Association between the presence of malignant ascites and survival outcomes in gastric cancer patients treated with nivolumab plus chemotherapy.

Although malignant ascites is associated with poor outcomes in gastric cancer (GC), its clinical value in first-line immune checkpoint inhibitor (ICI)-based chemotherapy remains unclear. This single-center study aimed to evaluate the impact of malignant ascites in patients with advanced GC treated with first-line nivolumab plus chemotherapy (n = 339) or chemotherapy alone (n = 422). Peritoneal involvement and malignant ascites grade were assessed using computed tomography imaging. Patients were classified into no peritoneal metastases (PM), PM without ascites, and PM with ascites groups. In the nivolumab plus chemotherapy group, patients with PM and ascites exhibited significantly worse survival (median PFS: 5.1 months; OS: 11.6 months) than those with no PM (median PFS: 10.2 months; OS: 22.0 months) or PM without ascites (median PFS: 10.8 months; OS: 19.7 months) (p < 0.001). While nivolumab plus chemotherapy was evidently associated with favorable survival outcomes for patients without malignant ascites compared to chemotherapy (PFS: p < 0.001; OS: p = 0.01), such survival benefits were not observed in those with PM and ascites (PFS: p = 0.09; OS: p = 0.39). This trend persisted in the PD-L1 combined positive score ≥5 and deficient mismatch repair subgroups. Our results reveal that malignant ascites is associated with poor survival with nivolumab plus chemotherapy, and limited benefit from adding nivolumab to chemotherapy in patients with GC. We suggest that malignant ascites may be considered a stratification factor in future clinical trials of ICI-based treatments for GC.