This report highlights the value of flow cytometry analysis, particularly in the setting of myeloproliferative neoplasms showing features of progression, as neoplastic plasmacytoid dendritic cell (PDC) proliferations may be present, representing either a clonal expansion of mature PDCs related to the underlying myeloproliferative neoplasm or transformation to blastic plasmacytoid dendritic cell neoplasm (BPDCN). BPDCN should always be considered in patients with myeloid neoplasms in progression and/or who develop new cutaneous findings, as it may prompt change of management.

A 40 years old woman, long-term smoker (10 pack years), presentedwith 6-month history of progressive fatigue, decreased effort tolerance and non-deliberate weight loss. Physical examination revealedsplenomegaly (24 cm on CT scan) and no lymphadenopathy. White blood cell count was 10.2 × 109/L (normal range [NR] 4.0–10.0) withabsolute lymphocytes of 8.7 × 109 L (NR 1.2–3.7) and circulating largebinucleated lymphocytes with abundant cytoplasm (Figure 1A–G).

A 72-year-old woman with no oncologic past medical history presentedto the emergency department with several weeks of progressivefatigue, dyspnea, night sweats, intermittent rash, and a 30 lb unintended weight loss. Initial imaging demonstrated splenomegaly (upto 22.6 cm) and attenuated bone marrow lesions, suspicious for bonemarrow involvement by an occult malignancy. Her complete bloodcount was significant for mild anemia (hemoglobin 10.0 g/dl), thrombocytopenia (platelets 103 × 109/L), with a normal white blood cell countof 10.0 × 109/L. Serum tryptase and histamine levels were markedlyelevated, >400 ug/L (normal range ≤ 10.9 ug/L) and 51,930 nmol/L(normal range 180–1800 nmol/L), respectively. Peripheral blood smearreview demonstrated 20% immature cells, medium-to-large in size,with abundant cytoplasm, prominent metachromatic granules, roundnuclei and immature chromatin (Figure 1A). Flow cytometry analysisof the blood revealed an aberrant mast cell population expressingCD2 (40%), CD13, CD25, CD33, and CD117; while negative for theexpression of CD34, HLA-DR, cytoplasmic MPO, and other lymphoid and monocytic markers. A bone marrow biopsy demonstratedextensive involvement by leukemic mast cells (Figure 1B–D), withcharacteristically round rather than spindle-shaped nuclear contours,abundant cytoplasm, some granulated and vacuolated, and immaturechromatin, with frequent mitotic figures (Figure 1D).