A rare, low-grade malignant stromal tumor that arises from the breast. It is characterized by the proliferation of atypical stromal spindle cells that surround benign tubules and ducts in a pericanalicular pattern. Mitotic activity is present. The tumor has a multinodular and infiltrative growth pattern and lacks the leaf-like architecture of phyllodes tumor. Wide excision with negative margins is the treatment of choice. [from
NCI]