Differential kinetics of thermal resistance (thermotolerance) between murine normal and tumor tissues

The kinetics of thermotolerance were studied in mouse normal and tumor tissues. Early generation isotransplants of a spontaneous fibrosarcoma, FSa-II, were used. Tumor cell suspension was transplanted into the C3Hf/Sed mouse foot. Hyperthermia was given by immersing animal feet into a water bath. The TG (tumor growth) time, the time required for half the treated tumors to reach 1000 mm3 from the first treatment day, was the end point. Foot reaction was studied as a normal tissue response. The RD50, the treatment time that induces a loss of one toe or greater reaction in half the treated animals, was analyzed. Thermotolerance developed rapidly and extensively in normal and tumor tissues. A significant difference was observed in the decay of thermotolerance between these tissues. The decay of thermal resistance was incomplete in the murine foot tissue, even at 14 days after the initial heat treatment. Although some experimental difficulties were involved in the tumor study, present results suggest a complete decay of thermotolerance in the FSa-II tumor in 8 days after a 7.5 min treatment of 45.5 degrees C. Thermal resistance again developed in both tissues following the second heat treatment, which was given 7 or 8 days after the first heat treatment. The development, maximum magnitude and decay of the second thermal resistance was comparable to those of the first thermal resistance in each tissue.