Effect of Thermotolerance and Step-down Heating on Thermal Radiosensitization in CHO Cells

Abstract

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/S_o) = αD + βD₂ and analysed in terms of TER_10%, α and β. Thermal enhancement ratio, TER_10%, was reduced when heat sensitivity was lowered either by chronic (pretreatment 40°C, 16h) or acute (43°C, 45 min–37°C, 10h) thermotolerance, but was enhanced after step-down heating (43–40°C or 45–40°C). It could be shown that thermal radiosensitization, as expressed by TER_10%, 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 α and β was found to be different for thermotolerance and step-down heating. For thermotolerant cells the values for α and β were reduced by about the same factor, whereas step-down heating caused an increase in both parameters, which was greater for α than for β. 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.