Your pathology report may include information about the rate of cell growth — what proportion of the cancer cells within the tumor are growing and dividing to form new cancer cells. A higher percentage suggests a faster-growing, more aggressive cancer, rather than a slower, “laid back” one. Tests that can measure the rate of growth include:
- S-phase fraction: This number tells you what percentage of cells in the sample are in the process of copying their genetic information, or DNA. This S-phase, short for “synthesis phase,” happens just before a cell divides into two new cells. A result of less than 6% is considered low, 6-10% intermediate, and more than 10% is considered high.
- Ki-67: Ki-67 is a protein in cells that increases as they prepare to divide into new cells. A staining process can measure the percentage of tumor cells that are positive for Ki-67. The more positive cells there are, the more quickly they are dividing and forming new cells. In breast cancer, a result of less than 10% is considered low, 10-20% borderline, and high if more than 20%.
Although the S-phase fraction and Ki-67 level may provide you and your doctor with useful information, experts don’t yet agree on how to use the results when making treatment decisions. Therefore, not all doctors order these tests routinely, so they may not appear in your pathology report. The other results in your report will be much more important in making informed choices. (If you decide to have an Oncotype DX test to check the likelihood of cancer coming back and whether you could benefit from chemotherapy, Ki-67 will be included in that panel of testing.)