A new variant of mastocytosis: report of three cases clinicopathologically mimicking histiocytic and vasculitic disorders

Cutaneous mastocytosis (CM) or urticaria pigmentosa is characterized by abnormal proliferation and accumulation of mast cells. Clinically, CM usually presents as symmetrically distributed red-brown macules or papules that develop weals, erythema and often pruritus on stroking (Darier's sign). The histological hallmark of the disease is an increase in oval to spindle-shaped mast cells in the dermis located around blood vessels and skin appendages. We describe three patients with a new clinicopathological type of CM, which clinically mimics a histiocytic disorder and histologically mimics leucocytoclastic vasculitis (LV). Three infants (two boys and one girl) developed generalized reddish-yellow-brown macules of 3-10 cm with occasional scaling and crusting on the trunk and extremities without further symptoms or organ involvement except variable itching. Histology revealed diffuse and dense dermal infiltrates of eosinophils, neutrophils and nuclear debris with perivascular accentuation, imitating LV. This infiltrate masked large epithelioid cells, positive for macrophage markers, which by special histochemical stains for metachromatic granules turned out to be mast cells. This is the first report of this new variant of CM, which may cause considerable diagnostic difficulties both clinically and histopathologically.