The effects of power on-off durations of pulsed ultrasound on the destruction of cancer cells.
OBJECTIVE
Low-intensity ultrasound irradiation is a potential method for suppressing cancer cell proliferation, inducing apoptosis and delivering specific cytotoxic genes or drugs into tumors topographically in future cancer therapies. However, ultrasound attenuates rapidly in tissue and produces heat. Pulsed ultrasound is frequently used to minimize pain and possible thermal damage to the surrounding normal tissue during therapy, since it results in smaller temperature increases. This study compared three pulsed-ultrasound strategies for destroying cancer cells, measuring their induced temperature increases to determine the optimal pulsing parameters.


METHODS
We performed three types of experiment, involving ultrasound with (1) a fixed duty cycle of 50% with variable on- and off-times, (2) a fixed off-time with variable on-times, and (3) a fixed on-time with variable off-times.


RESULTS
The results show that for different types of cultured cells (HeLa, HT-29, Ca9-22 and fibroblast) exposed to ultrasound of the same frequency (1 MHz) and energy, long pulses combined with off-times that are 5-10 times longer (on-/-off-times pairs of 5/25, 25/250, or 250/2500 ms/ms) cause significant cell destruction whilst avoiding temperature increases of more than 1.5 degrees C. Furthermore, the correlation between the temperature increase and the percentage of surviving cells is low.


CONCLUSIONS
Pulsed ultrasound with a long on-time and an even longer off-time exerts a high cytotoxic effect but a smaller temperature increase compared with non-pulsed ultrasound. This indicates that the cytotoxic effects observed in the current study were not purely due to the thermal effects of the ultrasound.
Fang
H Y
HY
Division of Medical Engineering Research, National Health Research Institute, Miaoli, Taiwan.
Tsai
K C
KC
Cheng
W H
WH
Shieh
M J
MJ
Lou
P J
PJ
Lin
W L
WL
Chen
W S
WS
D005347:Fibroblasts / Q000528:radiation effects*; D019073:HT29 Cells; D006367:HeLa Cells; D006358:Hot Temperature; D006801:Humans; D013997:Time Factors; D014407:Tumor Cells, Cultured / Q000528:radiation effects*; D014464:Ultrasonic Therapy