Bone Marrow - Systemic Mastocytosis

Question 1

Describe the morphology of normal mast cells in the bone marrow.

Answer 1

Normal mast cells are round and granulated.

(a) H&E 20x; (b) Giemsa 60x; The granules stain purple with toluidine blue (c, 20x) or orange with chloroacetate esterase (d, 20x).

Question 2

Describe the morphology of “neoplastic” mast cells.

Answer 2

Neoplastic mast cells are spindle shaped and may appear triangular or kite shaped on aspirate. They possess elongated or bilobed nuclei. (a) H&E
20, (b & c) Giemsa on aspirate 100.

In bone marrow aspirates, cytoplasmic granularity ranges from paucigranular to hypergranular with the granules often seen around bare nuclei

Question 3

Which immunohistochemistry markers are present on neoplastic mast cells but absent on normal mast cells?

Answer 3

CD25 and CD2

Question 4

Name immunohistochemistry markers used in the work-up of mastocytosis.

Answer 4

Immunohistochemistry of neoplastic mast cells. (a) Mast cell tryptase, (b) CD117 [this is usually strongly expressed in contrast to this image], (c) CD25, (d) CD2 (note strong expression in accompanying T-cells).

Question 5

The pathognomonic lesion for bone marrow involvement by mastocytosis is….

Answer 5

Nodular interstitial aggregate composed of a
core of delicate, spindle-shaped mast cells surrounded by a cuff of eosinophils and lymphocytes

Question 6

Patterns of bone marrow involvement by mastocytosis include:

Answer 6

(a) Nodular interstitial, (b) Perivascular, (c) Paratrabecular (d) Fibrotic.

Question 7

Diagnostic criteria for systemic mastocytosis

Answer 7

Major criteria: Multifocal dense infiltrates of mast cells (>= 15 mast cells in aggregate) detected in sections of bone marrow and/or other extracutatious organ(s)

Minor criteria: (1) Bone marrow or other etracutaneous organ with >25% of the mast cells in the infiltrate are spindle-shaped, atypical morphology or immature. (2) Detection of c-KIT mutation at codon 816 (usually D816V). (3) Expression of CD25 and/or CD2 on mast cells. (4) Serum total tryptase is persistently > 20ng/mL without other cause.

Diagnosis is made one the major criteria and at least one minor criteria are present or when >= 3 minor criteria are present.