Influence of Thermotolerance on the Interaction between Hyperthermia and Radiation in L1A2 Cellsin Vitro

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Abstract
The influence of a prior heating at 42.degree. 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.degree. C (thermotolerance) developed 4 h after preheating and reached its maximum at a 10-h postheating 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 h. If administered up to 24 h prior to either radiation alone or simultaneous treatment, preheating caused a reduction in the shoulders of the radiation survival curves. In addition, 10-16 h after preheating, the cellular sensitivity to radiation alone was slightly decreased by a factor of 1.2-1.3. Since such reduced radiosensitivity also resulted from simultaneous treatment given 10 and 16 h after preheating the thermal enhancement ratio was not affected significantly by preheating. Prior heating apparently did not influence the thermal radiosensitization observed with simultaneous heat and radiation.