Myeloid neoplasm with eosinophilia associated with isolated extramedullary FIP1L1/PDGFRA rearrangement.

Myeloid neoplasms with eosinophilia associated with PDGFRA rearrangement are very responsive to tyrosine kinase inhibitors (TKIs). Herein, we report a case of a 53-year-old man with eosinophilia and a well-differentiated extramedullary myeloid tumor with evidence of FIP1L1/PDGFRA rearrangement by fluorescent in situ hybridization in the extramedullary tissue. His bone marrow evaluation revealed a hypercellular marrow with eosinophilia but without evidence of a FIP1L1/PDGFRA rearrangement. The patient was treated with imatinib at a dose of 100 mg daily and responded with normalization of his peripheral eosinophil count. The case raises the possibility that an extramedullary myeloid tumor may represent a primary site for PDGFRA rearrangement, and highlights the importance of performing cytogenetic testing on extramedullary tissue. Detection of the chromosomal rearrangement is critical for initiation of effective targeted therapy that can improve patient outcomes.