Ki-67 is a protein that serves as a significant biomarker in breast cancer pathology. It provides insights into how quickly cancer cells are proliferating, which refers to their rate of growth and division. Understanding the level of Ki-67 helps medical professionals gain a clearer picture of a tumor’s biological characteristics.
The Role of Ki-67 in Cell Growth
The Ki-67 protein plays a specific role within the cell. It is present in the nucleus of cells during their active phases of growth and division, which include the G1, S, G2, and M phases of the cell cycle. Conversely, Ki-67 is absent in cells that are in a resting state, known as the G0 phase. This presence only during active proliferation makes Ki-67 a reliable indicator of a cell’s growth activity.
Consider it like a light that switches on only when a factory machine, representing a cell, is actively running and producing. The higher the presence of this protein, the more cells are actively engaged in the process of multiplication within the tumor.
Interpreting Ki-67 Test Results
The level of Ki-67 in a breast cancer tumor is measured through immunohistochemistry (IHC). In this process, a pathologist applies special stains to a tissue sample, usually from a biopsy. These stains bind to the Ki-67 protein, highlighting these cells. The pathologist then counts the percentage of cancer cells showing this staining, which is reported as the Ki-67 score or index.
A low Ki-67 score indicates a slower-growing tumor, while a high score suggests rapid cell division. For instance, a score below 5% is considered low, whereas 30% or higher is seen as high. Some pathology laboratories may use slightly different thresholds, such as 10% or 15%, to define low versus high proliferation. These cutoff points are not universally standardized across laboratories. Therefore, discuss your specific Ki-67 score with your medical team.
Impact on Breast Cancer Treatment
The Ki-67 score serves as both a prognostic and a predictive marker in breast cancer. A higher Ki-67 score indicates a more aggressive, faster-growing tumor. This increased proliferative activity can be associated with a greater likelihood of the cancer recurring after initial treatment.
Given that chemotherapy treatments are designed to target and destroy rapidly dividing cells, a high Ki-67 score can suggest that the cancer is more likely to respond favorably to chemotherapy. For example, studies have shown that tumors with Ki-67 values greater than 25% or even 35% may exhibit improved responses to neoadjuvant chemotherapy, which is administered before surgery.
Conversely, for certain types of breast cancer, especially those that are estrogen receptor-positive (ER-positive), a very low Ki-67 score might influence treatment decisions. In these cases, a low proliferation rate could support the use of hormone therapy alone, potentially allowing patients to avoid chemotherapy and its associated side effects. Clinical trials have explored how changes in Ki-67 levels after short-term endocrine therapy can help determine which patients might safely forego chemotherapy.
Ki-67 in the Context of Other Tumor Markers
Ki-67 is never evaluated in isolation; it is considered alongside other tumor markers to provide a comprehensive picture of breast cancer. These other markers include Estrogen Receptor (ER), Progesterone Receptor (PR), and Human Epidermal Growth Factor Receptor 2 (HER2). ER and PR indicate whether the cancer cells grow in response to hormones, while HER2 status identifies if the cells have too many HER2 proteins, which can promote growth.
For tumors that are hormone receptor-positive (ER-positive) and HER2-negative, Ki-67 helps to further classify the cancer into distinct subtypes. Luminal A tumors, for example, are ER-positive, PR-positive, HER2-negative, and exhibit a low Ki-67 index, often below 14% or 15%. These tumors grow more slowly and have a better prognosis.
In contrast, Luminal B tumors are also ER-positive and HER2-negative, but they are characterized by a higher Ki-67 index, usually above 14% or 15%, or by lower PR positivity. Some Luminal B tumors can also be HER2-positive. This higher proliferation rate suggests a more aggressive tumor behavior compared to Luminal A. The combined profile of ER, PR, HER2, and Ki-67 provides the oncology team with a complete understanding of the tumor’s biology, fundamental for developing the most effective and personalized treatment strategy.