Systemic Mastocytosis

Systemic mastocytosis (SM) refers to mast cell infiltration in extra-cutaneous tissues. The symptoms of systemic mastocytosis are due to degranulation of mast cells and/or accumulation of mast cells in target organs.

Degranulation of mast cells

Symptoms can be caused by secretion of the following factors:

• Histamine: Pruritus, urticaria, hypotension, gastric hypersecretion, bronchoconstriction.
• Heparin: Local anticoagulation, osteoporosis
• Leukotrienes: Bronchoconstriction
• Prostaglandins: Bronchoconstriction, flushing
• Platelet-activating factor:
• Proteases:
• Tumor necrosis factor:

Accumulation of mast cells in organs

Accumulation of mast cells in organs can cause organ dysfunction. The so-called B findings refer to organ involvement without organ dysfunction. C findings refer organ involvement with organ dysfunction. The example above shows hepatic involvement with cirrhosis (white arrow) and ascites (yellow arrow) and nodal involvement with bulky adenopathy (red arrow). We also have involvement with diffuse sclerosis. Interestingly, the non-radiology literature stresses the more common osteoporosis, with scarce mention of the sclerosis that tends to dominate the radiology literature.

Diagnosis systemic mastocytosis

The diagnosis of SM requires either, 1 major and 1 minor OR 3 minor criteria. Warning: Boring for radiologists

The one major criterion is: Multifocal, dense infiltrates of mast cells (≥15 mast cells in aggregates) in sections of bone marrow and/or other extra-cutaneous organ(s).

Minor criteria are:

• Bone marrow or other extra-cutaneous organs: >25% of 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.
• Activating point mutation at codon 816 of KIT in bone marrow, blood, or another extra-cutaneous organ.
• Mast cells in bone marrow, blood, or other extracutaneous organs express CD2 and/or CD25 in addition to normal mast cell markers.
• Serum total tryptase persistently > 20 mg/mL (unless associated w clonal myeloid disorder).

Subtypes

• Indolent (ISM): No C findings
• Smoldering (SSM): 2+ B findings, no C findings
• Aggressive (ASM): C findings, no MCL features*
• Mast cell leukemia (MCL): BMBx diffuse infiltration by atypical, immature mast cells. Aspirate smears ≥20% mast cells.
• SM with associated hematologic neoplasm (SM-AHN): SM + MDS, MPN, AML, lymphoma, other

References

Akin C, Gotlib J. Systemic mastocytosis: Determining the subtype of disease. UpToDate

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