The response of six mouse tumours to combined heat and X rays: implications for therapy

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Abstract
The response of 6 types of mouse tumor [lines of spontaneous origin] to single doses of X-rays alone or to X-rays in combination with moderate hyperthermia (42.5.degree. C/60 min) was assessed using delay in tumor regrowth. Thermal sensitization was observed in 5 of the 6 tumors. The degree of sensitization varied with the size of the X-ray dose, being larger at higher doses. The degree of sensitization also depended on the sequence and separation of the heat and irradiation. The thermal sensitization has been measured in terms of the X-ray doses to produce the same level of tumor damage with or without heat, i.e. thermal enhancement ratios. These TER [treatment effectiveness ratio] values, measured for X-ray doses in excess of 20 Gy [gray], are not greater in any of the tumors than in a range of normal tissues, if the X-rays and heat are given in close succession. Separation of the heat and X-rays reduces the TER values slightly, but some effect is still apparent at 3-24 h. In normal tissues the effect of heat is totally lost within 4 h. Comparison of these tumor data with published normal tissue data indicates a therapeutic advantage if the heat and X-rays are separated by more than 1 h. This therapeutic gain is most reliably achieved with heat given after irradiation.