New applications of ultrasonography: interventional ultrasound

Abstract

Since 1975–80, worldwide but mostly in Europe and Japan, sonography has become the imaging technique of choice for guiding percutaneous interventional procedures for diagnostic or therapeutic purposes. In the last 10 years, the most important advances in interventional ultrasound have occurred in therapeutic applications, fostering and facilitating the development of the so-called minimally-invasive techniques. Among all these new applications for the treatment of fluid collections, inflammatory diseases, benign and malignant tumors, the most recent dramatic advances have occurred in primary and secondary liver malignancies, which will be the focus of this review. The rationale for the local treatment of primary and secondary liver cancers differs according to the success of conventional treatments (e.g. surgery and chemotherapy) and to the survival rates of such conditions, depending on clinical stage, patient age, associated diseases, and so on. Ultrasound-guided percutaneous therapies for liver cancer can be divided into: (1) direct intratumoral injection of chemical agents, such as ethanol, hot saline and acetic acid; and (2) thermally-mediated techniques, such as radiofrequency ablation, interstitial laser photocoagulation, microwave therapy or cryotherapy. Through different mechanisms of action, all these methods induce cell death by coagulative necrosis. The clinical efficacy of some of these techniques has been clearly established, like percutaneous ethanol injection in the treatment of hepatocellular carcinoma. In contrast, experience with the other techniques is much more preliminary, mostly due to the limited availability of sufficient clinical trials and to the fascinating speed of technological development. We review the state-of-the-art of the two most promising techniques, namely radiofrequency ablation and interstitial laser photocoagulation, and the present preliminary clinical data in the treatment of hepatocellular carcinomas and liver metastases. Radiofrequency energy is delivered to the tumors by means of electrode-needles (14–17 G) electrically insulated along all but the distal portion of the shaft and percutaneously introduced into the targets under ultrasound guidance. In the short history of this method, several technologies have been tested and used, but ‘multiple-hook’ and ‘internal-cooling’ electrodes are currently the leading choices, providing large necrosis volumes (up to 5 cm in diameter for a single 12-min application with an array of three internal-cooling electrodes) in short sessions, under local or general anesthesia. Radiofrequency ablation has proved its usefulness in the treatment of hepatocellular carcinomas, achieving complete necrosis in over 85–90% of cases in lesions smaller than 5 cm in diameter and in single sessions, with a low rate (<10%) of local recurrences. In the treatment of liver metastases, whose very poor responsiveness to percutaneous ethanol injection and other ‘chemical’ therapies is extensively reported in the literature, radiofrequency ablation has 65–75% reported complete efficacy in the control of local tumor growth in lesions not exceeding 4 cm. Such great efficacy has been so far coupled with very low rates of major side-effects (<2%), most of them requiring no surgical repair. Interstitial laser photocoagulation induces thermally-mediated coagulation through thin optic (Nd:YAG) fibers percutaneously inserted into the target under ultrasound guidance. Multiple fibers (beam splitters) are usually required to achieve a sufficiently wide necrosis volume. The reported success rate (complete necrosis and local control of tumor growth) ranges 45–75% of the lesions, mostly liver metastates from colorectal carcinoma. An equally low rate of complications is reported for interstitial laser photocoagulation as for radiofrequency ablation, that is no more than 2–3% of cases. Therapeutic interventional procedures under ultrasound guidance and using other types of energy (microwaves, focused ultrasound, cryotherapy and so on) are currently on trial and, to date, do not seem to be as easily employable or to achieve as large necrosis volumes in short treatment sessions as radiofrequency ablation and interstitial laser photocoagulation.