Thermal ablation of liver tumors by high-intensity-focused ultrasound using a toroid transducer. Results of animal experiments

The aim of this study was to demonstrate in a rabbit liver tumor model that high intensity focused ultrasound (HIFU) produced with toroidal-shaped emitters may have a role in treating colorectal liver metastases. The HIFU device was composed of eight ultrasound emitters created by sectioning a single toroidal piezocomposite transducer. Each of the eight emitters was divided into 32 transducers operating at a frequency of 3 MHz. The toroidal transducer has a diameter of 70 mm and a radius of curvature of 70 mm. A 7.5 MHz ultrasound imaging probe (Vermon, Tours, France) was placed in the centre of the device. Using this transducer single lesions of 7 cm
³
were created in 40 seconds. Juxtaposition of single lesions was performed under ultrasound guidance. VX2 tumor segments (25 mg) were implanted into right lateral liver lobes of 45 New Zealand white rabbits. Fifteen rabbits were treated with toroidal HIFU ablation (Group 1). Fifteen rabbits were resected (Group 2). Fifteen rabbits were not treated and formed a control group (Group 3). Group 1 and 3 were compared to evaluate treatment efficacy. Group 1 and 2 were compared to evaluate if the toroidal HIFU treatment increases the risk of tumor dissemination. Total hepatectomy took place 11 days after treatment. The therapeutic response was evaluated with follow-up ultrasound imaging and the corresponding gross pathology and histology. HIFU ablation produced using the toroidal transducer allowed fast and homogeneous tumor treatments. Ablations were visible on sonograms. The VX2 tumors were completely coagulated and were surrounded by ablated liver tissue without secondary thermal lesions in surrounding organs. In the control group tumor volume was 225% higher at the time of autopsy when compared to the volume at the day of the treatment. Tumor dissemination was lower in the HIFU group (25%) compared with resected (67%) and control (38%) groups. Findings of ultrasound imaging, gross pathology and histology supported these outcomes. Successful rabbit liver tumor ablation can be achieved using a toroidal HIFU transducer under ultrasound imaging guidance and therefore could be an effective treatment of localized tumors. There was no complication related to the HIFU procedure. Its clinical usefulness has to be further proven.