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Ablative treatment of liver tumors: Overview
Image-guided ablation techniques have evolved considerably during the past 25 years and are increasingly used in the definitive treatment of small primary and secondary liver tumors. Image-guided ablation is recommended as the best therapeutic choice for patients with early-stage hepatocellular carcinoma (HCC)—defined as either a single tumor smaller than 5 cm or as many as three nodules smaller than 3 cm—when surgical options are precluded and is also offered as a first-line therapy, instead of surgery, for patients with very-early-stage tumors smaller than 2 cm (see Chapter 91 ). In addition, image-guided ablation is used to treat limited unresectable hepatic metastatic disease, especially from colorectal cancer.
Image-guided ablation must seek to provide predictable and contiguous cell-lethal ablation zones with a clinically effective, global ablation margin in all three planes, also termed A0 and broadly defined as tumor-free margin of 5 to 10 mm ( Fig. 96A.1 ). Volumetric assessment of the safety margin and image fusion of pre- and post-interventional scans can be used to evaluate local treatment success. When considering tumor ablation, the disease must be targetable, the treatment should be controlled and scalable, unnecessary collateral injury should be avoided, and, because ablation is an in-situ therapy, the lesion should be amenable to noninvasive verification of treatment response. Many interventional and imaging issues contribute to achieving these goals in liver tumor ablation, including ablative technologies, imaging guidance, and manipulation of the tumor setting.
Schematic model of thermal ablation.
The target diameter of the ablation zone (D*) ideally must be 2 cm larger than the diameter of the tumor that undergoes treatment (D).
Ablative technologies
Several methods for focal tumor destruction have been developed and clinically tested. Although radiofrequency ablation (RFA) has been the most popular technique to date, several alternate technologies, including thermal and nonthermal methods, have recently been adopted because they seem to overcome some of the specific limitations of RFA. ,
Radiofrequency ablation (see Chapter 96B )
Radiofrequency ablation involves the application of high-frequency (375–480 kHz) alternating current to the target tissue by using a needle-like applicator, with cell death resulting from frictional heating. Active tissue heating occurs only within a few millimeters of the exposed tines of the applicator, and larger-volume tissue destruction mainly relies on conductive heating. RFA probes are usually placed under ultrasound or computed tomography (CT) guidance either directly to or iteratively around the tumor to create a confluent ablation zone. Manufacturers of RFA instruments have adapted radiofrequency probes through internal cooling, pulsed application, and expandable multitined probes to overcome this limitation. However, the resultant ablation zone remains sensitive to related convective “heat sumping” to adjacent flowing vessels larger than 3 mm and perfusion-mediated background tissue cooling. These effects can result in compromised predictability of the ablation zone and, although RFA remains the most clinically prevalent ablation tool, this modality is increasingly superseded by other ablative technologies because of these limitations. A recent development in RFA technology is the introduction of a no-touch technique, which consists of activating, in bipolar mode, multiple electrodes inserted just beyond the tumor margins. This approach has been reported to be especially useful to treat tumors in subcapsular location.
Microwave ablation (see Chapter 96C )
Microwave ablation (MWA) achieves tissue heating through the point application of electromagnetic microwave radiation from a needle-like probe. These probes essentially contain a broadcast antenna within the “feed point,” toward the tip of the device. Most MWA devices are tuned to excite water within soft tissue by “broadcasting” at a frequency of 900 to 2,450 MHz that, by virtue of the inefficient oscillation of polarized water molecules, leads to localized tissue heating throughout an approximate 2 cm tissue sphere around the probe tip. This larger zone of active heating seems much more robust and less compromised by tissue-mediated factors than tissue heating by RFA. However, both RFA and MWA cause tissue vaporization and gas formation, which can obscure imaging of the tumor target during application of treatment. Treatment zones of 3 to 5 cm in diameter can be achieved in five to eight minutes with MWA. More predictable treatment outcomes are attained only through careful iterative or multiprobe techniques and depend on operator experience. ,
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