The effects of clamping and reoxygenation on repopulation during fractionated irradiation

Clamping of solid tumors is often used during radiobiological experiments to eliminate reoxygenation and hypoxia as confounding variables. The appropriateness of this procedure has been questioned recently.

This article describes experiments designed to examine the tumor surviving fraction (TSF) during fractionated treatment (up to 9 x 2 Gy, every 6 h or 24 h) given under normal, unclamped conditions (aerobic), and to compare it to TSF values for treatment given under clamped conditions (anoxic). The dose per fraction for the clamped procedure was 5.5 Gy, approximately equivalent to the unclamped doses of 2 Gy (based on an assumed OER of 2.75).

Overall, the survival in the unclamped experiments was greater than in the clamped experiments. When the contribution of repopulation to this phenomenon was analyzed, no significant difference was found for four (KHT-C, KHT-LP1, RIF-1, B16F1) of five transplantable murine tumor cell lines used. However, for the SCC-VII squamous cell carcinoma, following small numbers of fractions there appeared to be more repopulation during the aerobic treatment schedule. Reoxygenation during the treatment was also analyzed, and the results indicate that, even with 24 h between doses, reoxygenation was insufficient to prevent an increasing hypoxic fraction during the treatment, which can explain the enhanced survival seen for the aerobic vs. the anoxic irradiation conditions.

With the fractionation schedules used in this study, there was no evidence for a difference in repopulation kinetics when the treatment was given to clamped (anoxic) or unclamped (aerobic) tumors. In all five tumors studied, reoxygenation between the 2 Gy fractions was insufficient to prevent hypoxia from affecting the response of the tumors to the treatment.