Development of an interstitial ultrasound applicator for endoscopic procedures: animal experimentation

Biliary cancer is very difficult to treat, mainly because of the advanced stage at which such tumours are detected and the low efficacy of systemic therapeutic modalities like radiotherapy. Palliative measures designed to clear the duct (either by means of surgery or an endoscopic procedure) are presently performed. A relatively noninvasive alternative could be developed to fill this gap in the therapeutic arsenal. To this end, we have designed an interstitial ultrasound (US) applicator suitable for use with a digestive endoscope. This applicator is based on a water-cooled plane transducer that operates at 10 MHz. Although, because the target zone is cylindrical in shape, it might have seemed more logical to use a cylindrical transducer. Nevertheless, a plane transducer was chosen because the pressure field from this kind of emitter decreases less quickly, which means faster and deeper heating. However, to generate coagulation necrosis all around the duct, the applicator has to be rotated around its axis; this is achieved by means of a flexible metallic shaft (2 m in length and 3. 8 mm in diameter) that joins the device's active head (which contains the transducer) to the casing with all the connectors. A holder is fixed at the endoscope channel inlet; it controls the rotation of the applicator. Trials were conducted on pigs. The duodenoscope was introduced via the oesophagus down through the duodenum as far as the hepatopancreatic ampulla. Using a guide wire, the applicator was navigated into the duct via the endoscope instrument channel. Well defined, reproducible volumes of coagulation necrosis with a diameter of 20 mm were generated in the biliary tissue and the liver. These promising results indicate that this kind of endoscopic US delivery system may represent an effective tool for the treatment of biliary tumours in humans. An Independent Ethics Committee recently approved preliminary clinical trials of this applicator.