Mammary fibroadenoma with multinucleated stromal giant cells.

We have studied an otherwise typical mammary fibroadenoma in a 42-year-old female in which numerous, bizarre, mitotically inactive, multinucleated giant cells were present throughout the stroma. Immunohistochemical and ultrastructural studies confirmed the mesenchymal, specifically fibroblastic, nature of the giant cells. Cells of this type, which are more commonly an incidental finding within the interlobular stroma of the breast, are benign, and should not be mistaken for malignant cells on microscopic examination.