Muscular hamartoma of the breast: a rare breast lesion containing smooth muscle.

Hamartoma of the breast is an uncommon entity, usually presenting as a well-demarcated breast mass. Microscopically, the lesion is composed of mammary glandular component, fibrous stroma, adipose tissue, and smooth muscle in variable proportions. Among the variants of breast hamartoma, muscular hamartoma is rare. This lesion should be differentiated from other breast tumors that contain smooth muscle element. We report a breast lesion of a 36-year-old woman diagnosed as a muscular hamartoma in which the muscular component is cellular and some mitotic figures are present. The criteria to distinguish between benign and malignant smooth muscle lesions in the breast, emphasizing mitotic count, are also discussed.