#00063667

A 67-year-old man presented with fatigue, shortness of breath, and diarrhea. His peripheral blood smear showed pancytopenia and 24% large atypical cells with coarse basophilic cytoplasmic granules and bilobed nuclei (panel A – black arrows, 60X and panel B – green arrow, 100X). On a Wright-Giemsa stained bone marrow aspirate smear, it became easier to appreciate that these were atypical mast cells. These mast cells comprised 26% of the marrow cellularity and displayed spindled nuclei and abundant metachromatic granules in the cytoplasm (panel C – red arrows, 60X). The bone marrow core biopsy showed a hypercellular bone marrow for age ( 90% cellular) with frequent paratrabecular and interstitial aggregates of spindled mast cells (panel D – 60X). These aggregates were positive for CD117, Tryptase, and CD25 (partial) immunohistochemical stains (panels E, F, and G – 20X). ). Chromosomal analysis showed gains of chromosomes 8, 13, 14, and 21. Next-generation sequencing detected KIT D816V mutation as well as FLT3, RUNX1, SRSF2, and BCOR mutations. Based on these findings a diagnosis of Mast Cell Leukemia (MCL) was made. The patient died three weeks after the bone marrow biopsy.