Aim: Systemic inflammation has been associated with chemoresistance and prognosis in solid tumors. Systemic immune-inflammation index (SII) is a novel marker derived from complete blood count. We investigated whether differences between SIIs measured before and after neoadjuvant chemotherapy (NACT) are associated with tumor regression grade (TRG) and survival in gastric and gastroesophageal junction (GEJ) cancer patients.Methods: Records of gastric and GEJ cancer patients treated with NACT in two centers were evaluated retrospectively. Patients were categorized according to difference between pre-and post-NACT SII values (ΔSII). Association between clinicopathological factors and TRG was analyzed using logistic regression method. Predictors of diseasefree and overall survival (DFS and OS) were determined with Cox regression models. Results:The study included 140 patients. Patients with ΔSII<0 were more likely to achieve TRG 0/1 (45.2% vs. 19.1%, p = 0.003) and ΔSII<0 was an independent predictor of TRG 0/1 (OR = 6.05, p<0.001). DFS and OS of patients with ΔSII<0 were also significantly longer (p = 0.031 and p = 0.006, respectively). After adjustment for other variables, ΔSII≥0 was an independent prognostic factor for OS (Hazard ratio (HR) = 2.13, p = 0.008).Conclusions: Changes in SII, which is a low-cost and easily accessible marker, may be used to estimate prognosis, individualize postoperative treatment and optimize surveillance in gastric and GEJ cancer patients treated with NACT.

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