Extracorporeally induced ablation of renal tissue by high-intensity focused ultrasound

OBJECTIVE: To investigate the safety and the effects on healthy renal tissue of high-intensity focused ultrasound (HIFU) applied extracorporeally. PATIENTS, MATERIALS AND METHODS: Ultrasound waves (1.04 MHz) created by a cylindrical piezo-ceramic element were focused by a parabolic reflector to a physical focus size of 32 x 4 mm (-6 dB). For an in vivo study, HIFU was applied to the healthy tissue of 24 kidneys, monitored by ultrasonography, with a maximum power of 400 W and a spatially averaged intensity (ISAL) in the focus of 1192 W/cm(2). Fourteen kidneys were removed immediately after ablation to evaluate the side-effects and the effects in the focal zone, and 10 kidneys were removed delayed after 1, 7 and 10 days. The clinical study consisted of 19 patients requiring radical nephrectomy for a renal tumour. HIFU was applied to the healthy tissue of 19 kidneys (up to 1600 W, I(SAL) = 4768 W/cm(2)) before proceeding with the radical nephrectomy. RESULTS: There were no major complications after applying HIFU to the 43 kidneys. Side-effects included skin burns (grade 3) in two patients. During the follow-up there were no further HIFU-specific side-effects. In one case (in vivo study) there was a thermal lesion of the small intestine, which was due to mis-focusing. HIFU effects in the focal zone immediately after application were: interstitial haemorrhages, fibre rupture, shrinking of the collagen fibres, and coagulation necrosis. These effects occurred sporadically, and their number and size did not correspond to the number of HIFU pulses applied. After 7 and 10 days, there was a well-demarcated coagulation necrosis in vivo. CONCLUSION: Using this device, extracorporeally applied HIFU can ablate healthy kidney tissue in vivo in combination with diagnostic online ultrasonography. The technique is safe and resulted only in minor complications (skin burns). Refinements in the technology are essential to establish HIFU as a noninvasive treatment option that allows complete and reliable tissue ablation.