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

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Abstract
Survival curves of 9L rat brain tumor cells separated by centrifugal elutriation in early G1 (.gtoreq. 97%), mid-S (.gtoreq. 80%) and mid-G2 (.gtoreq. 70%) phases were measured after 0-1800 rad of 137Cs .gamma. rays. The Do [median lethal dose], Dq [quasi-threshold dose] and n values calculated from the cell survival curves at early G1, mid-S and mid-G2 were not significantly different (P > 0.1). Identical results were found with the following 3 different elutriation-irradiation procedures: elutriating, irradiation and plating for colony formation; irradiating, elutriating and plating for colony formation; 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 (.apprx. 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 Do. The absence of any substantial change in radiosensitivity throughout the cell cycle makes it unlikely that the [antineoplastic drug] BCNU[1,3-bis-(2-chloroethyl)-1-nitrosourea]-radiation interaction previously observed for 9L cells was caused by presynchronizing the cells into a radiation-sensitive phase of the cell cycle.

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