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Page 6 of 14 Abe et al. Mini-invasive Surg 2023;7:28 https://dx.doi.org/10.20517/2574-1225.2023.15

[10]
technique or a hand-sewn anastomosis. Both MIE and RAMIE abdominal manipulations were performed
in the same way.

Recording of clinical data and postoperative complications
Patient demographic data and clinical outcomes were compared between the MIE and RAMIE groups. Data
included age, gender, body mass index (BMI), American Society of Anesthesiologists (ASA) score, Charlson
comorbidity index (CCI), preoperative therapy (chemotherapy or CRT), tumor characteristics, operative
procedure, operative time, console time, intraoperative blood loss, duration of hospital stay, incidence of
postoperative complications, and short-term outcomes. The clinical staging of tumors was performed
according to the 8th edition of the TNM classification . Postoperative complications were classified
[11]
[12]
according to the Clavien-Dindo classification .
Propensity score matching
To control potential differences in patient characteristics between the two groups, we used propensity score
matching to assemble comparable groups. After estimating the propensity score of patients in the RAMIE
group, we matched each patient sequentially to a patient in the MIE group who had the closest propensity
score using simple 1:1 nearest neighbor matching. We imposed a 0.20 caliper of the propensity score logit
standard deviation. We included age, gender, cStage, CCI, ASA-physical status (ASA-PS), main tumor
location, neoadjuvant therapy, dissection field, and abdominal procedure as covariates.

Statistical analysis
The results are expressed as the median with a range for continuous variables. Differences between groups
were analyzed using the Fisher exact test or the Mann-Whitney U test, as indicated. A P-value < 0.05 was
considered statistically significant. All statistical computations, including propensity scores, were carried out
with SAS software (version 12; SAS Institute, Inc., Cary, NC, USA).

The cumulative sum (CUSUM) method was used to quantify the effect on the operative time learning curve.
We used CUSUM plots to analyze the RAMIE learning curve in patients with esophageal cancer . A
[13]
learning curve was considered complete at the point at which the surgical time decreased on the CUSUM
plot.

RESULTS
MIE was introduced in 2012, and 535 cases were performed until August 2022. RAMIE was started in 2019,
and 84 cases were performed by August 2022. Of the 535 MIE cases, 208 cases of MIE were performed from
January 2019 to August 2022.

To analyze the number of cases required for RAMIE standardization, the console time was calculated using
CUSUM analysis. The calculated CUSUM learning curve was identified graphically to consist of three
phases: phase I (28 initial cases), phase II (14 mid-term cases), and phase III (42 final cases; Figure 3). The
slope of the CUSUM for phase I was positive, indicating insufficient procedural proficiency. The slope for
phase II was variable but generally plateaued, suggesting that the console surgeon had achieved the learning
point. In contrast, the slope of CUSUM in phase III tended to decrease, indicating that the surgeon has
acquired technical proficiency.

Patient demographics
To compare the outcomes of MIE and RAMIE, we compared 208 MIE and 84 RAMIE cases performed
from 2019, at the time when RAMIE was introduced, to September 2022. Table 1 shows the background
factors between the MIE and RAMIE groups. There were no differences in background factors between the