Systemic Mastocytosis Infiltrating Lymph Nodes: Examples of Sneaky & Prominent Involvement

Lessons From the Friday Unknowns

CD20 and CD3 stains highlight B and T cells, respectively. The B cell and T-cell compartments do not seem to be expanded.

CD20 (left) & CD3 (right) stains highlight preserved lymph node compartments

On higher power, a subtle multifocal paracortical infiltrate is identified, composed of clusters of epithelioid cells some with pale cytoplasm, conferring a monocytoid look, associated with few scattered eosinophils and mature plasma cells.

Immunohistochemical studies show that these epithelioid cells are positive for CD117 and tryptase as shown below, in addition to CD25 and focally CD2 (not shown).


A concurrent flow cytometric immunophenotyping identified an aberrant mast cell population (0.12% of total events) that expresses CD117 (bright), CD2, CD25 and CD30 (dim, subset).

These findings are diagnostic of nodal involvement by systemic mastocytosis. The neoplastic cells are not easily identifiable by morphology alone, and could thus be easily missed or mistaken for reactive histiocytes or monocytoid B-cells.

Link to digital slides:| Slides labeled case 5

Case 2: In contrast to the above-described case, below is a case with prominent nodal involvement by systemic mastocytosis, in which the architecture is subtotally effaced by a pale neoplastic infiltrate.

The infiltrate has a distinguished monocytoid appearance, mimicking marginal zone lymphoma.

It is composed of numerous small cells with kidney bean–shaped nuclei and abundant pale cytoplasm, associated with many eosinophils and vascular proliferation.

Touch imprint showed numerous degranulated mast cells, admixed with eosinophils:

The pale cells are positive for tryptase, confirming the diagnosis of mastocytosis.

Link to digital slides:| Slides labeled case 3



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Siba El Hussein, MD

Hematopathology | Cytopathology | Molecular pathology | Digital pathology | Data science | Machine learning