Objective:To investigate the effect of pulsed high intensity focused ultrasound(PHIFU)combined with microbubbles(MBs)on apoptosis of VX2 tumors implanted in rabbits’ livers thnough non thermal conductivity. Methods:Liver VX2 tumor models were established in 36 rabbits

which were divided randomly into three groups:control group

PHIFU groups and PHIFU+MBs groups. PHIFU parameters used were as follows:ultrasound frequency 0.87 MHz

ISP(spatial peak intensity)5 900 W/cm2

pulse repetition frequency100 Hz

duty cycle 15%

and expose time 90 s. The rabbits were sacrificed one day after ultrasound exposure and the tumor tissues were taken. Apoptosis in situ was detected by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling(TUNEL);proliferating cell nuclear antigen expression(PCNA)was examined by immunohistochemical staining. The remaining tumor tissues were sent for 2

3

5-triphenyltetrazolium chloride(TTC)staining. Results:After HIFU exposure

liver tumors of all the three groups were uniformly stained red by TTC staining

suggesting that none of the animals had coagulated necrotic changes. TUNEL method detected a few positive staining tumor cells in control group;but in PHIFU group and PHIFU+MBs group

there were a lot of brow-colored positive apoptotic cells. PCNA detection showed more brow-colored positive cells in control group than in PHIFU group and PHIFU+MBs group. Apoptosis index(AI)of the tumor cells in the PHIFU group and PHIFU+MBs group was higher than that in the control group

while proliferating index(PI)of the tumor cells in PHIFU group and PHIFU+UCA group was lower than that in control group(P=0.000). AI of the tumor cells was the highest and PI was the lowest in PHIFU+UCA group(P=0.000). Conclusion:The non-thermal effects of PHIFU combined with MBs can promote apoptosis of VX2 tumors implanted in rabbits’ livers.