Ki-67 is a nuclear protein expressed in all active phases of the cell cycle (G1, S, G2, and mitosis), but absent in resting cells (G0 phase).
- What it measures: Proliferation rate of tumor cells.
- How it’s reported: As a percentage of positively stained tumor cells (the Ki-67 index).
- Clinical meaning:
- Low Ki-67 → slow-growing, less aggressive tumor.
- High Ki-67 (>20–30%) → Indicates a highly proliferative tumor, associated with aggressive behavior, poor prognosis, and potential benefit from more intensive systemic therapy. Commonly used in breast cancer and other solid tumors to guide prognosis and sometimes treatment decisions.
Cancers where Ki-67 is routinely used:
- Breast cancer – Prognostic marker; high Ki-67 (>20–30%) suggests aggressive disease, higher recurrence risk, and potential benefit from chemotherapy.
- Neuroendocrine tumors (NETs) – Central in grading (G1: <3%, G2: 3–20%, G3: >20%). Guides prognosis and therapy choice.
- Lymphomas – High Ki-67 (often >80–90% in Burkitt lymphoma, DLBCL) reflects rapid proliferation and aggressive clinical course.
- Prostate cancer – Sometimes used as an adjunct prognostic marker, but not standard in guidelines.
- Other solid tumors (lung, brain, GI, etc.) – May provide prognostic information, but clinical utility varies.
In short: Ki-67 = proliferation marker; the higher the value, the more aggressive the tumor biology.