Rate of Cell Growth

S-Phase Fraction and Ki-67

The pathology report may include information about the rate of cell growth; that is, the proportion of cancer cells within the tumor that are actively growing and dividing.  A higher percentage suggests a more aggressive, faster growing cancer.    The two most common tests used for this purpose are known as S-Phase Fraction and Ki-67.

S-Phase Fraction measures the percentage of cells in the sample that are in the "synthesis phase," the part of a cell's life that occurs right before it divides into two new cells.  A result of less than 6% is low; 6-10% is intermediate; greater than 10% is considered high.

Ki-67 measures a protein that increases as cells prepare to divide.  The pathologist stains the cancer cells to see how many are positive for Ki-67.  In breast cancer, a result of less than 10% is considered low, 10-20% is borderline, and greater than 20% is high.

The rate of cell growth and the grade of a cancer tend to be closely related, but the tests measuring them are not always accurate and are not always included in the pathology report.  If your tumor is tested by Oncotype DX or MammaPrint, Ki-67 will be included in that report.