Myoepithelial Marker Misadventure

A 61-year old woman underwent an ultrasound-guided biopsy of an asymmetric density noted on imaging. 

Image 1.  Core biopsy specimen showing diffusely infiltrative small nests and glandular structures.

Image 3.  Invasive carcinoma is composed of small, uniform cells with ‘basaloid’ appearance, but no mitotic activity.

Image 5.   The carcinoma expresses p63; note normal myoepithelial arrangement in benign gland, upper left.

Image 2.  Invasive carcinoma showing focal cribriform architecture, with diffuse infiltration into fat.  Note normal acinus, top left.

Image 4.  Immunohistochemistry using antibodies to p63.  Most tumor nests contain some p63 expression by the neoplastic cells.

Diagnosis:  Invasive carcinoma, no special type, low combined histologic grade.

Discussion:  The primary pathologist for this case was confident that the biopsy specimen contained an invasive carcinoma, but the presence of p63 expression made her question her diagnosis. 

    Invasive carcinomas, especially those that show any ‘basaloid’ cytology, may show focal myoepithelial differentiation, as in this case.  Myoepithelial markers may be helpful in supporting the histologic diagnosis of invasive versus non-invasive, but it is not always a ‘present/absent’ dichotomy.  The location of p63-expressing cells is crucial to the correct interpretation of this immunohistochemical adjunct.