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Ki-67 – Proliferation Marker in Tumor Diagnostics

Ki-67 is a cell proliferation marker used in pathology to assess the growth activity of tumor cells and guide cancer treatment decisions.

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Things worth knowing about "Ki-67"

Ki-67 is a cell proliferation marker used in pathology to assess the growth activity of tumor cells and guide cancer treatment decisions.

What is Ki-67?

Ki-67 is a cell proliferation marker – a protein found in actively dividing cells that serves as a key biomarker in tumor diagnostics. The name comes from its place of origin: the protein was first characterized in the 1980s at the University of Kiel (abbreviated Ki), and the antibody clone number 67 gave it its name.

Ki-67 is detectable only in cells that are actively undergoing division. In resting, non-proliferating cells, the protein is absent. This property makes Ki-67 a valuable tool for assessing how rapidly a tumor is growing and how aggressive it may be.

Biological Function

Ki-67 is a nuclear protein expressed during all active phases of the cell cycle – the G1, S, G2, and M phases. It is not detectable in the resting phase (G0 phase). Although its precise biological function is not yet fully understood, Ki-67 is thought to play a role in organizing chromatin during cell division.

Clinical Significance as a Tumor Marker

In clinical oncology and pathology, the Ki-67 index (also called the Ki-67 proliferation index) is used to determine the proportion of dividing tumor cells in a tissue sample. The index is expressed as a percentage, indicating how many cells in a tumor sample test positive for Ki-67.

Low Ki-67 index (e.g., below 10–15%): Suggests slowly growing, less aggressive tumor tissue.
High Ki-67 index (e.g., above 30%): Indicates rapidly growing, more aggressive tumor tissue.

Exact cut-off values can vary depending on the type of tumor and the clinical guidelines used.

Areas of Application

Ki-67 is measured in a wide range of tumor types, including:

Breast cancer: Ki-67 is a key parameter in classifying breast carcinomas into molecular subtypes (e.g., Luminal A vs. Luminal B) and influences therapy decisions.
Neuroendocrine tumors (NETs): Ki-67 is a central component of the WHO classification for neuroendocrine tumors and determines tumor grading (G1, G2, G3).
Lymphomas: Used to assess the aggressiveness of lymphomas.
Other tumors: Including prostate cancer, brain tumors, and gastrointestinal tumors.

Diagnosis: How is Ki-67 Determined?

Ki-67 is detected by immunohistochemistry (IHC) on tissue samples (biopsies or surgical specimens). Specific antibodies that bind to the Ki-67 protein are applied to the tissue, making positive cells visible under the microscope. A pathologist then evaluates how many cell nuclei in the sample stain positive and calculates the Ki-67 index as a percentage.

Prognostic and Predictive Significance

Ki-67 serves both as a prognostic marker (providing information about the likely course of the disease) and as a predictive marker (helping to predict how well a patient will respond to certain therapies):

A high Ki-67 value is associated with a worse prognosis in many tumor types.
At the same time, rapidly growing tumors with a high Ki-67 index may respond better to chemotherapy, as these treatments preferentially target dividing cells.
In breast cancer, Ki-67 influences the decision for or against chemotherapy in accordance with clinical guidelines.

Limitations

Despite its widespread use, Ki-67 has limitations. The assessment of the Ki-67 index can vary between different laboratories and pathologists (interobserver variability). International standardization efforts, such as those by the Ki-67 International Working Group (IKWG), aim to harmonize measurement methods and improve clinical reliability.

References

Scholzen T, Gerdes J. The Ki-67 protein: from the known and the unknown. Journal of Cellular Physiology. 2000;182(3):311–322.
Dowsett M et al. Assessment of Ki67 in Breast Cancer: Recommendations from the International Ki67 in Breast Cancer Working Group. Journal of the National Cancer Institute. 2011;103(22):1656–1664.
WHO Classification of Tumours Editorial Board. WHO Classification of Tumours: Digestive System Tumours. 5th ed. Lyon: IARC Press; 2019.

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