The prognostic value of the Systemic Immune-Inflammation Index at diagnosis in small cell lung cancer

Aims: The Systemic Immune-Inflammation Index (SII) is a blood-based marker of systemic inflammation with prognostic value in several cancers, but its role in small cell lung cancer (SCLC) relative to simpler indices remains unclear. We evaluated the association between the diagnostic SII and overall survival (OS) in SCLC and compared its prognostic performance with the neutrophil-to-lymphocyte ratio (NLR) and Prognostic Nutritional Index (PNI). Methods: In this single-centre retrospective cohort, 100 patients with histologically confirmed SCLC were analysed. The SII, NLR, and PNI were calculated at diagnosis. Discrimination was assessed using Harrell's concordance index, and patients were dichotomized at the median SII for the survival analyses. Survival was analysed using Kaplan-Meier, log-rank, and Cox regression, with the primary multivariable model stratified by stage. Results: The median age was 69 years; 70% had extensive-stage disease, and 89% died over a median observed time (from diagnosis to death or last contact) of 9.9 months. The SII showed modest discrimination (Harrell C-index 0.564). In the primary multivariable analysis stratified by disease stage, the SII was not independently associated with OS (adjusted HR 1.39, 95% CI 0.90-2.14, p=0.139), and it did not add significant value to a clinical model containing age, gender, and stage (ΔC-index 0.008, 95% CI -0.025 to 0.050). The SII, NLR, and PNI showed comparable and modest prognostic performance (C-index 0.56-0.60), with no significant differences. Conclusion: A higher diagnostic SII was associated with worse survival in SCLC but behaved as a modest marker, with no statistically significant superiority over the NLR or PNI demonstrated in this limited cohort, with its independent value limited by disease stage.