Eosinophils and the Efficacy of Immune Checkpoint Inhibitors Across Multiple Cancers: A Retrospective Study.

Background/Objectives: Immune checkpoint inhibitors (ICIs) have transformed cancer therapy; however, their efficacy varies among patients. Eosinophils have been reported as prognostic markers in chemotherapy-treated pancreatic cancer; however, these studies are limited mainly to single cancer types and relatively small cohorts. Therefore, we aimed to examine the relationship between eosinophil count and the effectiveness of ICIs across various cancer types. Methods: We retrospectively analyzed 138 patients treated with ICI monotherapy, ICI plus chemotherapy, or chemotherapy alone at our institution between December 2015 and September 2024. Peripheral blood parameters, including eosinophil counts and albumin levels, were collected at baseline and after two cycles of treatment. Associations between overall survival (OS) and progression-free survival (PFS) were assessed using Kaplan-Meier analysis and Cox proportional hazards regression. Results: In the ICI monotherapy group, patients with higher baseline eosinophil counts had significantly longer OS (HR 0.26, p = 0.007) and PFS (HR 0.30, p = 0.005). No significant associations were observed between the ICI plus chemotherapy and chemotherapy-alone groups. Changes in eosinophil counts between baseline and after two cycles were not associated with outcomes in any group. Conclusions: Baseline eosinophil counts were significantly associated with survival outcomes in patients receiving ICI monotherapy and may serve as a promising predictive biomarker.