Adjuvant chemotherapy in luminal breast cancers

Luminal breast cancers are traditionally considered to comprise of tumors expressing estrogen receptor (ER) and represent the majority of breast cancers. These tumors are characterized by significant heterogeneity in phenotype, molecular signature, relapse patterns and therapeutic response to endocrine and chemotherapy. Whilst adjuvant endocrine therapy is standard of care in patients with tumors that express either ER and/or progesterone receptor (PR), the indication for adjuvant chemotherapy is less clear-cut. On average, ER-positive breast tumors derive less benefit from chemotherapy compared to ER-negative tumors, however there is still clearly a subset of patients with ER-positive tumors that are chemosensitive. The basis for the addition of chemotherapy to adjuvant endocrine therapy is usually guided by the clinician's estimation of prognosis and assessment of the endocrine sensitivity of the tumor. The use of chemotherapy in this setting, however, is highly variable. There is tremendous value in identifying subgroups of patients who can expect favorable outcomes with endocrine therapy and who may not require any additional therapy. Similarly, it is equally important, if not more important, to characterize patients with ER-positive disease who will derive a substantial benefit from cytotoxic chemotherapy. In this article, we aim to discuss the utility of current biomarkers used to guide decisions regarding chemotherapy in ER-positive, HER2-negative breast cancers.