Predictive role of systemic immune-inflammation index in the prognosis of patients with advanced left-sided colorectal cancer: a retrospective study.

Purpose: This study aimed to evaluate the prognostic value of the systemic immune-inflammation index (SII) in patients with advanced left-sided colorectal cancer (CRC) receiving CAPEOX ± bevacizumab as first-line chemotherapy. Methods: A total of 231 patients with advanced left-sided CRC who received first-line CAPEOX ± bevacizumab therapy were included. Patients' blood cell counts, clinical, and pathological data were collected before treatment, and systemic inflammatory indices were calculated, including neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), monocyte-to-lymphocyte ratio (MLR), and SII. Optimal cutoffs for NLR, PLR, MLR, SII, and age were determined using X-Tile software, categorizing patients in to high- or low-value groups; clinicopathological characteristics were then compared between the high- and low-value groups within each systemic inflammatory index using chi-square tests. Survival curves were estimated using the Kaplan-Meier method, with log-rank tests applied to compare differences between groups. The Shapiro-Wilk test for normality and Spearman correlation analysis were used to evaluate the correlations among SII, NLR, PLR, and MLR. Univariable and multivariable analyses were performed with the Cox proportional hazards regression model. The prognostic value of systemic inflammatory indices was compared using the concordance index (C-index) and 5-fold cross-validation. The stability and generalizability of the C-index under varying data partitions were evaluated using mean square error (MSE). Interaction effects between treatment regimens and SII were further explored using multivariable Cox regression analysis. Results: Univariable and multivariable Cox regression analyses identified age, primary tumor resection, SII, NLR, PLR, and MLR as independent prognostic factors for overall survival (OS). Comparative analysis of the C-index and MSE in the training and validation datasets demonstrated that SII outperformed NLR, PLR, and MLR, exhibiting the highest average C-index and the lowest MSE across 5-fold cross-validation. Patients with elevated pre-treatment SII had significantly worse OS compared to those with lower values. Conclusion: SII is a robust prognostic marker in patients with advanced left-sided CRC receiving CAPEOX ± bevacizumab as first-line chemotherapy, demonstrating superior prognostic value compared to NLR, PLR, and MLR. Higher pre-treatment SII values were associated with worse OS, underscoring its clinical utility in prognostic stratification.