Mast Cell Activation Syndrome Mimicking Breast Cancer: Case Report With Pathophysiologic Considerations.

Mast cell (MC) activation syndrome (MCAS) represents the common variant (with a prevalence of at least 17% in Germany)1 of systemic MC activation disease (MCAD) (ie, a subclass of mastocytosis) (see Supplemental Figure 1 in the online version).2 MCAD comprises a heterogeneous group of multifactorial, polygenic disorders characterized by aberrant release of variable subsets of MC mediators together with accumulation of either morphologically altered and immunohistochemically identifiable mutated MCs owing to MC proliferation (systemic mastocytosis and MC leukemia) or morphologically ordinary MCs because of decreased apoptosis (MCAS and well-differentiated systemic mastocytosis; for details, see Ref. 2).