Effect of thermotolerance and step-down heating on thermal radiosensitization in CHO cells

Chinese hamster ovary cells were exposed to single or fractionated heat treatments followed by irradiation on ice with graded doses of X-rays. The dose-response curves obtained were fitted by the linear-quadratic equation -ln(S/S0) = alpha D + beta D2 and analysed in terms of TER10%, alpha and beta. Thermal enhancement ratio, TER10%, was reduced when heat sensitivity was lowered either by chronic (pretreatment 40 degrees C, 16 h) or acute (43 degrees C, 45 min-37 degrees C, 10 h) thermotolerance, but was enhanced after step-down heating (43-40 degrees C or 45-40 degrees C). It could be shown that thermal radiosensitization, as expressed by TER10%, is modified by thermotolerance or step-down heating only to the extent to which cellular survival is modified by the corresponding pretreatments. However, the relative change of alpha and beta was found to be different for thermotolerance and step-down heating. For thermotolerant cells the values for alpha and beta were reduced by about the same factor, whereas step-down heating caused an increase in both parameters, which was greater for alpha than for beta. Data analysis showed that the modification of thermal radiosensitization by thermotolerance can be interpreted as if the cells were heated at the given temperature for a shorter time, whereas after step-down heating the cells responded as if they were exposed to a higher temperature prior to irradiation.