Divergent Outcomes in Lung Adenocarcinoma with Paraneoplastic Hypereosinophilia in Patients Treated with Pembrolizumab: A Two-Case Report.

Paraneoplastic hypereosinophilia (PHE) is an underrecognized manifestation of solid tumors, particularly non-small cell lung cancer. We present two cases of metastatic lung adenocarcinoma with marked eosinophilia treated with PD-1 inhibitor immunotherapy, both of whom demonstrated changes in eosinophil counts but divergent clinical outcomes. While elevated eosinophil levels during immune checkpoint inhibitor therapy have been associated with improved treatment response in prior studies, the clinical significance of persistent or severe hypereosinophilia remains incompletely understood. These cases highlight the heterogeneous course of paraneoplastic hypereosinophilia and underscore the potential-yet uncertain-role of eosinophils as prognostic biomarkers in oncology, warranting further investigation to clarify the role of PHE as a prognostic or predictive biomarker in oncology.