[Experimental studies on thermotolerance in hyperthermia treatment of cancer]

Thermotolerance was investigated in hyperthermia using FM3A cells in vitro and in vivo. FM3A cells were heated at 42.0 degrees C, 43.0 degrees C and 44.0 degrees C and the survival rate of the cells was decreased in this order. In in vitro experiments, thermotolerance induced by heating at 43.0 degrees C for 30 min reached at maximum, when the heating interval was 12 hr and thermotolerance induced by heating at 44.0 degrees C for 30 min reached at maximum, when the heating interval was 12 to 24 hr. In in vivo experiments, thermotolerance induced by heating at 42.0 degrees C, 43.0 degrees C and 44.0 degrees C continued for 48 hr after initial heating and disappeared after 96 hr. In the repeated hyperthermia experiments, 7 times with 24 hr interval heating or 4 times with 48 hr interval heating did not show the significant inhibition of tumor growth as compared with the control group at 42.0 degrees C, 43.0 degrees C and 44.0 degrees C. However, twice with 96 hr interval heating indicated the significant inhibition of tumor growth at 42.0 degrees C, 43.0 degrees C and 44.0 degrees C. From these results it is suggested that hyperthermia treatment should be repeated after the disappearance of thermotolerance.