Significance of atypical peritoneal cytology in patients with locally advanced gastric cancer.

454 Background: While positive peritoneal cytology is associated with worse clinical outcomes in patients with gastric cancer, the significance of atypical peritoneal cytology remains unclear. This study evaluated the incidence of atypical peritoneal cytology and its association with survival and peritoneal recurrence. Methods: From a retrospective database of patients who underwent curative-intent surgery at our institution between 2011 and 2022, we identified patients with pT3 and pT4 stages of gastric cancer who had preoperative diagnostic laparoscopy with peritoneal washing and cytology results. Results: A total of 92 patients with preoperative peritoneal cytology results of benign (n=72, 78.3%), atypical (n=17, 18.5%), or malignant (n=3, 3.3%) were included. Baseline characteristics, comorbidities, labs, and clinical characteristics were not statistically different among the three groups. Median overall survival (OS) was 81 months (m) for the benign group, 30 m for the atypical group (hazard ratio (HR) 1.79; 95% confidence interval [0.76, 4.19]; p=0.2), and 22 m for the malignant group (HR 3.23 [0.74, 14.0]; p=0.12). Median peritoneal recurrence-free survival (RFS) was not reached for benign, 15 m for atypical (HR 3.09 [1.38, 6.89]; p=0.006), and 11 m for malignant (HR 6.46 [1.82, 22.8], p=0.004). The rate of peritoneal recurrence was 25% (n=18) for benign, 53% (n=9) for atypical, and 100% (n=3) for malignant. Conclusions: There is a high incidence of atypical peritoneal cytology (18.5%) among patients with locally advanced gastric cancer who underwent curative-intent surgery. The atypical group demonstrated outcomes between those of the benign and malignant groups, indicating underlying heterogeneity and diagnostic uncertainty of this category. Our analysis is limited by single-institutional data and a small sample and event size; the size of the malignant group was especially limited given the nature of the surgical database. Further investigation is needed to improve prognostication, stratify those at highest risk for peritoneal recurrence, and define optimal treatment for patients with atypical peritoneal cytology.