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Clinicopathological and prognostic significance of microsatellit instability (MSI) status and PDl-1 expression in Turkish patients with gastric cancer.

e15538 Background: The aim of this study was to evaluate the prognostic role of microsatellite instability (MSI) status and PD-L1 expression in surgically resected gastric cancer and the relationship of these parameters with clinicopathological features. Methods: Eighty six gastric cancer patients which had curative surgical resection at Acibadem Atakent and Maslak Hospitals between 2010 and 2017 were analysed. Tumor samples were evaluated with MSI and PD-L1 antibodies by immunohistochemical (IHC) methods. PD-L1 IHC scoring was performed using the combined positive score (CPS). Survival analysis was accomplished using the Kaplan-Meier method. Cox proportional hazard regression model was used to identify independent risk factors that affect patients' overall survival. Results: The rate of PD-L1 expression in tumor cells was 34.9% (n = 30) and the frequency of PD-L1 expression in immune cells with CPS (≥1%) was 57% (n = 49). MSI-H was detected in 11.6%(n = 10), and more observed in PD-L1 positive cases (p = 0.021). MSI-H status was significantly correlated with older age, increased tumor size, presence of PD-L1 expression, and adenocarcinoma subtype. PD-L1 expression was associated with lymph node metastasis, adenocarcinoma subtype, microsatellite instability, presence of preoperative treatment and improved response to preoperative chemotherapy. In our study, the impact of MSI status on survival was not demonstrated, but PD-L1 expression positivity(≥1%) in both tumor cells (15.7 vs 53.4 months, p = 0.008)and in immune cells (20.4 vs NR; p = 0.027) was associated with short overall survival. PD -L1 expression in tumor cells was an independent prognostic factor for overall survival in multivariate analysis (HR: 2.28, p = 0.047). Conclusions: PD-L1 expression was related to a poor prognosis in patients with gastric cancer and can represent a rational approach for PD-1/PD-L pathway-targeted immunotherapy.