Myeloid metaplasia presenting as a breast mass

A case of myeloid metaplasia presenting as an isolated breast mass in an elderly woman is described. The breast tumor developed 16 years after the initial diagnosis of myelofibrosis and was the only clinically significant sign of the disease necessitating treatment. Ultrastructural examination demonstrated that the lesion consisted of megakaryocytes and immature myeloid cells. This rare lesion, one of a number of manifestations of hematopoietic diseases involving the breast, should be included in the differential diagnosis of stromal breast lesions, especially those containing giant cells.