Treatment Outcomes for Gastric Cancer With Positive Peritoneal Lavage Cytology or Localized Peritoneal Metastasis

Importance In the era of modern systemic chemotherapy, the optimal treatment strategy for gastric cancer with positive peritoneal lavage cytology (CY1) and/or localized peritoneal metastasis (P1a) remains controversial. Objective To explore the optimal treatment strategy for patients with gastric cancer with CY1 and/or P1a by comparing the outcomes of various therapeutic strategies. Design, Setting, and Participants This cohort study was a retrospective multicenter study set in 48 institutions of the Stomach Cancer Study Group in the Japan Clinical Oncology Group. Participants were patients with gastric cancer with CY1 and/or P1a, but no other distant metastases, who were treated in 2016 to 2022. Data were analyzed from July 2025 to May 2026. Exposure Patients were stratified according to initial treatment: upfront gastrectomy (n = 838 [71%]) or upfront chemotherapy (n = 343 [29%]). The upfront chemotherapy group was further subdivided into chemotherapy alone (n = 205), gastrectomy after conversion to P0CY0 (n = 92), and nonconversion gastrectomy (n = 46) according to preoperative chemotherapy efficacy. Main Outcomes and Measures The primary outcome was overall survival (OS), and secondary outcomes included progression-free survival. Results Among 1181 patients, the median (IQR) age was 72 (65-78) years; 725 patients (61%) were male and 456 (39%) were female. The overall 5-year OS rate was 25.0% (95% CI, 22.3%-28.0%), with a median survival time of 24.3 months. Upfront gastrectomy showed a significantly longer OS than upfront chemotherapy (5-year OS: 27.6% vs 19.0%; hazard ratio, 0.80; 95% CI, 0.69-0.93; P = .004). Among the multimodal treatment strategies, upfront chemotherapy followed by gastrectomy showed a significantly higher 5-year OS rate than upfront gastrectomy with postoperative chemotherapy (39.9%; 95% CI, 31.7%-50.2%, vs 31.4%; 95% CI, 27.6%-35.8%; P = .01). Gastrectomy alone and chemotherapy alone showed poor prognoses (10.3% and 3.2%, respectively). Notably, survival after gastrectomy following conversion to P0CY0 was the most favorable (5-year OS: 50.5%), whereas that without P0CY0 was modest (5-year OS: 17.2%). Conclusions and Relevance This study found that for gastric cancer with CY1 and/or P1a, upfront chemotherapy followed by gastrectomy resulted in a slightly higher 5-year OS rate than upfront gastrectomy followed by postoperative chemotherapy. Because gastrectomy after conversion to P0CY0 was associated with the most favorable survival, the development of more effective preoperative chemotherapy is warranted.