Oxygen enhancement ratio as a function of dose and cell cycle phase for radiation-resistant and sensitive CHO cells.

There is still controversy over whether the oxygen enhancement ratio (OER) varies as a function of dose and cell cycle phase. In the present study, the OER has been measured as a function of survival level and cell cycle phase using volume flow cell sorting. This method allows both the separation of cells in different stages of the cycle from an asynchronously growing population, and the precise plating of cells for accurate measurements at high survival levels. We have developed a cell suspension gassing and sampling system which maintained an oxygen tension <20 ppm throughout a series of sequential radiation doses. For both radiation-resistant cells (CHO-K1) and a radiation-sensitive clone (CHO-xrs6), we could separate relatively pure populations of G1-phase, ${\rm G}_{1}/{\rm S}\text{-boundary}$, S-, and G2-phase cells. Each cell line showed a typical age response, with cells at the G1/ S-phase boundary being 4 (CHO-K1) to 12 (CHO-xrs6) times more sensitive than cells in the late S phase. For both cell...