Radiation Response of Synchronized 9L Rat Brain Tumor Cells Separated by Centrifugal Elutriation

Survival curves of 9L rat brain tumor cells separated by centrifugal elutriation in early G1 (≥97%), mid-S (≥80%), and $\text{mid-}{\rm G}_{2}$ (≥70%) have been measured after 0-1800 rad of 137 Cs γ rays. The D0, $D_{{\rm q}}$, and n values calculated from the cell survival curves at early G1, mid-S, and $\text{mid-}{\rm G}_{2}$ were not significantly different (P > 0.1). Identical results were found with three different elutriation-irradiation procedures: (1) elutriating, irradiating, and plating for colony formation; (2) irradiating, elutriating, and plating for colony formation; (3) elutriating to obtain G1 cells, allowing them to progress, then irradiating in S or G2 and plating for colony formation. After doses of 600 and 900 rad given to every fraction separated by centrifugal elutriation from an asynchronous population of 9L cells, a very small but significant increase in resistance (∼1.4) was found in late G1 cells. The survival curve obtained with these resistant cells after elutriation showed a significant (P < 0.05) increase in the value of D0. The absence of any substantial change in radiosensitivity throughout the cell cycle makes it unlikely that the BCNU-radiation interaction previously observed for 9L cells was caused by presynchronizing the cells into a radiation-sensitive phase of the cell cycle.