Systemic immune-inflammation index as a prognostic biomarker to predict overall survival after primary stereotactic radiosurgery for brain metastases

To evaluate the usefulness of the systemic immune-inflammation index (SII) and systemic inflammation response index (SIRI) in predicting local tumor control (LC) duration and overall survival (OS) in patients with brain metastases treated with primary stereotactic radiosurgery (SRS). All consecutive patients who underwent fractionated SRS between April 2018 and December 2022 were retrospectively analyzed. Statistical analyses included Cox regression and Kaplan-Meier curves. A total of 132 metastases in 65 patients were analyzed. The median SII was 909 (IQR 524–1696), and the median SIRI was 1.6 (IQR 0.7–3.1). The median LC duration was 10 months, with 66.7% of lesions under control, 13.6% progressed, and 19.7% had unknown status due to insufficient imaging. The median OS was 11 months; 83% of patients had died by the last follow-up. SII and SIRI were not associated with LC duration in univariable Cox regression analysis. However, higher SII (p = 0.05) was significantly associated with worse OS in multivariable Cox regression analysis, along with male sex (p < 0.01) and gastrointestinal cancer origin (p = 0.02). These findings were also supported by Kaplan-Meier curves with log-rank tests. The optimal SII cutoff for predicting OS was 2600 (concordance index 0.72; hazard ratio 2.65). SIRI was associated with OS in univariate analysis (p = 0.03), but not multivariable analysis. This study suggests that higher SII was associated with worse OS and that it can be useful markers to predict OS, but not LC duration.