A breast cancer pathology report contains specific details about the tumor, one of which is the Ki-67 score. The Ki-67 protein is a marker found within the nucleus of cells that are in the process of dividing. Its presence indicates that a cell is active and multiplying, which is a fundamental process of tissue growth and a hallmark of cancer.
In the context of breast cancer, measuring the Ki-67 protein provides a window into the behavior of cancer cells. By identifying the proportion of cells containing this protein, pathologists can gauge the proliferative activity, or growth rate, of a tumor. This information is a valuable piece of the puzzle for understanding the disease in each individual.
The Role of Ki-67 in Breast Cancer
Oncologists measure Ki-67 to determine the proliferation rate of a breast tumor. Because the Ki-67 protein is present only when cells are actively multiplying and is absent in resting cells, it serves as a direct indicator of this activity. This measurement is performed on tumor tissue collected during a biopsy or surgical procedure.
In the laboratory, a technique called immunohistochemistry (IHC) is used. This process involves applying a specific antibody that binds to the Ki-67 protein within the cancer cells. When the antibody attaches, it creates a stain that can be seen under a microscope, allowing a pathologist to count the positive cells.
Understanding the Ki-67 Score
The result of the Ki-67 test is presented as a percentage, often referred to as the Ki-67 score or labeling index. This figure represents the proportion of cancer cells in the tissue sample that stained positive for the Ki-67 protein. For example, a score of 25% means that one-quarter of the cancer cells examined were actively dividing when the tissue was collected.
Pathology labs group these scores into categories to describe the tumor’s proliferative activity. While there are no universally standardized cutoff points, a general framework is often used. A score below 15% is typically considered to indicate low proliferation. A score greater than 30% is usually classified as high proliferation, while those in between can be more complex to interpret.
Ki-67’s Impact on Prognosis and Treatment
The Ki-67 score is a significant factor when determining a patient’s prognosis, as it provides insight into the aggressiveness of the tumor. Generally, a higher Ki-67 score is associated with a tumor that is growing more rapidly. This increased proliferation can mean there is a greater likelihood of the cancer spreading to other parts of the body.
The score has a direct impact on treatment recommendations, particularly for patients with the most common subtype of breast cancer: estrogen receptor-positive (ER-positive) and HER2-negative. In these cases, the Ki-67 level helps oncologists decide whether chemotherapy would be beneficial. Because chemotherapy works by targeting and destroying rapidly dividing cells, a tumor with a high Ki-67 score is more likely to respond well to this type of treatment.
Conversely, a low Ki-67 score suggests that the cancer cells are dividing slowly. For these tumors, the risks and side effects of chemotherapy may outweigh the potential benefits. In such instances, hormone therapy (also called endocrine therapy) alone might be recommended as the primary course of adjuvant treatment. This approach blocks the hormones that fuel the cancer’s growth and can be highly effective for slow-growing, ER-positive tumors.
The Ki-67 score can also be used to monitor how a tumor is responding to initial treatment. For some patients who receive therapy before surgery, a decrease in the Ki-67 score can indicate that the treatment is working effectively to slow down cell proliferation. This dynamic use of the test provides real-time feedback that can help guide ongoing therapeutic strategies.
Limitations and Context of the Ki-67 Test
While the Ki-67 score provides valuable information, it is just one component of a comprehensive analysis and has recognized limitations. The most notable is the variability in scoring that can occur between different pathology labs and even between individual pathologists. This lack of universal standardization means that a score from one lab cannot always be directly compared to another.
Therefore, oncologists create a complete diagnostic profile by integrating the Ki-67 score with several other established prognostic factors. A patient’s final treatment plan is a holistic decision, synthesized from all these data points to create a strategy tailored to their specific cancer. The primary factors considered alongside Ki-67 include:
- Tumor grade: Describes how abnormal the cancer cells appear under a microscope.
- Tumor size and stage: The size of the tumor and whether it has spread to nearby lymph nodes.
- Hormone receptor status (ER/PR): Indicates if the cancer’s growth is fueled by estrogen or progesterone.
- HER2 status: Identifies the presence of a protein that promotes cancer cell growth.