Once cancer is diagnosed, there are several tests performed on lymph node or tumor tissue that can be useful in determining a woman's prognosis and for assessing the type of treatment that will be most effective for her specific breast cancer. The issue of which factors are the most reliable at determining a woman's prognosis and predicting her outcome to certain treatments is perpetually under study. As research progresses, certain factors will fall in and out of favor. Only when found to be accurate and reliable does a factor become a part of standard practice. Commonly assessed prognostic and predictive factors include lymph node status, tumor size, and tumor grade, type of cancer, hormone receptor status, proliferation rate, and HER2/neu (also known as erbB2 expression).
HER2 (human epidermal growth factor receptor 2) is a gene found in every cell of the human body, and its purpose is to help a cell divide. The HER2 gene tells a cell to form the HER2 protein on the cell surface. HER2 protein then receives a signal to send a message to the center of the cell, known as the nucleus, that it is time to divide. The HER2 protein is also called the HER2 receptor.
Each healthy breast cell contains two copies of the HER2 gene, which contribute to normal cell function. When a change occurs that causes too many copies of the HER2 gene to appear in a cell, the gene, in turn, causes too many HER2 proteins, or receptors, to appear on the cell surface. This is referred to as HER2 protein overexpression. Patients who are considered HER2-positive have cancer that grows and spreads more rapidly.
HER2 protein overexpression affects about 25% of breast cancer patients and results in a more aggressive form of the disease and earlier disease reappearance; in these cases the disease may not be as responsive to standard therapies. The HER2 status of a tumor is determined by testing tissue removed during a biopsy.
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