Pathology

The major feature of systemic mastocytosis is the finding of compact dense multifocal mast cell infiltrates in a bone marrow trephine biopsy. These are most usefully identified by means of staining for mast cell tryptase. Such infiltrates may also be identified in other organs and lymph nodes. The individual morphology of the mast cells is also important. Typically they are spindle shaped, but may demonstrate atypia, including nuclear lobulation, smaller or absent

Table 46.2 WHO classification of mast cell disorders

■ Cutaneous mastocytosis

■ Indolent systemic mastocytosis

■ Systemic mastocytosis with associated clonal haematologic non-mast cell lineage disease (including AML, MDS, MPD, CMML, NHL)

■ Aggressive systemic mastocytosis

■ Mast cell leukemia

■ Mast cell sarcoma

■ Extracutaneous mastocytoma.

granules, and a primitive nuclear chromatin pattern; they also aberrantly express CD2 and CD25.32-35 37 Reticulin is usually increased, and collagen and new bone formation may be seen. "Pseudo-granulomas" may also be apparent and commonly are associated with eosinophils, lymphocytes (sometimes lymphoid nodules), and plasma cells in addition to the mast cell infiltrate.

The determination of serum tryptase level may be a useful index of burden of mast cells and is usually elevated in systemic mastocytosis. For patients with mast cell leukemia, the infiltrate comprises immature mast cell precursors which may also be identified in the blood film. It is perhaps somewhat of an anomaly that this entity appears with the MPD in the WHO classification rather than with the acute myeloid leukemias.

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