Ablation in Hepatology
Radiofrequency Ablation (RFA)
Radiofrequency Ablation is one of several minimal invasive techniques against liver tumours. Many patients with a malignant liver tumour are not good candidates for surgery, because their tumour is too widespread or inaccessible or so much liver tissue would have to be removed with the tumour that not enough would be left to provide adequate liver function. Some liver tumours that have failed to respond to chemotherapy or have recurred after initial surgery may be treated by RFA.
A needle electrode is placed in the tumour under the guidance of an imaging method (US or CT). A radiofrequency current then is passed through the electrode to heat the tumour tissue near the needle tip and ablate it. The heat from radiofrequency energy also closes up small blood vessels, thereby minimizing the risk of bleeding. Radiofrequency Ablation may be done as an outpatient procedure without general anaesthesia. Admission to the hospital is not usually necessary.
RFA may be done by placing ablation needles through the skin; by placing needles through a tube inserted through small holes in the skin (laparoscopy); or during open surgery. Most radiologists prefer the percutaneous approach because it is much less invasive, produces few complications and requires only sedation rather than general anaesthesia. In addition it is relatively inexpensive.
Radiofrequency Ablation is most effective if the tumour or tumours are less than two inches in diameter. Although RFA can be used to treat larger tumours, the results are not as good. Typically RFA is not used to treat liver tumours if there is active cancer outside of the liver.
In most studies, more than half the liver tumours treated by radiofrequency ablation have not recurred and treatment-related serious complications are infrequent. RFA may be used repeatedly to treat recurrent liver tumours.
Radiofrequency Ablation is a minimally invasive treatment that is rapidly completed and often does not require hospital admission. Patients are usually able to resume their usual activities within a few days. In addition, radiofrequency ablation is less expensive than other treatment options.
In microwave ablation or coagulation therapy, microwaves emitted from the distal segment of a percutaneous probe vibrates and rotates molecular dipoles (water molecules) generating heat and resulting in thermal coagulation of the target tissue. The rotation follows the alternating electric field component of the ultra-high-speed (2,450-MHz) microwaves.
In laser ablation, light at optical or near-infrared wavelengths from a laser fiber scatters within tissue, is converted into heat and causes a necrosis around the fiber tip.
Two methods have been developed for producing large volumes of necrosis. The first consists of firing multiple bare fibers arrayed at 2-cm spacing throughout a target lesion. The second uses cooled-tip diffuser fibers that can deposit up to 30 W over a large surface area, thus diminishing local overheating.
Ultrasound-guided laser interstitial thermotherapy
The combination of high resolution ultrasound and laser therapy provides a minimally invasive but elaborate treatment. Sonographic imaging allowes exact placement of the laser probe and verification of thermocoagulation by a resulting hyperechogenic signal enhancement.
Cryoablation is a method of in situ tumour ablation in which subfreezing temperatures are delivered through penetrating or surface probes in which a cryogen is circulated. Irreversible tissue destruction occurs at temperatures below -20°C to -30°C. Cryolesions as large as 6-8 cm in diameter can be created safely.
Ethanol ablation causes a dehydration of the cytoplasm within neoplastic cells and subsequent coagulation necrosis, followed by fibrous reaction. Within neoplastic vessels, ethanol causes thrombosis and tissue ischemia. The size and shape of the induced necrosis depends on histologic characteristics, degree of vascularization, presence of capsule or septa, and tissue consistency. Hepatocellular carcinoma is the most responsive tumour.
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