The faces of mast cell disease: bone marrow infiltrates in 3 patients with systemic mastocytosis

The clinical spectrum of mast cell disease ranges from relatively innocuous and histologically subtle urticarial skin lesions to an aggressive and fatal leukemic form of mast cell proliferation. Not surprisingly, mast cell infiltrates may show significant microscopic heterogeneity, particularly in the bone marrow, the most common site of involvement in systemic mastocytosis (SM). Herein, 3 cases are presented to illustrate the clinical and morphologic heterogeneity of mast cell disease: the first patient, with long standing urticaria pigmentosa, developed anemia and thrombocytopenia; the second patient presented with a pathologic fracture; and the third patient was suspected to have refractory anemia. Upon bone marrow examination, all 3 patients showed mast cell infiltration with distinct morphologic features and all met the WHO criteria for aggressive systemic mastocytosis. Histochemical methods continue to play a role in the identification of mast cells, with some limitations depending on the degree of differentiation of the mast cells and tissue processing methods. Immunohistochemistry has contributed to the identification of mast cells. Coexpression of CD117 and CD25, as well as expression of the more specific immunohistochemical marker tryptase, is seen in systemic SM. The latter may also be employed as a serum marker in the diagnosis and follow-up of patients with SM. The mast cells, in the majority adults with SM, have somatic point mutations of KIT.