PMC7549112_01
Peripheral blood (A and B), the aspirate smears (C and D), and core biopsy. The peripheral blood shows circulating immature cells with fine chromatin, some with metachromatic coarse azurophilic granules (myelomastocytic blasts), and some without granules (typical myeloblasts) (A and B). The aspirate smears also show similar blasts with metochromatic granules (myelomastocytic blasts) and some blasts without granules (typical myeloblasts) as well as fewer immature mastocytic cells (promastocytes, metamastocytes) (C and D). The bone marrow is markedly hypercellular (100%; E) with diffuse infiltrate by immature cells with fine chromatin and conspicuous nucleoli (F).
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A peripheral blood differential (Fig. 1A and 1B) showed 4% blasts with no obvious cytoplasmic granules (typical myeloblasts) and 75% cells with metachromatic granules, one third of which showed immature chromatin (myelomastocytic blasts). The bone marrow aspirate smears (Fig. 1C) showed 5% blasts with no obvious cytoplasmic granules (typical myeloblasts) and 38% immature cells with metachromatic granules (myelomastocytic blasts). A few immature mastocytic cells (promastocytes, metamastocytes) were also observed (Fig. 1D). The core biopsy showed markedly hypercellular marrow (approaching 100%; Fig. 1E) with sheets of immature cells (Fig. 1F).
The Immunophenotype. Immunohistochemical studies showed a diffuse increase in CD117+ immature cells (A), some of which were strong-positive (myelomastocytic blasts) and others weak-positive (typical myeloblasts). The CD117+ myelomastocytic blasts were positive for tryptase, whereas the typical myeloblasts were negative for tryptase (B). CD34 was positive in a very small population of the blasts (C). Flow cytometry (D) showed 2 populations of CD45 dim blasts. One population (the red-colored cluster) was strong-positive for CD117 (myelomastocytic blasts), with a small subset positive for CD34; the other population (the blue-colored clusters) was weak-positive for CD117, compared with the control lymphocytes (the green-colored cluster in green), and negative for CD34. Both populations were positive for CD33 and negative for myeloperoxidase.
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Immunohistochemical studies were performed, showing that CD117 (Fig. 2A) was strong-positive in approximately 30% to 35% of the cells (myelomastocytic blasts) and weak-positive in approximately 20% (typical myeloblasts). Tryptase (Fig. 2B) was positive in a subset of the blasts, presumably the myelomastocytic blasts. CD34 was positive in a small subset of the blasts (Fig. 2C). Flow cytometry immunophenotyping (Fig. 2D) also showed 2 distinct populations of immature cells or blasts.