Systemic Mastocytosis : Bone Marrow

Comments:
Bone Marrow Involvement in Systemic Mastocytosis (SM): The most common finding in bone marrow is multifocal or disseminated, paratrabecular and perivascular infiltrates of spindle-shaped mast cells (as shown here). There may be other cell types mixed in, including eosinophils, plasma cells, histiocytes, and fibroblasts.

A dense reticulin fibrosis is frequently present. Collagen fibrosis develops in long-standing cases of SM. Primary myelofibrosis should be excluded by doing tryptase immunostain. Focal osteosclerosis is usually seen in associated with paratrabecular infiltrates. There is neoangiogenesis associated with mast cell infiltrates.

Some cases, especially indolent systemic mastocytosis, show a striking reactive lymphoid infiltrate in the bone marrow that may lead to the mistaken diagnosis of a low-grade non-Hodgkin lymphoma (lymphoplasmacytic lymphoma, CLL).

The image is a higher magnification showing a compact infiltrate of spindled mast cells in the bone marrow from a case of indolent systemic mastocytosis. See slide 40 for clinical history.