The prognostic role of positive peritoneal cytology in gastric cancer

Background Locally advanced non-metastatic gastric cancer is primarily managed with surgical resection and peri-operative oncological therapies. Positive peritoneal cytology is often considered to be indicative of disseminated malignancy. We have previously shown that conversion from positive (Cy+) to negative (Cy0) cytology with systemic therapy, and absence of macroscopic disease, improves survival. With the advent of novel chemo- and radiotherapy regimes as well as immunotherapy, we recognise a lack of current understanding of the role of peritoneal cytology as a prognostic factor. Our study aims to determine the role of positive peritoneal cytology as a prognostic factor in gastric cancer. Methods This is an international multi-centre retrospective cohort study including high-volume, gastric cancer tertiary referral centres. Adult patients without macroscopic metastatic disease who underwent peritoneal cytology, and subsequently multi-modality treatment including surgical resection between 2011 and 2021 were included. The primary outcome measure is overall survival at 1, 3 and 5 years. Secondary outcome measures include recurrence-free survival at 1, 3 and 5 years; 30-day and 90-day major postoperative morbidity; and subset survival analyses were performed in patients who converted to Cy0 in restaging laparoscopy after neoadjuvant therapy. Statistical analysis was performed using R software. Results This is an interim analysis of the data of 280 patients with a mean age of 64.9 (0.72). 231 patients received pre-operative chemotherapy. 51.0% underwent open gastrectomy (41.2% laparoscopic and 4.64% robotic). 55% of patients underwent a total gastrectomy. 25.9% patients had positive cytology on laparoscopy, while 9.71% had indeterminate cytology. There was significant variation in the sites of peritoneal washings (Figure 1). So far, only 1 patient has shown conversion from positive to negative cytology after oncological therapy. At 1 year, the mortality rate was 41.5% (figure 2), while the recurrence rate was 41.4%. Conclusions In this interim report, we demonstrate significant heterogeneity in the way peritoneal cytology is performed, and the peri-operative management of this cohort. The POPEC study closes to patient recruitment at the end of August 2023, and thus at AUGIS we plan to report on the final findings of this study with an anticipated sample size of over 1000 patients.