First report of B-lymphoblastic leukemia harboring LRRFIP1::FGFR1 fusion: expanding the clinical spectrum of myeloid/lymphoid neoplasms with tyrosine kinase gene fusions.

Myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions (MLN-TK) are rare hematologic malignancies defined by recurrent kinase gene rearrangements. FGFR1 is a well-recognized partner in this category, but de novo B-lymphoblastic leukemia (B-ALL) as the initial presentation remains exceedingly rare. We report the first case of B-ALL with an LRRFIP1::FGFR1 fusion, identified by whole transcriptome sequencing in a 62-year-old male. The patient achieved sustained complete remission following intensive chemotherapy without hematopoietic stem cell transplantation. Only two prior cases of LRRFIP1::FGFR1 fusion have been reported, both presenting as acute myeloid leukemia. All three cases share an identical fusion structure. This case expands the clinical and molecular spectrum of FGFR1-rearranged neoplasms and underscores the importance of comprehensive molecular profiling for accurate classification, risk assessment, and individualized therapeutic planning in MLN-TK.