Invasive micropapillary carcinoma of the breast.

Nine examples of a rare variant of invasive breast carcinoma characterized by the formation of micropapillae within clear spaces separated by a fibrocollagenous or delicate fibrovascular stroma are presented. Designated as invasive micropapillary carcinoma, the unusual morphology of this tumor requires its distinction from metastatic ovarian serous papillary carcinoma and extensive vascular/lymphatic invasion by either a primary or metastatic breast carcinoma. The micropapillary pattern is retained in metastatic foci and areas of recurrence. This pattern also occurs admixed with regular infiltrating duct carcinoma either in the primary mammary tumor or when it recurs.