The relationship between heating time and temperature for inhibition of growth in baby rat cartilage by combined hyperthermia and X-rays.

The relationship between the Thermal Enhancement Ratio (TER) for X-ray damage and time of heating has been investigated in epiphyseal rat cartilage. The TER at each temperature rises steeply with increasing heating time. Data obtained using various heat treatments with 8 Gy of X-rays have been analysed in terms of stunting "rate' as measured by the slope of the dose-effect curve obtained for each temperature. The "rate' of stunting per unit heating time, induced by thermally enhanced X-ray damage is compared with the "rate' of stunting induced by heat alone. The two are similar each having an activation energy of approximately 550kJ mole-1, as determined using the Arrhenius equation. Halving the heating time requires at 1 degrees C temperature increase to achieve the same degree of thermal enhancement of X-ray damage. Similar results have been reported previously for damage caused by heat alone. Over a range 42 degrees C-45 degrees C, the threshold heating time to cause direct thermal injury falls within the range of times used to enhance X-ray damage. It is suggested that a component of damage due to direct thermal injury, indistinguishable from radiation damage and thermally enhanced radiation damage, will contribute to TER assessments in some experimental systems.