This 8-month-old boy presented with scattered brown macular (and few papular) lesions on the back. There was a clinical concern for histiocytosis. Cutaneous mastocytosis can come in a variety of clinical settings - urticaria pigmentosa, solitary mastocytoma, diffuse cutaneous mastocytosis and TMEP. The histology is along a spectrum, with mastocytoma at one end, forming a nodule of mast cells, and TMEP at the other, with a mild increase of spindled mast cells around vessels. This patient is best characterized as urticaria pigmentosa and the histology is typical. Childhood cases usually do well, with clearance after a few years.
There is an infiltrate in the dermis. The epidermis is uninvolved. The infiltrate appears in a perivascular and interstitial distribution.