Influence of thermotolerance on the interaction between hyperthermia and radiation in L1A2 cells in vitro.

The influence of a prior heating at 42 degrees C for 90 min on the response of L1A2 cells to a subsequent, simultaneous treatment with hyperthermia and radiation was investigated. Resistance to heat alone at 42 degrees C (thermotolerance) developed 4 hours after preheating and reached its maximum at a 10-hour post-heating interval. Without prior heating, maximal thermal radiosensitization occurred with simultaneous treatment (X-rays in mid heat treatment) and irrespective of the sequence, this radiosensitization was completely lost at an interval of 4 hours. If administered up to 24 hours prior to either radiation alone or simultaneous treatment, preheating caused a reduction in the shoulders of the radiation survival curves. In addition, 10 to 16 hours after preheating, the cellular sensitivity to radiation alone was slightly decreased by a factor of 1 . 2-1 . 3. However, since such reduced radiosensitivity also resulted from simultaneous treatment given 10 and 16 hours after preheating, the thermal enhancement ratio was not affected significantly by preheating. Thus, prior heating apparently did not influence the thermal radiosensitization observed with simultaneous heat and radiation.