Supplementary Material for: Role of Up-Front Primary Tumor Resection and Tumor Sidedness in the Survival of Synchronous Metastatic Colon Cancer Patients

<b><i>Introduction:</i></b> Uncertainty exists about a possible survival benefit of primary tumor resection (PTR) in synchronous metastatic colon cancer (mCC). Since sidedness of the primary tumor is regarded as an important prognostic factor, our objective was to study the interaction between PTR and sidedness in synchronous mCC. <b><i>Methods:</i></b> In this retrospective study, we used data from 2 first-line phase 3 randomized controlled trials (RCTs). A mixed Cox regression model was used to study the multiplicative interaction between PTR and sidedness. We adjusted for age, treatment arm, WHO performance status, number of affected organs by metastases, serum lactate dehydrogenase, and year of enrollment. <b><i>Results:</i></b> We found that PTR is associated with better survival in both right-sided (hazard ratio [HR] 0.59 [95% confidence interval 0.42–0.8 2]) and left-sided mCC (HR 0.70 [95% confidence interval 0.52–0.93]). The interaction between PTR and sidedness was not significant (<i>p</i> = 0.45). <b><i>Conclusion:</i></b> Our data suggest that the prognostic value of PTR is independent of sidedness. Validation of these results will be performed in ongoing RCTs.