Breast carcinoma with basal differentiation: a proposal for pathology definition based on basal cytokeratin expression

Aims: To assess the expression and coexpression of a range of different biomarkers that have been used to define breast carcinomas with a basal phenotype (BP) and their relationship with prognosis in an attempt to refine the definition of BP and to evaluate the reliability of using a single biomarker to identify these tumours. Methodsa and results: The expression pattern of basal cytokeratins (CK5/6 and CK14), oestrogen, progesterone and androgen receptors, epidermal growth factor receptor, HER2, BRCA1, P-cadherin and myoepithelial markers (smooth muscle actin and p63) were studied in a well-characterized series of invasive breast carcinoma (1872 cases) with long-term follow-up using immunohistochemistry and tissue microarray. Although the additional markers were associated with basal CK expression, they did not serve to improve recognition of cases with differing outcome when compared with basal CKs alone and, if used to define cases, reduced considerably the proportion of cases allocated to this poor prognostic type of breast cancer. Conclusions: BP can be defined based on the expression of basal CKs regardless of the expression of other markers.