Theoretical comparison of two interstitial ultrasound applicators designedto induce cylindrical zones of tissue ablation

Although interstitial techniques are invasive, they are still the first-lin e therapeutic modalities for certain types of tumour. They are mainly relev ant to tumours that are either inoperable or located so deep that access is complicated. Of the various types of radiation that can be delivered by th e interstitial route, ultrasound is the most suitable for deep heating. The study compares the efficacy of two types of applicator with respect to the ir ability to induce cylindrical zones of coagulation necrosis. The transdu cer of the first applicator is tubular, whereas the second is plane and can rotate around its axis. Both have an external diameter of 4 mm, are fitted with surface cooling systems and operate at 10.7 MHz and 14 W.cm(-2). Comp arison involves mathematical modelling of ablated tissue in the targeted ar ea by resolving the bioheat transfer equation (BHTE) using an algorithm bas ed on finite differences. The BHTE gives a temperature Value from which the thermal dose can be determined. It is shown that tissue ablation by tubula r transducers is slow, and, in consequence, perfusion disturbs the heating pattern: in vivo, irradiation with a tubular transducer lasting 1081 s woul d be required to ablate a tissue mass with a radius of 8 mm. The correspond ing period using a rotating plane transducer with 20 firing angles is only 618 s. The mean exposure time of each shot lasts 37 +/- 7 s. Therefore perf usion would have much less impact in the case of therapy administered using a plane transducer than that using a tubular one.