What is Ki-67?

Ki-67 is a protein found in the nucleus, which is the control center of every cell. This protein only appears when a cell is preparing to divide and make new cells. Cells that are resting and not dividing do not show Ki-67. As a result, Ki-67 is regarded as a marker of cell growth and activity.

Why is Ki-67 important?

The presence of Ki-67 tells pathologists how quickly cells are dividing. In normal tissues, only a small number of cells show Ki-67 at any one time. However, in tumors, a high number of cells may be positive for Ki-67, meaning the tumor is growing and dividing more rapidly. This information can help doctors determine the aggressiveness of a tumor.

How do pathologists test for Ki-67?

Pathologists test for Ki-67 using a technique called immunohistochemistry (IHC). This test uses special antibodies that attach to the Ki-67 protein in the nucleus of dividing cells. When viewed under the microscope, cells that show Ki-67 appear stained (positive), while cells that are not dividing remain unstained (negative).

Pathologists often calculate a Ki-67 index, which is the percentage of tumor cells that test positive. For example, if 30 out of 100 tumor cells show staining, the Ki-67 index is 30%.

What does the Ki-67 index mean?

The Ki-67 index helps pathologists assess how actively a tumor is growing:

• Low Ki-67 index – A smaller number of cells are dividing. The tumor may be slow-growing.

• High Ki-67 index – Many cells are dividing. The tumor may be more aggressive and grow faster.

The exact cut-off values for “low” or “high” Ki-67 can vary depending on the type of tumor. For example, in breast cancer, Ki-67 is sometimes used to help separate slow-growing cancers from fast-growing ones. In brain tumors and neuroendocrine tumors, Ki-67 plays an important role in grading the tumor.

Why is Ki-67 included in my pathology report?

Ki-67 is often included in pathology reports for certain types of cancer because it provides information about the behavior of the tumor. A higher Ki-67 index may suggest the need for closer follow-up or more aggressive treatment, while a lower index may support less intensive treatment. Your doctor will interpret the Ki-67 results in the context of other findings, such as the tumor type, grade, and stage.

Ki-67 in non-tumor cells

It is important to know that Ki-67 is not only found in tumors. Many normal, healthy tissues naturally have groups of dividing cells that show Ki-67. For example, in lymph nodes, special areas called follicles contain immune cells that are actively dividing to help the body fight infections. These follicles often show a high level of Ki-67 staining, even though they are completely normal.

This means that a high Ki-67 result does not always indicate cancer. The result must be interpreted by a pathologist in the context of what tissue is being examined and whether the sample contains a tumor.

Questions to ask your doctor

• What is the Ki-67 index in my tumor?

• Does the Ki-67 level mean my tumor is slow-growing or fast-growing?

• How does Ki-67 affect the treatment options you are recommending?

• Is Ki-67 used to grade or stage my type of tumor?

• Will the Ki-67 result help predict my prognosis?