Thermal Radiosensitization in CHO Cells by Prior Heating at 41–46°C

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Abstract
CHO cells were exposed to heat at temperatures ranging from 41°C to 46°C followed by irradiation on ice with graded doses of X-rays. The dose–response curves obtained were analysed in terms of D10%, D0 and Dq and thermal enhancement was expressed by the corresponding values TER10%, TEF and TEQ, respectively. TER10% and TEF were shown to increase linearly with heating time, the increase being steeper at higher temperatures. The dose–response curves were also analysed using the equation − ln(S/S0) = αD + βD2; the values of α and β obtained from curve-fitting were found to increase with heating time. For temperatures below 43°C the relative increase in α was greater than that in β; the Arrhenius activation energies were Ea = 890 kJ mol−1 for α and Ea = 1830 kJ mol−1 for β. At temperatures exceeding 43°C the relative increase of α and β was similar and the corresponding activation energies were about the same (Ea ≈ 700 kJ mol−1). The increase in the α-term was attributed to a depressed repair of double-strand breaks, whereas the increase of β was assumed to be a consequence of an insufficient repair of base damage.