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