Mast cell leukemia with complex genomic alterations in an elderly patient with prior hematologic and solid malignancies: a case report.

Mast cell leukemia (MCL) is the rarest and most aggressive variant of systemic mastocytosis (approximately 1% of cases), with a median survival of under 2 years. Diagnosis requires ≥20% atypical mast cells in the marrow aspirate, and the disease frequently overlaps with myeloid neoplasms. An 86-year-old man with paranasal sinus diffuse large B-cell lymphoma in remission since 2017 after R-CHOP and methotrexate, and prostate adenocarcinoma treated in 2019, presented with acute pancytopenia, presumed to represent lymphoma relapse. Serum tryptase exceeded 11 999 ng/mL; the aspirate showed 20% pleomorphic mast cells (CD117+, weak CD25, CD2-), confirming aleukemic MCL. Formal CMML criteria could not be confirmed due to the unavailability of monocyte differential data; however, the findings raised suspicion for an associated myeloid neoplasm, with SM with an associated hematological neoplasm remaining an alternative classification. Karyotyping was normal; next-generation sequencing revealed pathogenic variants in TP53, RB1, DAXX, ASXL1, TET2, and SRSF2, and a rare extracellular-domain KIT p.D419del. He declined inpatient midostaurin, deteriorated rapidly, and died 2 weeks later. This case illustrates a therapy-related MCL (plausible but unconfirmed given the non-leukemogenic profile of methotrexate and the focal nature of prostate stereotactic body radiation therapy) with a suspected associated myeloid neoplasm and complex pathogenic mutations. The KIT p.D419del extracellular domain variant is a rare non-D816V mutation; the canonical D816V was not detected on NGS, though the presence of a low-variant allele fraction D816V cannot be fully excluded due to assay sensitivity. Despite midostaurin, the disease remained aggressive. Persistent unexplained cytopenias warrant heightened suspicion of MCL, and comprehensive genomic profiling clarifies diagnosis, distinguishes overlapping myeloid disease, and informs prognosis in this aggressive, refractory neoplasm.