A 60-year-old woman presents with a large breast mass that she first detected 3 months ago. Mammography reveals a well-circumscribed mass measuring 8 cm in diameter. A breast biopsy shows loose fibroconnective tissue with a sarcomatous stroma, abundant mitoses, and nodules and ridges lined by cuboidal epithelial cells. Which of the following is the appropriate diagnosis?

Phyllodes tumor of the breast is a proliferation of stromal elements accompanied by a benign growth of ductal structures. These tumors usually occur in women between 30 and 70 years of age. Phyllodes tumors resemble fibroadenomas in their overall architecture and the presence of glandular and stromal elements. Like fibroadenoma, benign phyllodes tumor is sharply circumscribed, and the cut surface is firm, glistening, and grayish white. Microscopically, the stroma of a benign phyllodes tumor is hypercellular and has mitotic activity. The distinction from fibroadenoma is made not on the size, but on the histologic and cytologic characteristics of the stromal component. Malignant phyllodes tumors have an obviously sarcomatous stroma with abundant mitotic activity, and the stromal component is increased out of proportion to the benign duct elements. They are usually poorly circumscribed, with invasion into the surrounding breast tissue. Sarcomatous elements are not features of the other choices.Diagnosis: Phyllodes tumor of the breast