Objective   To investigate the effect of ultrasound irradiation at different low doses on the curative effect of high intensity focus ultrasound (HIFU) on rabbit VX2 liver tumor.    Methods   Forty-five rabbits bearing VX2 tumors at a diameter of 1.2 to 2.2 cm were randomly and equally divided into 3 groups. Group A received only HIFU irradiation (180 W) without firstly low-dose irradiation. Group B and group C both received firstly low-dose irradiation (80 and 100 W respectively) and then received HIFU ablation (180 W) in 24 h later. The tumor mass was observed with light and electron microscopy before and after ablation. TCC staining was used to evaluate the volume of coagulation necrosis and to calculate the energy efficiency factor (EEF).    Results   Group B spent lesser time than group A for the same volume of coagulation necrosis (P0.05)

but group B and C had no significant difference in the time for coagulation necrosis though group C had a larger volume of coagulation necrosis (P0.05). Compared the 3 groups

the EEF of group A (46.43±13.67) was maximal (P0.05)

and that of group C (7.01±1.33) was minimal (P0.05).    Conclusion   The greater power of the first irradiation is

the more obvious efficiency will be achieved. However

it is not wise to blindly increase the power of the first irradiation in oreder to guarantee the safety in clinical HIFU therapy.