Continuous-wave ultrasound and neoplastic transformation in vitro.

C3H/10T1/2 cells in suspension were assayed using an initiation-promotion protocol for neoplastic transformation induced by continuous-wave ultrasound. Cells were insonated at 1.765 MHz for 40 min. Two ultrasonic intensities were used: 1.3 and 2.6 W/cm2 spatial average. The first intensity was found to be noncytotoxic; the second resulted in immediate lysis of 20% of the cells, followed by the clonogenic survival of 64% of the remaining cells. Ultrasound was delivered alone or in combination with X-rays (2 Gy, 240 kVp given before ultrasound), and/or 12-O-tetradecanoyl-phorbol-13-acetate (TPA, 0.1 microgram/ml post-irradiation). Under all treatment conditions, there was no effect of ultrasound on transformation at the 95% confidence level.