Thermal radiosensitization in CHO cells by prior heating at 41-46 degrees C.

CHO cells were exposed to heat at temperatures ranging from 41 degrees C to 46 degrees 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) = alpha D + beta D2; the values of alpha and beta obtained from curve-fitting were found to increase with heating time. For temperatures below 43 degrees C the relative increase in alpha was greater than that in beta; the Arrhenius activation energies were Ea = 890 kJ mol-1 for alpha and Ea = 1830 kJ mol-1 for beta. At temperatures exceeding 43 degrees C the relative increase of alpha and beta was similar and the corresponding activation energies were about the same (Ea approximately 700 kJ mol-1). The increase in the alpha-term was attributed to a depressed repair of double-strand breaks, whereas the increase of beta was assumed to be a consequence of an insufficient repair of base damage.