Answer: Juvenile fibroadenoma

Histology: On low power, the lesion is well-circumscribed with a pseudocapsule. The stroma is more cellular than normal, but lacks atypia and mitoses. The ducts are hyperplastic with pleomorphic cells and mitotic figures can be seen, however, necrosis is absent. The epithelial hyperplasia shows various patterns, including laciform, micropapillary, solid and cribriform. The hyperplastic ducts contain a hyperplastic myoepithelial zone, and overlapping and streaming of epithelial cells.

Discussion: Juvenile fibroadenomas are characterized by stromal cellularity and epithelial hyperplasia. The tumor border is well defined, often with a pseudocapsule of compressed breast tissue. As its name implies, these tumors tend to be younger than the average age for the usual fibroadenomas, most are younger than 20 years old. However, tumors with this histology have been reported in patients as old as 72.

Juvenile fibroadenomas do not show the myxoid mucopolysarccharide seen in usual fibroadenomas. Unlike phyllodes tumors, they do not show stromal overgrowth, stromal cellular atypia or stromal mitoses. The epithelial portion of the tumor is usually hyperplastic. The hyperplasia may be ductal, lobular or combined. Several patterns of epithelial hyperplasia may be seen, including laciform (fenestrated proliferations that resemble cribriform DCIS, but contains a hyperplastic myoepithelial zone and have streaming and overlapping cells), papillary, solid and cribriform. The epithelium in juvenile fibroadenomas may show cellular pleomorphism, atypia and mitoses, but necrosis is not a feature. The hyperplastic changes are not usually seen in the surrounding breast tissue. In unusual cases, carcinoma may be found in juvenile fibroadenomas. Despite the epithelial hyperplasia, often with atypia, seen in these lesions, follow-up of patients with juvenile fibroadenomas has not shown a predisposition for subsequent development of phyllodes tumors or carcinoma. These tumors should be managed by excision to preserve as much breast tissue as possible to allow normal breast development.