Detection of FIP1L1–PDGFRA fusion gene-positive cells in the skin lesion of a patient with hypereosinophilic syndrome

Hypereosinophilic syndrome (HES) is a heterogeneous group of diseases, characterized by persistent hypereosinophilia and end-organ damage. The FIP1L1–PDGFRA (F/P) fusion gene is found in 3–25% of patients with HES and is an oncogenic driver of myeloid neoplasms with clonal eosinophilia. Although cutaneous symptoms are the most common type of symptom in patients who have F/P fusion gene-positive HES (F/P HES), histological reports are limited. We herein present the case of a 78-year-old man with erythematous macules and severe pruritus on his trunk and extremities. Laboratory investigations revealed marked eosinophilia and elevated serum vitamin B12. A histological examination showed massive infiltration of eosinophils and mast cells around the vessels in the upper dermis. Fluorescence in situ hybridization revealed F/P fusion genes in nuclei in the peripheral blood and the skin lesion. The patient was diagnosed with F/P HES, and showed an excellent clinical and haematological response to imatinib.