The effect of ultrasound-induced hyperthermia and X irradiation on skin.

Local hyperthermia for up to 60 min at 42.5-43.0 degrees C was induced in the skin of the mouse leg using ultrasound (780 kHz, 0.5-2.0 W/cm2). X radiation at doses of 10 to 30 Gy was delivered either before, during, or after the hyperthermia and the skin reactions were followed for 50 days. The thermal enhancement factor (TEF) was estimated using three criteria: (1) the maximum skin reaction; (2) the skin reaction integrated over the 50-day experimental period; (3) the skin reaction integrated over Days 8 to 32, which have a larger estimate. The TEF was independent of the sequence of heat and X radiation for intervals up to 1 hr. For 1 hr of hyperthermia, the TEF as measured by the maximum skin reaction did not change with radiation doses of 10 to 20 Gy. When the skin reaction integrated over 50 days or Days 8 to 32 was used as the criterion of response, the TEF varied with radiation dose from 4.7 at 10 Gy to 1.6 at 20 Gy. For a fixed radiation dose of 20 Gy, the TEF was not increased significantly by extending the duration of the hyperthermia from 30 to 60 min. The TEF for a radiation dose of 20 Gy delivered in three fractions over 5 days was smaller than that for a single 10-Gy fraction.