Immunohistochemistry is a laboratory technique that identifies specific proteins within tissue samples, providing insights into abnormal cells. In breast cancer, this method has transformed how the disease is diagnosed and how treatment plans are developed for each patient.
Understanding Immunohistochemistry
Immunohistochemistry works by leveraging the specific interaction between antigens and antibodies. Antigens are unique proteins on or within cells, and antibodies are specialized proteins that recognize and bind to them. This technique uses these tools to pinpoint the presence and location of particular proteins in a tissue section.
To perform the test, a tissue sample, often obtained through a biopsy, is prepared and placed on a slide. Specific antibodies are then applied to the tissue. If the target protein (antigen) is present, the antibody will attach to it.
A detection system then visualizes this antibody-antigen binding. An enzyme attached to the antibody, when exposed to a specific chemical substrate, produces a visible color change in the tissue. This color change highlights the exact areas where the target protein is located, allowing a pathologist to examine its presence under a microscope.
Key Biomarkers in Breast Cancer
Immunohistochemistry is routinely used in breast cancer diagnosis to identify several specific biomarkers, which are molecules that indicate the presence or characteristics of a disease. These biomarkers provide crucial information about how a breast cancer might behave and respond to different therapies. Each invasive breast cancer is typically tested for these markers.
Estrogen Receptor (ER) is a protein found inside some breast cancer cells. A positive ER test indicates that estrogen can attach to these receptors and stimulate cancer growth, making it a “hormone receptor-positive” cancer. Approximately two-thirds of breast cancers are hormone receptor-positive.
Progesterone Receptor (PR) is another protein found in breast cancer cells, similar to the estrogen receptor. When progesterone binds to PRs, it can also promote cancer cell growth. A positive PR test, often seen alongside a positive ER test, further confirms that the cancer is hormone-sensitive.
Human Epidermal Growth Factor Receptor 2 (HER2) is a protein on the surface of breast cells that plays a role in cell growth and division. In about 15% to 20% of breast cancers, an excess of HER2 receptors means the cancer cells are growing and dividing more rapidly.
Ki-67 is a protein present in actively dividing cells. It serves as a proliferation marker, indicating how quickly cancer cells are multiplying. A low Ki-67 index, typically below 10-14%, suggests a slow-growing tumor. Conversely, a high Ki-67 index, often above 20% or even 30%, indicates a rapidly dividing tumor.
Personalizing Breast Cancer Treatment
The results from these immunohistochemistry tests directly influence treatment strategies for breast cancer patients. Understanding a tumor’s specific characteristics allows oncologists to select therapies that are most likely to be effective.
For Estrogen Receptor-positive (ER+) and/or Progesterone Receptor-positive (PR+) breast cancers, hormone therapy is a primary approach. These therapies lower the body’s estrogen levels or block estrogen receptors on cancer cells, preventing growth. Hormone receptor-positive cancers typically respond well to these targeted treatments.
HER2-positive breast cancers, despite their aggressive nature, respond favorably to specific targeted therapies. These treatments, such as monoclonal antibodies or antibody-drug conjugates, are designed to specifically attack the HER2 protein, slowing or stopping the growth of cancer cells. The effectiveness of these targeted drugs has significantly improved the prognosis for patients with HER2-positive disease.
The Ki-67 index helps inform chemotherapy decisions. Tumors with a high Ki-67 score, indicating rapid cell division, are generally more responsive to chemotherapy because these drugs primarily target fast-growing cells. While a high Ki-67 score suggests a more aggressive tumor, it also means chemotherapy may be more effective. The Ki-67 index, along with other factors, helps determine the necessity and intensity of chemotherapy.