Absence of synergistic enhancement of non-thermal effects of ultrasound on cell killing induced by ionizing radiation

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 degrees 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 degrees 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.