Fibroadenomas are benign, spherical, well-demarcated tumors of varying size tending to occur in young women. They arise from the epithelium and stroma of the terminal duct-lobular unit. The proliferating stroma distorts the epithelium creating a typical picture as seen here. In the inset, one sees the clear demarcation of the fibroadenoma (upper right) from breast tissue with cystic change (lower left). Tumors resulting from proliferation of fibroepithelial breast tissue include fibroadenoma, cellular fibroadenoma (a variant of fibroadenoma seen mostly in juvenile patients), benign and malignant phyllodes tumors. Fibroadenoma and phyllodes tumor are thought to be separate entities rather than points on a spectrum.