Predictive value of baseline eosinophil counts for immune-related adverse events in metastatic NSCLC treated with immune checkpoint inhibitors

Background: Immune checkpoint inhibitors (ICIs) have become a cornerstone in the treatment of metastatic non-small cell lung cancer (mNSCLC). Immune-related adverse events (irAEs) remain a significant challenge, and reliable biomarkers for predicting their occurrence and severity are lacking.  Objective: To evaluate whether baseline eosinophil counts are associated with the onset and severity of immune-related adverse events in patients with metastatic non-small cell lung cancer treated with immunotherapy. Materials and methods: This retrospective single-center study included 52 patients with mNSCLC treated with ICIs. Baseline absolute eosinophil counts were collected prior to treatment initiation. Patients were categorized into three groups based on the highest grade of toxicity observed during therapy: no irAEs, grade 1–2 irAEs, and grade 3–5 irAEs, according to CTCAE v5.0. Comparisons between groups were performed using the Kruskal–Wallis test with post-hoc pairwise analysis. Results: Among the included patients, 28 (53.8%) experienced no irAEs, 15 (28.8%) developed grade 1–2 irAEs, and 9 (17.3%) developed grade 3–5 irAEs. Baseline eosinophil counts increased progressively across toxicity groups, with median values of 110 cells/µL (IQR 67.5–242.5) in patients without irAEs, 180 cells/µL (IQR 115–410) in those with mild irAEs, and 470 cells/µL (IQR 380–650) in patients with severe irAEs. The overall difference between groups was statistically significant (Kruskal–Wallis test, p = 0.026). Pairwise comparisons showed significantly higher eosinophil levels in patients with grade 1–2 (p = 0.034) and grade 3–5 irAEs (p = 0.019) compared with patients without irAEs. Conclusion: Higher baseline eosinophil counts were associated with both the occurrence and severity of immune-related adverse events in patients with metastatic non-small cell lung cancer receiving immunotherapy. Baseline eosinophils represent an accessible and clinically applicable biomarker that may support pretreatment risk stratification and individualized toxicity monitoring. Prospective validation is warranted.