Study based on existing databases investigating the causal oncological treatment effects of complete mesocolic excision on UICC stage I-III right-sided colon cancer.

Population-based cohort study, predominantly prospective based, on the same population as the investigator's previously study comparing short-term outcome after CME with conventional colon resections. The COMES database combines the prospectively registered colon cancer database in Hillerød (CME data), and data from the national database of the Danish Colorectal Cancer Group (DCCG) covering patients undergoing conventional resection (non-CME) in the other three centers. The medical records of all the patients in the non-CME group (control group) were reviewed by colorectal surgeons from Hillerød. Data audit for all CME patients was performed by various co-authors employed at the other centers. A similar audit of data for non-CME patients having postoperative complications or recurrence was performed by the co-author representing the department treating the specific patients.

Statistical analysis plan

Continuous data are presented as median and interquartile ranges, and categorical data as frequencies and proportions. Kruskal-Wallis test and Fisher's exact test were used as appropriate. Death is a competing risk to recurrence and time-to-event analyses were performed as competing risk analyses obtaining the cumulative incidences for recurrence or death using the "cmprsk" R-package.

Unbiased estimation of marginal or population-averaged treatment effects in observational and non-randomized studies can be obtained through different propensity score methods. Inverse Probability of Treatment Weighting (IPTW) uses the propensity score to weight each patient's data based on the inverse probability of receiving the treatment actually received. IPTW gives unbiased estimates of average treatment effects in time-to-event analyses if no differences in observed baseline covariates exist between the treatment groups. To account for baseline differences between patients in the two groups, stabilized weights truncated at the 0.99 interval were calculated using the "IPW" R-package. The following baseline covariates will be used: age, sex, ASA score, neoadjuvant chemotherapy, tumor location, tumor morphology, perineural invasion, extramural venous invasion, tumor stage, and serosal invasion. All covariates used and UICC stage, two-way interactions, and squared terms of continuous covariates will be assessed for balance between the CME and the non-CME group after IPTW using the "cobalt" R-package. Absolute mean differences in mean (using standardized mean difference) and proportions (using raw mean difference) below 0.1, variance ratios between 0.5 and 2, and Kolmogorov-Smirnov tests equal or below 0.05 will be accepted. Graphical inspection of the distribution of covariates will be also performed.

The cause-specific hazards and overall survival will be analyzed using Cox regression. Binary outcomes will be analyzed using logistic regression. Lymph node yield will be analyzed using linear regression after logarithmic transformation. Number of metastatic lymph nodes will be analyzed using negative binominal regression. All analyses of primary and secondary outcomes will be performed after IPTW. The 95% confidence intervals for the estimates from the original Cox regression IPTW analyses will be constructed after 1000 bootstraps with replacement, and a robust sandwich estimator will be used for the logistic regression analyses in order to ensure correct variance estimation.

All available data will be used. Model assumptions will be checked. A p-value below or equal to 0.05 will be considered significant. All analyses will be performed using R statistical software, version 3.5.1 (R Foundation for Statistical Computing, Vienna, Austria).

Inclusion Criteria:

• Right-sided colon cancer was defined as primary adenocarcinomas located in the cecum, ascending colon hepatic flexure, right or mid third of the transverse colon.
• UICC stage I-III - Gastroepiploic and infrapyloric lymph node metastases are not considered as distant metastases

Exclusion Criteria:

• Synchronous colorectal cancer - even in the right colon
• No residual tumor in the specimen after neoadjuvant chemotherapy
• Metachronous colorectal cancer
• Appendix cancers
• Resections in Hillerød not performed according to the principles of CME
• Non-macroradical (R2) resections (peroperative assessment)

Data for all patients undergoing elective surgery for UICC stage I-III colonic adenocarcinoma in the Capital Region of Denmark. The population of this region is approximately 1⋅75 million, more than 30 per cent of the population of Denmark, and it is served by only four public university colorectal cancer centres.