Myxoma of the breast: report of a case with unique histological and immunohistochemical appearances.

A case of myxoma of the breast is reported. The patient, a 19 year old Japanese woman, showed a lump in the left breast which had enlarged gradually over 3 years. A tumor measuring 5 x 5 x 4.5 cm was located mainly in the mammary parenchyma, but partially involved the overlying subcutaneous tissue. Histologically the tumor was multinodular and each nodule consisted of an abundant myxoid substance with a few spindle or stellate mesenchymal cells. The presence of hyaluronic acid was observed in the myxoid area, and a few constituent cells showed immunoreactivities for S-100 protein and alpha 1-antichymotrypsin. Electron microscopic studies revealed that some constituent cells looked like undifferentiated mesenchymal cells, while others showed a differentiation similar to fibroblast or histiocyte. These findings suggest that the constituent cells might derive from totipotential primitive mesenchymal cells.