tumor, Nearness of tumor’s deepest portion to the collecting system or sinus, Anterior/posterior
descriptor, and the Location relative to the polar line.

Statistical analyses
[20]
Statistical analyses and interpretation of the results were performed according to established guidelines .
Continuous variables are presented as median and interquartile range (IQR) or mean and standard
deviation. Categorical variables are presented as frequency and proportion. Differences in the distribution
of continuous and categorical variables between the RAPN and OPN groups were compared using the
Mann-Whitney and chi-square tests, respectively.

Adjustments were made using 1:1 nearest-neighbor PSM to account for possible baseline differences
between patients who underwent OPN and RAPN . Propensity scores were calculated using a logistic
[21]
regression model with odds of receiving RAPN as a dependent variable and age at treatment, sex, BMI,
CCI, preoperative eGFR, clinical stage, clinical tumor size, tumor side (right or left), individual RENAL
score item, and total RENAL nephrometry score as independent variables. After balanced matching of
covariates, the effects of the surgical procedures on outcomes were estimated using the Mann-Whitney and
chi-square tests for continuous and categorical variables, respectively.