Study On Radiosensitivity To X-ray And Carbon Ion Irradiation Under Different Statuses Of Oxygenation In CHO-K1Cells

Background of Objective:Hypoxia is known to contribute to radiation resistance by directly depriving cells of oxygen and by indirectly influencing cell physiological factors. This study is to investigate the radiation sensitivity in anoxic, hypoxic and reoxygenated cells to X-rays and carbon ion in CHO-K1cells, and the possibility of reducing this radioresistance by carbon ion irradiation.Material and Methods:The Chinese hamster ovary cell line (CH0-K1) was used for this stuy, and was investigated under different statuses oxygen, including oxia (air), hypoxia (0.5%O2,94.5%N2and5%CO2) or anoxia (95%N2;5%CO2). To achieve a defined acutely anoxic or hypoxic condition, five specially chambers for producing different oxygen statuses were designed. The chambers were connected in parallel to the gas supply and then gassed for2hours at200ml/min with the corresponding mixed gas (95%N2plus5%C02, or0.5%02and94.5%N2plus5%CO2). For experiments under chronic conditions, cells were kept for24hours under hypoxia or anoxia condition. For the measurements of reoxygenation, the cells were exposed to air again for additional periods of time. Irradiation was done using250kVp x-ray and103keV/μm carbon-ions (average LET) with a1cm of spreading-out-Bragg peak. Cell survival was measured by a colony forming assay, and oxygengen enhancement ratio (OER) and relative bioloigical effect (RBE) were calculated. Cell cycle distribution was analysed by FCM under anoxia, hypoxia and after reoxygenation from chronic anoxia and hypoxia. The counterpart studies were done in oxic cells as the control. Results:From survival curves under different oxygenation statuses, we found that CHO cells had a similar survival under both acute and chronic hypoxia. In contrast, survival after irradiation under chronic anoxia was slightly reduced compared to that under the acute anoxia. A similar reduction of survival could be seen for cells irradiated within1hour after reoxygenation from chronic anoxia compared to that for oxic cells. This was true for x-ray as well as for carbon irradiation. The measurement of cell cycle distribution showed a fairly uniform distribution of cell cycle under hypoxia, and a block in G1and a nearly complete depletion of S-phase cells directly after chronic anoxia. The distribution after chronic anoxia was changed rapidly in the first12hours, which was comparable to that in oxic cells after15hours. Cell survival measured15hours after reoxygenation showed decreased sensitivity compared to reoxygenated cells of0and1hour after reoxygenation. For chronic hypoxia, a fairly uniform distribution of cell cycle under reoxygenation resulted in similar survival for1hour and24hours of reoxygenation. Conclusion:The study illustrated that for measurement in cell cultures, changes in survival and OER after chronic oxygen depletion were due to the changes in cell cycle distribution. The OER from carbon ion irradiation under different oxygenation statues was significantly decreased, which also confirmed the effectiveness of high-LET particles in cell inactivation, especially for hypoxic cells. The influence of the cell cycle on radiosensitivity for carbon ion was not found compared to that in X-ray irradiation.