Abstract Background: The efficacy of conversion therapy for gastric cancer with peritoneal metastasis (P1) and/or positive peritoneal lavage cytology (CY1) treated with immunotherapy remains unclear. This study aimed to clarify clinical outcomes in this setting. Methods: In this retrospective, single-institution cohort study, 144 patients with P1 and/or CY1 confirmed by staging laparoscopy at initial diagnosis received first-line immunotherapy with chemotherapy and/or trastuzumab between November 2019 and March 2024. Patients were divided into a conversion therapy group and a palliative therapy group. Results: Of 144 patients, 75 showed a clinical response and underwent second-look laparoscopy. Negative peritoneal metastasis and cytology were confirmed in 36 patients, all of whom underwent conversion surgery. Compared with the palliative group, the conversion group had significantly longer median progression-free survival (22.9 vs . 5.8 months; P 2 (not reached vs . 28.5 months; P <0.006). No serious treatment-related adverse events led to treatment discontinuation or death. Conclusions: Conversion surgery following immunochemotherapy provided a significant survival benefit for gastric cancer patients with P1 and/or CY1. When immunochemotherapy is effective, active staging laparoscopy should be performed to assess surgical feasibility.
Liang et al. (2026) studied this question.