Multicenter retrospective study on treatment outcomes of gastric cancer with positive peritoneal cytology in Japan.

309 Background: Cytology-positive (CY1) gastric cancer is associated with poor prognosis, with a 5-year overall survival (OS) rate of approximately 25%. However, there is still no established consensus on the optimal therapeutic strategy. This study explored perioperative treatment and long-term outcomes for CY1 gastric cancer using a large-scale dataset in Japan. Methods: A total of 1190 patients without prior therapy who were diagnosed as CY1 and/or P1a at initial surgery between 2016 and 2022 were retrospectively collected from 48 institutions of the JCOG Stomach Cancer Study Group. Patients with other distant metastases and those who did not undergo gastrectomy or chemotherapy were excluded. Treatment details and long-term outcomes were evaluated according to treatment strategy. Results: At initial surgery, 853 patients were P0CY1, 135 were P1aCY0, and 202 were P1aCY1. Initial treatment was gastrectomy in 847 (71.2%) patients (S-group) and chemotherapy in 343 (28.8%) patients (C-group). In the C-group, S-1 + oxaliplatin (SOX) was most common (46.6%), followed by S-1 + cisplatin (SP) (9.9%), SOX + nivolumab (7.3%), and SOX + trastuzumab (5.5%), and 138 underwent subsequent gastrectomy, in whom 92 achieved conversion to P0CY0. Among 985 patients with gastrectomy, distal, total, and proximal procedures accounted for 46.9%, 51.4%, and 1.7%, respectively, resulting in R0 resection in 18.2%, R1 in 73.7%, and R2 in 8.1%. Postoperative chemotherapy was administered in 783 patients (79.5%), mainly S-1 (42.5%), SOX (21.6%), and S-1 plus docetaxel (DS) (17.9%). The 5-year OS was 27.5% in the S-group and 18.4% in the C-group (p=0.005). According to postoperative chemotherapy regimen, 5-year OS was 30.0% with S-1, 32.8% with SOX, and 37.6% with DS. By treatment strategy, the 5-year OS was 31.4% for gastrectomy plus postoperative chemotherapy, 10.0% for gastrectomy alone, 48.4% for gastrectomy after conversion to CY0, 13.8% for gastrectomy without conversion, and 2.4% for chemotherapy alone. Conclusions: Although prognosis remains poor of gastric cancer with CY1 and/or P1a, the most common treatment strategy with upfront gastrectomy followed by post-operative chemotherapy in Japan achieved better outcomes than those with chemotherapy or gastrectomy alone. Relatively favorable survival in patients achieving conversion to P0CY0 or receiving intensive postoperative chemotherapy highlights potential benefits of multimodal treatment strategies.