Advances in gene expression and molecular profiling have clarified that breast cancer is not a single disease entity, but a heterogeneous disease with multiple subtypes. Breast cancers can now be categorized into the luminal subtypes, which express estrogen receptors (ERs), the human epidermal growth factor receptor 2 (HER2)-positive subtypes, which express ERBB2, and the basal subtypes, which tend to be negative for ERs and ERBB2 expression;^1–3 each subtype is associated with a different prognosis. As a result, the adjuvant treatment of breast cancer has also evolved to reflect the heterogeneous nature of the disease. This evolution of treatment options illustrates the complexity of adjuvant therapy selection for patients with newly diagnosed breast cancer. This article will highlight the evolution of treatments and risk stratification models over time and introduce possible developments for the future of breast cancer therapy.

Selection of Adjuvant Therapy
The goals of systemic adjuvant therapy are to reduce the risk for recurrence and improve survival while causing minimal additional toxicity. The discovery of specific breast cancer subtypes that express different molecular markers has allowed the selection of breast cancer therapies to be tailored to the individual profile of a patient’s tumor (including tumor size and lymph-node status), risk for recurrence, patient comorbidities, patient menopausal status, and patient desires.

Personalized Medicine in Breast Cancer in 2009
This individualized approach utilizes the presence of hormone receptors in breast cancer to provide specific anti-estrogen therapy, with or without systemic chemotherapy. The presence of HER2 overexpression is targeted by a chemotherapeutic regimen that includes trastuzumab. Patients with basal subtype breast cancers who lack specific targets for anti-estrogen or anti-HER2 therapy are candidates for chemotherapy only.

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