A Review of Triple-Negative Breast Cancer

Background: An estimated 1 million cases of breast cancer are diagnosed annually worldwide. Of these, more than 170,000 are described as triple-negative. Triple-negative breast cancer (TNBC) is defined by the lack of protein expression of estrogen receptor (ER) and progesterone receptor (PR) and the absence of HER2 protein overexpression. TNBC is a subtype of breast cancer that overlaps with the "basal-like" breast cancer. TNBC has significant clinical implications. Methods: The epidemiology, diagnosis, clinical course, prognosis, and pathology of this subtype of breast cancer are reviewed. The authors compare the "triple-negative" and "basal-like" definitions of breast cancer. A discussion of both standard and experimental treatments for TNBC is included. Results: The poor prognosis of high-grade TNBC relates to poor disease-free interval in the adjuvant setting, shortened progression-free survival in the metastatic setting, and the lack of targeted therapy. However, not all TNBCs are associated with a poor prognosis. Conclusions: Although chemotherapy is the main current treatment of this subtype of breast cancer, new agents such as PARP inhibitors, which show promise in the treatment of TNBC, are currently in clinical trials.