Why are these tests important?
Breast cancer is the most frequently diagnosed cancer in women in the U.S., excluding skin cancer, and is the second leading cause of cancer death. The American Cancer Society estimates that 230,480 new cases of invasive breast cancer and 57,650 cases of in situ breast cancer (i.e., ductal carcinoma in situ or DCIS) are diagnosed in women each year and that 39,520 women and 450 men will die of breast cancer.
The keys to successful breast cancer treatment are early detection and appropriate treatment. The ultimate goal is to tailor treatments for the affected individual and the specific cancer type. For example, some breast cancers are more aggressive, some are estrogen or progesterone receptor responsive, promoting growth in the presence of these hormones, some show overexpression of HER2/neu (ERBB2), and some show enhanced sensitivity to chemotherapy while others are resistant to standard chemotherapy regimens. Depending on the characteristics of a tumor, treatment typically includes a combination of surgery, radiation, chemotherapy, hormone therapy, and/or targeted therapy.
The combination of treatments selected for an individual are based on several factors. Guidelines established by the National Comprehensive Cancer Network outline resources and parameters for estimating the risk of relapse and/or death as well as the benefits of certain treatments in people with breast cancer. The traditional parameters used for these purposes include:
- Patient's age and health
- Tumor size
- Presence or absence of progesterone and/or estrogen receptors (hormone status)
- Presence of HER2/neu overexpression and/or amplification
- Cancer grade (how normal or abnormal the tumor's cells are)
- Stage of the cancer (whether it has remained localized, spread to nearby tissues, or spread to other organs)
- Whether cancer cells are present in lymph nodes close to the tumor
For example, based on these parameters, patients with small localized breast cancers that have not spread to their lymph nodes may or may not be treated with specific adjuvant therapies, such as chemotherapy and/or tamoxifen (for hormone-sensitive cancers), to lower the risk that some cancer cells have escaped detection and treatment, causing recurrence of the cancer later. Some patients with low risk of relapse and good prognosis may not require adjuvant therapies. These patients might forgo this therapy and avoid associated side effects. Additionally, some breast cancer patients benefit from adjuvant therapies while others do not.
Gene expression tests are used primarily to help evaluate the patient's likely course of disease and risk of recurrence, and secondarily to provide information as to which patients may benefit from adjuvant chemotherapy. The use of gene expression profiling has led to the development of new breast cancer classifications. For example, breast cancer can be classified according to response to hormones like estrogen, and HER2 amplification. These categories can be used to predict overall survival. Breast cancer that is estrogen receptor positive may be more likely to respond to hormone therapy and have improved survival. Similarly, breast cancer that is HER2/neu-positive may be more likely to respond to Herceptin® and have an improved prognosis. A health care provider may decide on a case-by-case basis whether to offer gene expression profiling and, based on the results (as well as other factors), whether to offer additional treatment.