Transitional Increase in Growth Fraction Estimated by Ki-67 Index After Irradiation to Human Tumor in Xenograft

Abstract

The cell kinetics of human tumor cells (HSG) in vivo were estimated by Ki-67 index to analyze the biological behavior of quiescent cells after irradiation. The increment of Ki-67 index was dose-dependent up to 15 Gy irradiation. The Ki-67 index of tumor cells increased gradually from 35±5.0% before irradiation to 56±5.0% at day 5 after single irradiation of 15 Gy. Flow-cytometric study showed that the G1 population was decreased and the G₂M population was elevated at day 5 after the irradiation. In fractionated irradiation, the time-course in the change of the Ki-67 index was similar to that of single irradiation. It was likely that tumor cells were recruited into the cell cycle from quiescent phase after irradiation. We consider the Ki-67 index to be a reliable indicator of growth fraction and useful to estimate the cell kinetics of irradiated tumors in vivo. The growth fraction estimated by Ki-67 index during radiotherapy is expected to be useful in predicting tumor response to radiation therapy.