The response of the mouse ear to heat applied alone or combined with X-rays

The effects of heat either alone of combined with X-irradiation were investigated in the skin of the mouse ear. Ears were heated for times ranging from 7.5 minutes to four hours in a water-bath at temperatures ranging from 390C to 460C. Severe heat treatment caused necrosis during the first few days after heating. Mild hyperthermia, which itself caused early transient reddening but no further visible changes, when given immediately after X-rays enhanced the radiodermatitis which was observed during 10 to 50 days after treatment. From a level of heating which was slightly less than that required to produce necrosis in any mice, an increase in temperature of 0.50C caused an increase in the incidence of necrosis from 0 to 100%. For hyperthermia which enhanced radiodermatitis, an increase of 0.50C was equivalent to an increase in X-ray dose of 10 to 15%. For heat alone, the time required to produce necrosis in half the treated mice was reduced by one half for a rise in temperature of 10C. For heat combined with X-rays, the time required to produce a given enhancement of radiodermatitis was reduced by one third for a rise in temperature of 10C. Activation energies calculated using the Arrhenius equation were 580kJ/mole for necrosis after heat alone and 1030kJ/mole for thermal enhancement of radiodermatitis. These results suggest that different processes are involved in direct heat necrosis and thermal enhancement of radiodermatitis. (author)