Radiation Survival of Cells from Spheroids Grown in Different Oxygen Concentrations

Abstract

The position of the internal, chronically hypoxic Chinese hamster V79-1716 lung fibroblast cells in spheroids was varied by alterations in the O2 concentration in the growth medium. Such alterations were expected to cause large changes in the size of the radiobiologically hypoxic fraction. This was tested by growing and irradiating spheroids in O2 concentrations between 5 and 20.3%, ensuring that the irradiation and growth conditions were similar. The survival curves appeared linear below a surviving fraction of 3 .times. 10-2, and the slopes were intermediate between the slopes of control curves for cells from spheroids irradiated in N2 or fully oxygenated. Direct estimates of the hypoxic fractions could not be made. Two models of O2 diffusion might explain the data. Model 1 assumes that a large fraction of cells was fully hypoxic (radiobiologically) and that the internal, G1-confined, chronically hypoxic cells had a lower inherent radioresistance than the outer proliferating cells. Model 2 assumes that the inherent radioresistance was equal throughout the spheroid, and that the innermost cells died before the O2 concentration was reduced sufficiently to cause full hypoxic protection. Theoretical survival curves based on model 2 were generated using the measured geometries of the spheroids and multitarget single-hit survival theory. Acceptable agreement with the postulate that the innermost cells of spheroids die between 0.2-0.4% O2 was obtained. Implications rgarding the relative contributions of chronic and acute hypoxia to the fraction of hypoxic cells in tumors were suggested.