Supplementary Material for: The association between number of retrieved lymph nodes and survival in gastric cancer surgery: a Dutch population-based study

Background: This study aimed to evaluate whether the retrieval of 15 or more lymph nodes (LN) during gastrectomy for cancer is associated with better survival and more accurate pathological staging. Methods: Patients that underwent gastrectomy between 2011 and 2016 were reviewed from the Dutch Upper Gastrointestinal Cancer Audit. Patients with <15 and ≥15 LN retrieved were compared after propensity-score matching based on patient and tumor characteristics. The primary endpoint was 3-year overall survival. Results: A total of 2047 patients were included in the study. After propensity score matching, 522 patients with ≥15 LNs were matched to 522 patients with <15 LNs. There was no statistically significant difference in overall survival between both groups, with 3-year survival rates of 56% versus 59%, respectively. Patients with ≥15 LNs had a more advanced pN-category. While median survival was higher for patients with ≥15 LNs versus <15 LNs in the subgroups pN2, pN3a, and pN3b, no statistically significant differences were found. Similar results were found in the propensity score matched cohort using 23 LNs as cut-off. Conclusions: ≥15 LNs retrieved during gastrectomy for cancer was associated with higher pN-stage, likely as a result of stage migration. Three-year overall survival was comparable for patients with ≥15 LNs and patients with <15 LNs retrieved.