p63 immunohistochemical expression in tumor cells of high-grade invasive breast carcinomas on core biopsy: a potential diagnostic pitfall

In breast pathology, p63 is a highly specific myoepithelial marker, crucial for distinguishing in situ from invasive lesions. Its expression is characteristically absent in the neoplastic cells of invasive carcinoma. However, in our diagnostic experience focal p63 expression in neoplastic cells of some high-grade breast tumors has been observed. This study aimed to describe the expression pattern of p63 in high-grade versus low and intermediate-grade invasive breast carcinomas. We performed a retrospective immunohistochemical analysis for p63 on a cohort of 60 breast core biopsies. The cases included 20 Grade 3 (G3), 20 Grade 2 (G2) and 20 Grade 1 (G1) invasive breast carcinomas, graded according the Nottingham grading system. Nuclear p63 expression in neoplastic cells was assessed and described. Positive p63 staining in neoplastic cells was identified in 16 out of 20 (80 %) high-grade invasive carcinomas. The staining pattern was typically focal and moderate in intensity. Conversely, all but one case of invasive breast carcinoma G2 showed absolute negativity for p63. All cases of low-grade invasive carcinoma also showed clear negativity for p63. A clear association between p63 expression in neoplastic cells and G3 grading was observed. Our findings suggest that p63 expression can be a feature of high-grade invasive breast carcinoma. Although this findings represents a potential diagnostic pitfall, especially on small core biopsy samples, the awareness of this possibility can also allow p63 to serve as a helpful ancillary clue for identifying high-grade disease.