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WHO Classification of Tumours
Systemic mastocytosis
Tumours of haematopoietic and lymphoid tissues


Definition

The diagnosis of systemic mastocytosis (SM) can be made when the major criterion and one minor criterion or at least three minor criteria are present:

Major criterion: Multifocal, dense infiltrates of mast cells (≥15 mast cells in aggregates) detected in sections of bone marrow and/or other extracutaneous organ(s).

Minor criteria: 1. In biopsy sections of bone marrow or other extracutaneous organs, >25% of the mast cells in the infiltrate are spindle-shaped or have atypical morphology or, of all mast cells in bone marrow aspirate smears, >25% are immature or atypical. 2. Detection of an activating point mutation at codon 816 of KIT in bone marrow, blood or another extracutaneous organ. 3. Mast cells in bone marrow, blood or other extracutaneous organs express CD2 and/or CD25 in addition to normal mast cell markers. 4. Serum total tryptase persistently exceeds 20 ng/mL (unless there is an associated clonal myeloid disorder, in which case this parameter is not valid) (Table 2.10).

There are four major subtypes of systemic mastocytosis (Table 2.12):
Indolent systemic mastocytosis
Systemic mastocytosis with AHNMD
Aggressive systemic mastocytosis
Mast cell leukaemia

> Related Topics
Overview: Mastocytosis