Juvenile fibroadenoma with features of phyllodes tumor showing intraductal growth and prominent epithelial hyperplasia in an 11-year-old girl

Breast tumors in children are uncommon, with the majority of them being adult-type fibroadenoma (FA). We report a case of juvenile FA (JFA) with features of a benign phyllodes tumor (PT) in an 11-year-old girl, showing very unusual intraductal/intracystic growth. The tumor was located at the outer peripheral portion of the right breast apart from the nipple. Histologically, the tumor showed extensive leaf-like papillary structures with a broad fibrous stroma, protruding into multiple contiguous cystic spaces lined by flat ductal epithelium, and closely resembled PT but the stroma of the tumor was only slightly cellular, showing no nuclear atypia and very few mitotic figures. In contrast, epithelial cells covering the fronds exhibited marked hyperplasia, forming a thick multilayered epithelium. The histology of the tumor with intracystic papillary structures and epithelial hyperplasia showed some similarities with intraductal papilloma (IDP). The mechanism of such unusual intraductal growth of fibroepithelial tumors, including FA/JFA and PT, and their possible common histogenesis with IDP are discussed.