Systemic mastocytosis with plasma cell dyscrasia: report of a case.

Systemic mastocytosis (SM) comprises a heterogeneous group of disorders characterized by infiltration of bone marrow and other tissues by neoplastic mast cells. A subset of patients with SM has associated hematologic malignancy usually of myeloid origin and comprises an entity termed systemic mastocytosis with associated clonal hematological non-mast cell lineage disease (SM-AHNMD) by the current WHO classification. Reports of clonal lymphoid malignancies associated with SM are rare. We describe a patient who was simultaneously diagnosed with indolent SM and a plasma cell dyscrasia fitting the definition of monoclonal gammopathy of undetermined significance (MGUS). We also discuss the pathologic interaction between the neoplastic mast cells of SM and the lymphoid/plasma cell malignancy when these two entities coexist.