A 23 year old woman noted a firm,mobile,2 cm mass in her right breast.Alarmed,she consulted her GP,and it was removed.A representative section of the mass is shown in the accompanying photomicrograph.Which one of the following is the best diagnosis?

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Fibroadenomas are relatively common tumours in young women.They are usually well-circumscribed,and firm.They do not invade surrounding tissue,so they are mobile,and do not cause retraction of the skin.The histological appearance is quite distinct.Two patterns of growth are seen,often in the same lesion.In the pericanalicular pattern "P" the epithelial component takes the form of rounded ducts that remain small and undistorted,with the stroma arranged round them in a roughly symmetrical and regular manner.By contrast,in the intracanalicular pattern "In" the ducts appear elongated,but actually represent sections cut through flattened spaces compressed by nodular proliferation of the stromal component; in general,this latter pattern is more prominent in larger fibroadenomas.In both patterns of fibroadenoma,hormonal changes,such as occur in pregnancy and lactation,may induce hyperplasia of the epithelial component.

A 23 Year Old Woman Noted a Firm,mobile,2 Cm Mass