RADIATION-INDUCED DNA-DAMAGE IN TUMORS AND NORMAL-TISSUES .3. OXYGEN DEPENDENCE OF THE FORMATION OF STRAND BREAKS AND DNA-PROTEIN CROSS-LINKS

Results from several laboratories, including ours, have suggested that measurements of radiation-induced DNA strand breaks and DNA-protein c rosslinks (DPCs) may be used to estimate the hypoxic fraction or fract ional hypoxic volume of tumors and normal tissues. This suggestion has been predicated on both published and unpublished information that (1 ) the oxygen dependence of the formation of strand breaks in irradiate d mammalian cells is similar to the oxygen dependence of radiation-ind uced cell killing, and (2) the oxygen dependence of the formation of D PCs in irradiated mammalian cells is the mirror image of the oxygen de pendence of radiation-induced cell killing, However, the published stu dies that attempted to determine the relationship between the oxygen d ependence of the formation of strand breaks and the radiation sensitiv ity of mammalian cells were not performed at 37 degrees C, the exact o xygen concentrations were not always known, and the results were confl icting. In addition, most of the data on the oxygen dependence of the formation of DPCs are unpublished, Consequently, we have undertaken a comprehensive investigation of one cell line, 9L/Ro rat brain tumor ce lls, to determine if the shape of the oxygen dependence curve and the K-m value for radiation-induced strand breaks and DPCs were similar wh en 9L cells were irradiated under both ideal gas-liquid equilibrium co nditions at 4 degrees C and nonideal gas-liquid equilibrium conditions at 37 degrees C. At 4 degrees C under ideal gas-liquid equilibrium co nditions, the K, for the formation of strand breaks was approximately 0.0045 mM, and the K-m for radiation sensitivity was approximately 0.0 05 mM. A similar comparison for the formation of DPCs at 4 degrees C c ould not be made, because the efficiency of the formation of DPCs was much lower at 4 degrees C than at 37 degrees C. At 37 degrees C under nonideal gas-liquid equilibrium conditions, the apparent K-m for the f ormation of strand breaks and radiation sensitivity was approximately 0.032 mM, and the K-m for the formation of DPCs was approximately 0.02 mM. The data for strand breaks are in agreement with the published da ta of Chapman et al. (Int. J. Radiat. Biol. 26, 383-389, 1974), and th e data for DPCs are in agreement with the unpublished data of Meyn (pe rsonal communication). These results support the suggestion that measu rements of radiation-induced strand breaks and/or DPCs may be used to detect hypoxic cells and estimate the hypoxic fraction or fractional h ypoxic volume of tumors and normal tissues. (C) 1995 by Radiation Rese arch Society