Triple-negative breast cancer: what the radiologist needs to know.

Approximately 10% to 15% of breast carcinomas do not express estrogen receptor (ER) or progesterone receptor (PR) and do not exhibit overexpression or gene amplification of human epidermal growth factor receptor 2 (HER2). These tumors are called triple receptor-negative or triple-negative (TN) breast cancers. TN breast cancer has recently been recognized as a subtype of breast cancer with aggressive clinical behaviorandapoorclinicaloutcome.TNbreastcancershave a propensity to occur in premenopausal women and Hispanic and African-American women, and TN tumors are associated with mutations in p53 and BRCA1. Therapies targeting the estrogen receptor or the HER2/neu oncogene are likelytobeineffectiveagainstTNtumors.Thisarticlereviews the molecular, pathologic, and imaging features of TN breast cancer. Breastcancersareagroupofdiseaseswithawidespectrum of clinical, pathologic, and molecular characteristics. By the use of gene expression profiles, 4 breast cancer subtypes with different prognoses have been identified: 2 ER-negative subtypes: basal-like (BL) and human epidermal growth factor receptor 2 (HER2)-positive, and 2 ER-positive subtypes: luminal A and luminal B. 1-4 Breast cancers are classified as TN by the use of established immunohistochemical (IHC) assays for ER, PR, and HER2, and fluorescence in situ hybridization (FISH) assays for HER2. TN breast cancers are characterized bytheabsenceofall3predictive/prognosticmarkers:ER,PR, and HER2. Most TN breast cancers are high grade, and the tumors are usually large with relatively well-circumscribed pushing margins, lymphocytic infiltration and a high mitotic rate. 5,6 BL breast cancer is a molecular phenotype defined on the basis of cDNA microarray studies. 7 Approximately 70% of TN breast cancers are BL tumors. 1,8-12 The term BL comes from the resemblance between the cytokeratin expression pattern in the cancer cells and the myoepithelial cells of the breast. 9,13,14 ThisdoesnotmeanthatBLbreastcancercellsare derived from myoepithelial cells. Despite the great interest in BL and TN cancers, there are no universally accepted definitions for these tumors. In this review, we use the term BL when cDNA or more sophisticated technology was applied for tumor classification and TN when clinical assays for ER, PR, and HER2 were used.