Thermal enhancement of radiation damage in previously irradiated skin.

The responses of the mouse ear to hyperthermia alone, X rays alone or X rays combined with heat were investigated at various times up to one year after prior irradiation. The initial X-ray treatments, either a single dose of 17 Gy or 34 Gy given as 10 fractions in 11 days, caused the same skin reactions (a small area of moist desquamation in a few ears) and had the same effects on the responses to subsequent treatments. The response to heat alone was increased at 3-12 months after prior irradiation, so that the heating time at 43.5 degrees C required to cause necrosis was about 70% of that in age-matched controls. The responses to X rays alone were also increased. Prior irradiation, however, had different effects on the susceptibilities to develop acute radiodermatitis and late deformity. For acute radiodermatitis, the X-ray dose required to obtain a given response in previously irradiated ears was about 90% of the dose required in age-matched controls. For deformity at 6 months, the dose was about 70%. Prior irradiation had the same effects on the responses to X rays given 6 min before mild hyperthermia (43.5 degrees C, 15 min) as on those to X rays alone. As a consequence, the thermal enhancement ratio (TER), i.e., the dose of X rays required alone to cause an effect divided by that required with heat, did not depend on previous irradiation. The TER was 1.4 for both acute radiodermatitis and late deformity.