Hyperthermic sensitization of mouse intestine to damage by X rays: the effect of sequence and temporal separation of the two treatments.

Adult mice were irradiated with between 7 and 15 Gy of X rays to the abdomen either before or after immersion of the lower half of the body in water at various temperatures. The thermal enhancement of X-irradiation damage to the intestine was estimated using survival at five days as the endpoint. Thermal enhancement ratios (defined as the ratio between the dose of X rays and the dose of X rays plus heat to produce the same level of damage) were calculated at the 50% survival level. If only a few minutes separated the application of heat and irradiation, the values obtained were similar to those reported for other normal tissues. There was only a slight dependence on the sequence of the two treatments. As the time interval between the two treatments was increased the effect of heat was reduced and a normal irradiation response was obtained when the separation was greater than about four hours. The results are compared with those for other normal tissues and tumours and the relevance of these findings in the design of optimal therapeutic schedules of combined heat and X rays is discussed.