Single cutaneous plasmacytoma with crystalloid inclusions.

A patient had a primary cutaneous plasmacytoma of the cheek. Histologic studies showed dense aggregates of plasma cells admixed with occasional histiocytes. The cytoplasm of the histiocytes contained needle-like structures. Electron microscopy showed these structures to be completely enclosed within smooth membranes. They often were rhombic, with occasional broken-off corners. It is hypothesized that these structures represent protein that has been secreted by the plasma cells and subsequently phagocytosed and stored in crystalloid form by the neighboring histiocytes. The patient is asymptomatic 1 1/2 years after removal of the tumor.