Systemic immune-inflammation index as a predictor of venous thromboembolism in NSCLC patients treated with ICIs: a retrospective study

Venous thromboembolism (VTE) is a serious complication in non-small cell lung cancer (NSCLC), potentially exacerbated by immune checkpoint inhibitors (ICIs). The systemic immune-inflammation index (SII) is a simple biomarker associated with cancer outcomes, but its value for predicting VTE in NSCLC under ICIs remains unclear. We conducted a retrospective analysis of 264 patients with NSCLC who were treated with ICIs at Zhongshan Hospital (Xiamen), Fudan University between September 2020 and January 2025. The association between baseline SII and VTE risk was evaluated using Spearman correlation, Restricted cubic splines (RCS), receiver operating characteristic (ROC) curve analysis, and Fine–Gray competing risk models. Additional analyses included multinomial logistic regression for early- (≤3 months) versus late-onset (>3 months) VTE, landmark analyses, and subgroup analyses across different clinical and treatment characteristics. VTE occurred in 35 patients (13.3%). High SII correlated with hematologic parameters and showed a linear association with VTE . ROC analysis identified a cutoff of 847.5 (AUC =0.60). The incidence of VTE was higher in the high-SII group (21.4% vs. 8.1%, P = 0.002). Elevated SII independently predicted VTE (SHR=3.20, 95% CI: 1.67–6.12), affecting both early (RRR=2.74) and late events (RRR=2.12). Sensitivity analyses confirmed robustness. SII is an independent predictor of VTE in NSCLC patients on ICIs and may serve as a practical tool for risk stratification. Prospective validation is warranted.