Rare case links KIT D816V bone marrow mastocytosis with FIP1L1::PDGFRA eosinophilic leukemia

Researchers have reported an unusual case in which a patient carried two distinct myeloid neoplasms at the same time: a KIT D816V-positive bone marrow mastocytosis and a FIP1L1::PDGFRA-positive chronic eosinophilic leukemia. The case report, published as an open-access article in Annals of Hematology, describes a combination that the authors note is rarely, if ever, seen, because eosinophil-rich associated hematologic neoplasms carrying the FIP1L1::PDGFRA fusion gene are usually not detected in patients with KIT D816V-positive systemic mastocytosis.

Systemic mastocytosis is a condition in which, in a subset of patients, an associated hematologic neoplasm (AHN) is identified alongside the mast cell disorder. In most of these cases the second neoplasm is a myeloid malignancy, such as a myeloproliferative neoplasm or a myeloid leukemia. Some associated neoplasms also present with eosinophilia or as eosinophilic neoplasms. In the reported patient, the mast cell component was classified as KIT D816V-positive bone marrow mastocytosis, while the FIP1L1::PDGFRA-positive myeloid neoplasm was classified as chronic eosinophilic leukemia.

Clinically, the patient’s picture was dominated not by the eosinophilic neoplasm but by the mast cell disorder. He suffered from severe mediator-related symptoms due to wasp venom allergy, yet did not show clinical signs or symptoms of a hypereosinophilic syndrome. This dissociation between the molecular findings and the clinical presentation underscores how the two coexisting neoplasms contributed differently to the patient’s condition.

Treatment with imatinib at a low dose of 100 mg per day produced a striking but partial response. The eosinophilia resolved rapidly, and molecular testing showed a complete molecular remission, with the FIP1L1::PDGFRA fusion transcript becoming undetectable. However, the bone marrow mastocytosis itself and the mediator-related symptoms persisted despite treatment, indicating that the KIT D816V-positive mast cell component remained unaffected by the drug.

The case, reported by Max Vincent John, Veronika Buxhofer-Ausch, and colleagues with senior author Peter Valent, is published as a peer-reviewed, accepted case report under a permanent DOI, with the article shared early to provide faster access to the findings. The report illustrates that FIP1L1::PDGFRA-positive eosinophil neoplasms can, in rare instances, coexist with KIT D816V-positive bone marrow mastocytosis.

Subject of Research: A rare case of concurrent KIT D816V-positive bone marrow mastocytosis and FIP1L1::PDGFRA-positive chronic eosinophilic leukemia treated with imatinib

Article Title: Concomitant KIT D816V+ bone marrow mastocytosis and FIP1L1::PDGFRA+ eosinophil neoplasm: a rare liaison

Article References: John, M. V., Buxhofer-Ausch, V., Andorfer, A., Erdel, M., Burghofer, J., Haitchi-Petnehazy, S., Maier, E., Gleixner, K. V., Sperr, W. R., Webersinke, G., Zach, O., Beham-Schmid, C., & Valent, P. (2026). Concomitant KIT D816V+ bone marrow mastocytosis and FIP1L1::PDGFRA+ eosinophil neoplasm: a rare liaison. Annals of Hematology. https://doi.org/10.1007/s00277-026-07325-1

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DOI: 10.1007/s00277-026-07325-1

Keywords: systemic mastocytosis, bone marrow mastocytosis, KIT D816V, FIP1L1::PDGFRA, chronic eosinophilic leukemia, associated hematologic neoplasm, imatinib, eosinophilia, wasp venom allergy, case report, Annals of Hematology, myeloid neoplasm