Absence of Synergistic Enhancement of Non-thermal Effects of Ultrasound on Cell Killing Induced by Ionizing Radiation

Summary

The present study was performed to elucidate the role of non-thermal effects (cavitation and direct effects) of ultrasound, in simultaneous combination with X-irradiation on the cytotoxicity of mouse L cells. Firstly, mouse L cells were exposed to X-rays and ultrasound (1 MHz continuous wave, spatial peak temporal average intensity; 3·7 W/cm²) simultaneously at 37°C under O₂ or Ar saturated conditions to examine the cavitational effect of ultrasound. Secondly, cells were exposed to X-rays and ultrasound at 37°C under N₂O saturated conditions, which suppresses the cavitation, to examine the direct effects of ultrasound. The cavitational effect under O₂ and Ar saturated conditions induced an exponential decrease in cell survival, and resulted in an additive effect on cell killing with the combination of X-rays and ultrasound. The direct effect in the N₂O conditions induced no cell killing and did not modify the cell killing induced by X-rays. These results suggested that the non-thermal effects of ultrasound did not interact synergistically with X-rays for cell killing.