#00066295

A 72-year-old man presented with mild anemia and leukocytosis (white blood cell count, 15 × 10⁹/L) with eosinophilia (78%; absolute eosinophil count, 11.7 × 10⁹/L) (panel A, Wright-Giemsa stain, 20× lens objective). A bone marrow biopsy showed that the bone marrow was hypercellular (∼95%) with myeloid hyperplasia and prominent eosinophilia (panel B, hematoxylin and eosin [H&E] staining, 20× lens objective; panel C, H&E staining, 40× lens objective) accompanied by a paratrabecular proliferation of spindled mast cells (panel D, Giemsa stain, 100× lens objective), highlighted by CD117 immunostain with aberrant CD25 expression (panels E and F, respectively; 20× lens objective) without the expression of CD2 and CD30. Next-generation sequencing (assay sensitivity of 1%-2%) did not identify mutations, including KIT D816V. As expected, an RNA translocation panel identified that FIP1L1::PDGFRA was undetectable on routine karyotype (cryptic fusion). Fluorescence in situ hybridization confirmed the presence of CHIC2 deletion with FIP1L1::PDGFRA fusion.

This interesting case illustrates an exuberant mast cell proliferation with aberrant CD25 expression in the setting of a myeloid/lymphoid neoplasm with PDGFRA rearrangement. In this context, these mast cells are a component of the underlying myeloid/lymphoid neoplasm and should not be interpreted as systemic mastocytosis (SM), despite their morphologic features and aberrant CD25 expression, as has been rarely reported before. This case highlights the need for a comprehensive molecular profiling including assessment for tyrosine kinase fusion genes in cases with morphologic suspicion for mastocytosis without KIT D816V mutation. An accurate diagnosis is crucial, because unlike SM, myeloid/lymphoid neoplasms with PDGFRA rearrangement respond well to tyrosine kinase inhibitors, namely imatinib.

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