Effect of Step-down Heating on the Interaction between Heat and Radiation in a C3H Mammary Carcinoma in Vivo

Summary

The effect of step-down heating (SDH) on the interaction between heat and radiation was investigated in a C3H mammary carcinoma in vivo. SDH consisted of an initial sensitizing treatment (ST) performed at 44·5°C or 43·5°C followed by a lower temperature test treatment (TT) in the range 41·0–43·0°C. Step-up heating (SUH), i.e. TT followed by ST, and single heating were used as controls. The end-point was the radiation dose needed to control 50% of the tumours (TCD₅₀). The results were evaluated by calculating the thermal enhancement ratio (TER) defined as TER = TCD₅₀ (radiation alone)/TCD₅₀ (radiation and heat). For a simultaneous application of TT and radiation a significant enhancement of direct heat radiosensitization was observed with increasing ST time or ST temperature using SDH. In contrast, only a minor increase was seen with SUH. A comparison between TCD₅₀ values for the corresponding SUH and SDH schedules revealed that the SDH effect was largest at 41·0–42·0°C and decreased with increasing TT temperature. The radiosensitizing effect of SDH also decreased if an interval was allowed between ST and TT or between TT and radiation. However, as a result of an increased cytotoxicity towards hypoxic tumour cells, the TCD₅₀ value for SDH remained significantly smaller than for SUH, even with a sequential combination of radiation and heat.