RESUMEN
AIMS: Triple-negative breast cancers (TNBCs) are often poorly differentiated tumours that can present clinically as metastases of an unknown primary. Immunohistochemical panels are frequently used to determine the likelihood of a breast primary, but in this tumour subset cytokeratin (CK)7 may be the only positive finding. In this study we aimed to evaluate a commonly employed immunohistochemical panel using a large group of TNBCs (both basal-like and unclassified), and to analyse the CK7 staining patterns. METHODS AND RESULTS: Tissue microarrays containing 138 TNBCs were stained with antibodies against CK7, CK20, gross cystic disease fluid protein 15 (GCDFP-15), and mammaglobin. CK5/6 staining was used to identify basal-like tumours. CK7 staining was notably heterogeneous, with 14.5% of all cases demonstrating ≤20% tumour cell staining. A greater proportion of basal-like TNBCs than of unclassified TNBCs showed focal staining. GCDFP-15 and mammaglobin were not expressed in the majority of TNBCs, and were less frequently positive in basal-like than in unclassified TNBCs. CONCLUSION: TNBCs are commonly negative for most immunomarkers indicative of breast origin, with the exception of CK7. As about one in five TNBCs showed only focal CK7 positivity, use of this marker must be interpreted with caution, especially in small samples, so that the possibility of a breast primary is not overlooked.