Prognostic Significance of Post-Radiotherapy Neutrophil-to-Lymphocyte Ratio and Systemic Immune-Inflammation Index in De Novo Metastatic Breast Cancer

Purpose To determine whether post-radiotherapy (RT) neutrophil-to-lymphocyte ratio (NLR) and systemic immune-inflammation index (SII) predict survival in women with de novo metastatic breast cancer (dnMBC). Methods In this single-centre retrospective study, 132 dnMBC patients treated with external-beam RT between 2012 and 2022 were reviewed. Complete blood counts were obtained a median of 2 weeks after RT and before systemic therapy. Receiver-operating-characteristic analysis and published data set cut-offs at NLR > 3 and SII > 600. Progression-free survival (PFS) and overall survival (OS) were estimated with Kaplan–Meier; group differences were tested with the log-rank and Mann–Whitney U tests. Progression risk was assessed by binary logistic regression. Results Median follow-up was 29 months. Mean PFS was 46.0 ± 30.8 months in the NLR ≤ 3 group versus 36.8 ± 29.3 months in the NLR > 3 group ( p  = 0.008). Two-year PFS rates were 55% and 38%, respectively. NLR > 3 increased the odds of progression (OR = 1.20, 95% CI 1.02–1.42, p  = 0.026) but had no impact on OS (26.6 ± 25.7 vs 27.9 ± 27.7 months, p  = 0.797). SII (≤ 600 vs > 600) was not associated with either PFS ( p  = 0.723) or OS ( p  = 0.458). Conclusion A single NLR measurement obtained soon after RT independently identifies dnMBC patients at higher risk of early progression, supporting its use as a low-cost clinical stratifier. SII conferred no additional prognostic value in this setting. Prospective multicentre studies are required to validate these findings and to evaluate serial NLR dynamics.