Oxygen equilibration problems in the determination of K for HeLa S3 (OXF)

HeLa S 3 (OXF) cells attached to polystyrene Petri dishes could not be used to investigate radiosensitivity at low oxygen concentrations because of the oxygen dissolved in the polystyrene matrix. An estimation of K in the formula R 1 = S max /S = M([O] + K)/M[O] + K was made for mammalian cells attached to glass Petri dishes, but it was dependent on various factors, the most important being the depth of medium over the cells at the time of irradiation. Oxygen equilibration was via diffusion, therefore a correction for the oxygen concentration differences between the gas phase and the cells was made; a K value of 12 w moles/l was obtained which is appreciably higher than other reported values. Although an estimate of the errors on this value was not made, all measurements fell within the range 8·5 and 17·5 w moles/l. These results highlight the difficulties of making precise radiobiological observations at low oxygen concentrations for mammalian cells attached to Petri dishes.