Biphasic lesions of the breast.

Biphasic lesions of the breast are characterized by integral epithelial and stromal components. Determination of the stromal composition facilitates diagnosis. Lesions with predominantly specialized stroma include fibroadenoma and phyllodes tumor, and lesions with a mixture of nonspecialized and specialized stroma include fibroadenoma variant, hamartoma, and pseudoangiomatous stromal hyperplasia. Diagnostic difficulty may be encountered when a biphasic lesion has cellular stroma, raising the differential diagnosis of phyllodes tumor. Distinctive histologic features of phyllodes tumor include architectural frond formation, stromal cytological atypia, stromal mitotic activity, irregular gland dilatation, and morphologic heterogeneity. These histologic features contrast with those of other biphasic lesions.