A 35-yrs-old woman presents with a 2.2 -cm mass in her left breast. The mass is excised, and histologic sections reveal a tumor composed of a mixture of ducts and cells. The epithelial cells within the ducts are not atypical in appearance. There is a marked increase in the stromal cellularity, but the stromal cells are not atypical in appearance and mitoses are not found. Which of the following is the most likely diagnosis?

Benign phyllodes tumor * Neoplastic proliferations of the stroma of the breast may lead to the formation of either fibroadenomas or phyllodes tumors. * Fibroadenomas are characterized histologically by a mixture of fibrous tissue and ducts, with no increase in cellularity or mitoses. * Only the stromal cells, not the glandular cells, are clonal proliferations. Another neoplastic tumor that arises from the stromal cells is the phyllodes tumor. * It is distinguished from fibroadenomas by a more cellular stroma and the presence of stromal mitoses. The phyllodes tumor, which has been called a cystosarcoma phyllodes, may either be benign or malignant. * A benign phyllodes tumor is characterized by increased stromal cells with few mitoses, while a malignant phyllodes tumor has increased numbers of stromal cells that are atypical along with numerous mitoses.