Prognostic impact of positive peritoneal cytology (POPEC) in gastric cancer: Multi-centre European retrospective cohort study

Objective: Positive peritoneal cytology is traditionally viewed as representative of metastatic disease and a poor prognostic factor. The objective of this multi-center study was to define the prognostic role of peritoneal cytology in curative gastrectomy, evaluate international variation in cytology sampling, and assess the impact on positive peritoneal cytology yields. Methods: This was a multi-center international retrospective cohort study of 16 tertiary gastric cancer centers. Adult patients who underwent peritoneal lavage cytology at staging laparoscopy and subsequent gastrectomy between 2009 and 2023 were included. The primary outcome measure was overall survival at five years. Multivariable Cox regression provided hazard ratios (HRs) with 95 % CIs, adjusted for relevant confounding factors. Results: 837 patients with no radiological or macroscopic M1 disease were included, with a mean age of 66 (IQR 58–73) and 71 % were male. Non-distal gastric cancer was most common (47 %), with 59 % and 43 % of tumors staged pT3/4 and pN2/3, respectively. 66 patients (7.9 %) had positive cytology. Positive cytology was not associated with overall survival in multivariable analysis, controlled for stage and neoadjuvant treatment (HR=1.0; 95 %CI 0.51–2.0). Higher T and N stages were associated with positive cytology (p < 0.001). The proportion of patients with positive cytology was variable, depending on how many quadrants were sampled. Conclusion: Positive peritoneal cytology with otherwise M0 disease was not associated with decreased survival after curative intent gastrectomy in this study, meaning prospective study is needed. The technique of performing peritoneal washings influenced cytology yield and thus must be standardized in a much-needed prospective evaluation of peritoneal cytology. Synopsis: The POPEC multicenter international retrospective cohort study included 837 patients receiving curative gastrectomy. This study showed the technique of performing peritoneal washings influenced cytology yield, however positive peritoneal cytology was not associated with decreased survival. Therefore, positive peritoneal cytology should not be considered an absolute contradiction to curatively intended gastrectomy.