Definition

A way to measure how fast cancer cells in a tumor are dividing. Ki-67 is a protein that is found only in cells that are dividing. A high Ki-67 proliferation index means many cells are dividing quickly and that the cancer is likely to grow and spread. A Ki-67 proliferation index over 30% is typically considered high. The Ki-67 proliferation index may be used to help predict how well some cancers, such as breast cancer, will respond to certain treatments, such as chemotherapy. Also called Ki-67 score.