| Abstract: | The effect of step-down heating (SDH), consisting of an initial sensitizing treatment (ST) performed at either 44·5°C or 43·5°C followed by a lower temperature test treatment (TT), was investigated in a C3H mammary carcinoma in vivo. A linear relationship between heating time and tumour growth delay was observed for all temperature combinations applied. At a given TT temperature, SDH increased the slope of the dose-response curve compared to the curve for tumours, single-heated without an initial ST. The slope of the SDH curves increased asymptotically towards a plateau value as the ST time at 44–5°C was increased. The time-temperature relationship for single heating was described by a biphasic Arrhenius curve with activation energies of 1361±34 and 666±54 kJ/mol below and above an inflection point at 42·5°C, respectively. For SDH, the Arrhenius curve gradually became straight with increasing ST time, and the activation energy saturated at a value of 425 ±25 kJ/mol. The reduction of the activation energy at an ST temperature of 43·5°C was due rather to the extent of ST heat damage than to the ST time or temperature used. These results may be relevant for calculations of thermal doses, since even a short temperature peak (e.g. 44·5°C/5 min) significantly changed the time-temperature relationship. |
