Pathology of extramedullary mastocytosis

Mastocytosis encompasses a group of clinically and pathologically heterogeneous disorders most commonly involving the skin, which typically takes the form of urticaria pigmentosa. Mastocytosis may also involve other organs, most often bone marrow, followed by gastrointestinal tract, liver, spleen, and lymph nodes. The presence of extracutaneous involvement by mastocytosis is a major diagnostic criterion for systemic disease. However, mast cell infiltrates are often subtle in skin and extracutaneous organs, and the histologic features of mastocytosis at different anatomic sites may be variable. This article reviews the pathologic features and clinical correlates of mastocytosis involving skin and other extramedullary sites.