Systemic mastocytosis presenting with a prominent B lymphocyte proliferation in the bone marrow and extensive fibrosis of the spleen.

Systemic mastocytosis is a disease characterized by multifocal mast cell proliferation in the bone marrow or other extracutaneous organs. Because of loosely scattered and hypo-/agranular mast cells, the diagnosis is sometimes very difficult. In the bone marrow, mast cell infiltration may be associated with prominent lymphoid infiltration leading to a misdiagnosis of a low grade non-Hodgkin lymphoma. A 49-year-old woman presented with right arm and leg pain, psychiatric symptoms, and diarrhea for four years. Physical examination and laboratory investigation revealed hepatosplenomegaly, anemia, mild thrombocytosis, mild leucocytosis and lymphocytosis. In the bone marrow biopsy, there was a prominent B lymphocyte proliferation reminiscent of a low grade non-Hodgkin lymphoma/leukemia and there were some spindle cells aggregates in paratrabecular location. The consecutive bone marrow biopsies were similar to the first. The subsequent splenectomy specimen exhibited striking fibrosis. In the lymph node sections, there was marginal zone hyperplasia. Multifocal accumulations of mast cells were strongly positive with mast cell tryptase and CD117 on immunohistochemical staining, though no metachromasia was identified in Giemsa and Toluidine Blue stained aspirates and tissue sections, probably due to hypo-/agranulation of mast cells. The case was presented to emphasize the importance of the antibody to mast cell tryptase in the diagnosis of mastocytosis and to discuss problems of differential diagnosis of systemic mastocytosis.