While the relationship between eosinophilia and tumors has been extensively studied, eosinophilia is primarily observed clinically after tumor development, with no prior reports of eosinophilia occurring before tumor onset. This case report presents a unique instance of eosinophilia preceding the development of gastric cancer in an old male patient.

The patient was evaluated after being diagnosed with gastric cancer. Investigations included physical examinations, hematological evaluations, molecular assays for genetic variations associated with eosinophilia, tumor marker assessments, staging using the AJCC system, histopathological examinations, and comprehensive sequencing of tumor tissue.

The patient exhibited disseminated erythematous macules and had a history of eosinophilia. Genetic testing for germline variations associated with eosinophilia was negative, and molecular assays did not detect significant genetic alterations linked to clonal eosinophilia. Staging revealed T1N0MX gastric cancer, with histopathology showing high-grade intraepithelial neoplasia and positive PD-L1 expression. Notably, sequencing identified mutations in the NOTCH1 and NOTCH2 genes, which are known to regulate eosinophil migration. After surgical intervention, pathological examinations confirmed high-grade squamous cell carcinoma in the esophagus and high-grade intraepithelial neoplasia in the stomach, without eosinophil infiltration. Surprisingly, the peripheral blood eosinophilia dramatically decreased following tumor resection.

This case underscores the potential for eosinophilia to occur prior to tumor development, challenging the current understanding of the relationship between eosinophilia and cancer. Further research is warranted to explore the implications of eosinophilia in cancer pathogenesis and its clinical significance.