The Ki67 test is a diagnostic tool used in cancer pathology to measure the rate at which tumor cells are dividing. This measurement provides physicians with an estimate of how aggressively a tumor is growing, which is a significant factor in determining prognosis. The test identifies the Ki67 protein, a biomarker that serves as a proxy for the speed of cell proliferation. Understanding the Ki67 score helps oncologists categorize tumors and decide on the most appropriate course of treatment, especially where the proliferation rate directly influences therapeutic choices.
Understanding the Ki67 Protein and Staining Process
The Ki67 protein is a nuclear protein whose presence is strictly regulated by the cell cycle. This protein is actively produced and detectable only when a cell is engaged in division (G1, S, G2, and M phases). Conversely, the Ki67 protein is absent in cells that are resting or quiescent (G0 phase). Because of this precise timing, the protein functions as an effective marker for the fraction of cells that are actively proliferating within a tumor mass.
To visualize this protein, pathologists use immunohistochemistry (IHC) on a tissue sample, such as a biopsy. The tissue is prepared, sliced into thin sections, and specific antibodies are applied. These antibodies are designed to bind directly to the Ki67 protein inside the tumor cells.
A subsequent chemical reaction produces a visible color change, most often a brown stain, localized to the nucleus of any cell containing the Ki67 protein. When viewed under a microscope, actively dividing tumor cells will have a stained nucleus, while resting cells will not. This process allows the pathologist to create a clear visual distinction between the cycling and non-cycling tumor cell populations.
Interpreting the Proliferation Score
The Ki67 score, also referred to as the proliferative index, is calculated by a pathologist as a percentage. This percentage represents the ratio of tumor cell nuclei that have stained positive for the Ki67 protein compared to the total number of tumor cells examined. For an accurate measurement, pathologists typically count several hundred to a thousand tumor cells across the sample.
A high Ki67 score indicates a rapidly dividing, more aggressive tumor, which is likely to grow and spread quickly. Conversely, a low score suggests a slower-growing tumor with a smaller fraction of cells currently in the process of division. For example, in many contexts, a score below 10% or 14% is considered low, while a score exceeding 20% or 30% is often classified as high.
Calculating the score often involves focusing on “hot spots,” which are areas exhibiting the highest density of positive staining. However, there is no single, universally agreed-upon percentage cutoff that applies to all cancer types, which contributes to variability in interpretation across different institutions and studies.
Ki67 in Guiding Cancer Treatment
The Ki67 score is integrated into clinical practice primarily as a prognostic factor, helping to estimate the risk of cancer recurrence after initial treatment. In breast cancer, where its use is most common, the Ki67 score plays a significant role in classifying tumors into molecular subtypes. Hormone receptor-positive breast cancers are often divided into Luminal A (low Ki67) and Luminal B (high Ki67) based on this proliferation rate.
A low Ki67 score, typically associated with the Luminal A subtype, suggests the tumor will likely respond well to hormone therapy alone. These tumors are generally slow-growing, and the risk of recurrence may not warrant the addition of chemotherapy. The International Ki67 in Breast Cancer Working Group (IKWG) has suggested that a score of 5% or less can be used to estimate a favorable prognosis in early-stage, estrogen receptor-positive patients.
In contrast, a high Ki67 score, characteristic of the Luminal B subtype, indicates a higher risk of recurrence and a greater potential for benefit from chemotherapy. Chemotherapy agents work by targeting rapidly dividing cells, making tumors with a high proliferation rate more susceptible to this type of treatment. High Ki67 levels may also prompt the use of targeted agents, such as CDK4/6 inhibitors, which specifically block the cell cycle signals that drive proliferation.
Beyond breast cancer, the Ki67 index is routinely used to grade neuroendocrine tumors (NETs). In this context, the Ki67 score is used to categorize NETs into distinct grades, which directly determines whether the treatment strategy will focus on surgical resection, chemotherapy, or targeted therapies. The proliferation rate acts as a primary determinant for the tumor’s grade and the subsequent clinical management plan in these cancers.
Factors Affecting Ki67 Reliability
Despite its value, the Ki67 test faces several challenges that can affect the reliability and reproducibility of the results. One major area of concern is pre-analytical variability, which relates to the handling of the tissue specimen before staining. Issues such as the time elapsed before fixation or the type of fixative used can alter the protein and affect the staining quality.
Another significant limitation is the inter-observer variability in scoring, meaning different pathologists may arrive at different percentage scores for the same slide. This subjectivity arises because the visual counting process can be inconsistent, even with standardized protocols in place. International groups have worked to standardize the visual scoring method, but differences between laboratories persist.
While consensus guidelines exist for breast cancer, the thresholds for defining scores vary by institution and study, making it difficult to compare results directly. For these reasons, the Ki67 score is rarely used as a standalone factor but is instead considered alongside other clinical and pathological information to inform treatment decisions.