Two-toned Tumor

A 92 year old woman underwent an ultrasound guided biopsy for a mass in the 4:00 position of the right breast.

Image 1.  Small epithelial nests diffusely infiltrate stroma and fat.

Image 2. Higher power shows infiltrative nests extending into fat. The infiltrating nests are composed of bland cells, and there is no mitotic activity.

Image 3.  Higher power shows two distinct cell types; small glands are composed of bland luminal epithelial cells,  and small rounded nests are composed of myoepithelial cells stuffed with abundant eosinophilic basement membrane material.    The basement membrane material is so prominent that the associated cells are inconspicuous in some areas. 

Image 4 .  Immunohistochemical study using antibodies to p63 highlights dual cell population, and prominent myoepithelial component

Diagnosis:  Invasive mammary carcinoma with adenoid cystic features.

Discussion:  The dual cell population, pattern of infiltration, and low grade are characteristic of adenoid cystic carcinoma (ACC).  Individual cellular nests of ACC usually contain both cell types, with the formation of true and false lumens, the latter created by collections of basement membrane material.  This case is unusual in that the dual cell population seems to be restricted to individual nests, with luminal cells in some nests, and myoepithelial cells containing basement membrane material in other nests. Not surprising,  this low grade carcinoma was negative for expression of estrogen and progesterone receptors, as well as HER2.