PMC6854920_01
Peripheral smear showing immature cells with blast morphology (arrows) and atypical mast cells.
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Subsequent PS (Figure 1) showed marked leukopenia with some circulating abnormal blastoid cells (13%) of medium size with fine nuclear chromatin and unevenly distributed deeply basophilic/metachromatic coarse granules.
BM aspirate smear (100x) showing a variety of neoplastic MCs. The majority of blasts show abundant cytoplasm and coarse giant metachromatic blasts (a). Neoplastic MCs (Type I) (A-I) with eccentric round/oval nuclei, uneven granule distribution/hypogranulation, and spindle-shaped MCs with elongated cytoplasmic projections (b). Neoplastic MCs Type II (promastocytes) (A-H) with indented, bilobed nuclei, abundant hypogranulated cytoplasm, or with coalescent granules with uneven distribution (c). There is trilineage dysplasia with macronormoblasts, giant neutrophils, and dysplastic nuclear lobulation in megakaryocytes (arrows) (d). Increased mitotic figures (e). Prominent erythrophagocytosis exhibited by neoplastic mast cells (blue arrow) and histiocytes (f).
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The majority of blasts showed eccentric nuclei with abundant cytoplasm and coarse giant metachromatic/basophilic granules (metachromatic blasts) (Figure 2(a)). The majority of neoplastic MCs exhibited marked atypia and pronounced pleomorphism (Figures 2(b) and 2(c)) of (Type I) (Figure 2(b), A-I) with eccentric round/oval nuclei, uneven granules distribution/hypogranulation, and spindle-shaped mast cells with elongated cytoplasmic projections. Neoplastic MCs Type II (promastocytes) (Figure 2(c), A-H), with indented, bilobed nuclei and abundant degranulated/hypogranulated cytoplasm or with coalescent granules and uneven distribution were frequent. There was trilineage dysplasia with prominent megaloblastic changes mostly in the form of giant granulocytes, megakaryocytes with multilobulation, and some megaloblastic erythroid precursors (composite, Figure 2(d)) with some mitotic figures noted (Figure 2(e)). There was an evident hemophagocytic activity (predominantly erythrophagocytosis) exhibited by histiocytes and neoplastic mast cells (Figure 2(f)).
BM biopsy (H&E) showing diffuse infiltration by atypical mast cells 20x (a). 50x magnification of BM biopsy showing increased mitotic figures (arrow) and evident erythrophagocytosis (b). Immunohistochemistry on BM biopsy: the neoplastic mast cells are positive for (c) CD117, (d) tryptase, (e) CD25, and (g) CD4 and show partial positivity for (j) CD7 and (f) CD30. (i) CD3 immunostain was included to show the normal CD3-positive T cells. (h) Toluidine blue highlighted the more differentiated MCs.
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fig3
On H&E, BM biopsy was markedly hypercellular, largely replaced by sheets of neoplastic mast cells showing abundant clear cytoplasm with round/indented nuclei (including many forms with immature nuclear chromatin), and admixed with a reactive background composed of fibroblasts, eosinophils, small lymphocytes, and polyclonal plasma cells (Figure 3(a) and 3(b)). By immunohistochemistry (IHC)(Figure 3), the neoplastic MCs were positive for CD45, CD117, mast cell tryptase, CD25, CD43, CD4, and partially positive for CD30 and CD7 while negative for CD2.
FCM shows an abnormal population of mast cells (red) in the blast gate; approximately 26% (of the total cells) expressing bright CD117, and the majority is positive for CD33 and HLADR with partial aberrant CD25 on a subpopulation (~11% of total). The cells show aberrant expression of CD7 and CD4, partial expression for CD36, and negative for CD2.
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Flow cytometry on BM (Figure 4) showed an abnormal population of cells in the blast gate comprising 25%, expressing CD45 (moderate), CD117 (bright), CD33, and HLA-DR with partial aberrant CD25 expression.