Breast cancer is the most common cancer in American women, with 1 in 8 women developing breast cancer during their lifetime in the United States. It is 1 of the most common forms of cancer, representing 14.6% of all new cancer cases in the United States, with an estimated 246,660 new cases of breast cancer diagnosed in women and 40,450 deaths in 2016. Women most often develop cancer from the ages of 55 to 64 years, with a median age at diagnosis of 62 years. While breast cancer is common, if it is identified early enough (when localized), the 5-year survival rate for women is 98.8%. However, the survival rate is lower if the cancer has spread to the lymph nodes (i.e., node-positive, 85.2%) and drops dramatically to 26.3% if it has metastasized.

When a person is diagnosed with breast cancer, a thorough clinical and pathologic evaluation is performed to determine the stage of the disease, assist in the estimation of disease recurrence, and help predict response to therapy. For early-stage invasive breast cancer, the pathological assessment typically includes estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) status; axillary lymph node status, tumor size, presence of inflammatory changes; tumor proliferation; and histologic and nuclear tumor grade. Clinical history is also important, including age, menopause status, pregnancy status, and cancer history.

Treatment options for breast cancer include, but are not limited to, surgery, radiation therapy, and endocrine therapy. Adjuvant chemotherapy is used in the treatment of many patients with a high risk for breast cancer recurrence to reduce the risk of recurrence following surgical resection. This has resulted in improvements in both disease-free survival (DFS) and overall survival (OS) of women with breast cancer. In addition, adjuvant endocrine therapy such as the antiestrogen tamoxifen and aromatase inhibitors (AIs), including anastrozole, letrozole, and exemestane, reduce the risk of breast cancer recurrence in women with ER-positive (ER+) breast cancer.  However, not all women who receive these adjuvant treatments will benefit from them; therefore, physicians and patients must balance the benefits along with the risks of adjuvant chemotherapy and endocrine therapy, such as toxicities. HER2-positive (HER2+) breast cancers can grow faster and quicker and women with HER2+ breast cancers may benefit from treatment with HER2-targeting therapies, such as trastuzumab.

Due to the potential benefits and risks of adjuvant therapies, a patient-specific approach is recommended. To aid physicians and patients in making complex treatment decisions, several guidelines, including those published by the National Institutes of Health (NIH), St. Gallen, and the National Comprehensive Cancer Network (NCCN), as well as the computer-based models such as Adjuvant! Online and Nottingham Prognostic Index (NPI), are available. In addition, several gene-based assays are also clinically available, including MammaPrint (Agendia Inc.), Prosigna Breast Cancer Prognostic Gene Signature Assay (NanoString Technologies Inc.), Breast Cancer Index (bioTheranostics Inc.), and Oncotype DX (Genomic Health Inc.). These assays differ in the intended patient populations, sample requirements, genes evaluated, platforms, results, and validation. This evaluation reviews evidence of the analytical validity, clinical validity, and clinical utility of EndoPredict, a gene-based prognostic test for women with early stage, ER+, HER2-negative (HER2-) breast cancer that is used to predict the benefit of chemotherapy and risk of relapse.

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