Tumor Ablation Using Radiofrequency Energy

Abstract

Radiofrequency ablation is used for the treatment of a variety of neoplasms including: osteoid osteoma, hepatocellular carcinoma, renal cell carcinoma, bronchopulmonary carcinoma, parathyroid adenoma;1 hepatic and retroperitoneal metastases from a variety of primary tumors. The size of the coagulation zone is a crucial factor, as only a complete coagulation of the tumor including a sufficient safety zone inhibits local recurrence. Thus many efforts have been made to enlarge the coagulation zone using multiprobe arrays, saline perfusion, internal cooling, bipolar technique, pulsed application or a combination of these mentioned techniques. Tumors up to 5 cm can now be effectively treated, taking inclusion and exclusion criteria into account. Lately published data suggests that RF ablation is far more than an electro-physical tool to generate a thermal tumor destruction, it also induces a significant activation of tumor-specific T lymphocytes.

Percutaneous, image-guided, tumor ablation using thermal energy sources such as radiofrequency (RF) have received increasing attention as promising techniques for the treatment of focal malignant diseases. Often these therapies can still be used when more invasive surgical techniques are no longer feasible due to concomitant disease or tumor localisation. Several studies showed an impressive long term survival for patients with primary and secondary malignant tumors of the liver2 comparable with the data published for surgical resection.3 Unfortunately large prospective, randomised studies are missing. Potential benefits of percutaneous tumor ablation include: decreased cost and morbidity; the possibility of performing the procedure on outpatients and, the possibility of treating patients who would not be considered candidates for surgery due to age, comorbidity or disease spread. Additionally, recent studies support the idea that RFA induces a tumorspecific T-cell activation.4

An important limitation of RF tumor ablative techniques was the extent of coagulation that could be produced with a single RF application, i.e., the tumor size which could be practically treated in a single session.5 As many tumors show an advanced size at the time of their detection either the use of multiple treatment probes, multiple treatment sessions, or both was required. A major focus of research has therefore been on the development of techniques to achieve single session, large-volume tissue necrosis in a safe and readily accomplished manner.

After preliminary animal studies,6–10 radiofrequency ablation has been used for the treatment of a variety of neoplasms including: osteoid osteoma, hepatocellular carcinoma, renal cell carcinoma, bronchopulmonary carcinoma, parathyroid adenoma;1 hepatic and retroperitoneal metastases from a variety of primary tumors. The procedures are generally performed using thin (14–21 gauge), partially insulated electrodes which are placed under imaging guidance (CT, MRI, or ultrasound) into the tumor to be ablated (see Fig. 1). When attached to an appropriate radiofrequency generator, the RF current flows from the exposed tip of the RF needle, through the bio tissue of the human body, either to a neutral grounding pad (monopolar application) or to a second inserted RF needle (bipolar technique).

Using monopolar technique, a large dispersive electrode (grounding pad) is usually placed on the patient’s back, belly or thigh. A second needle electrode is used, instead of the grounding pad, for bipolar ablation. Current passing through tissue leads to ion agitation, which is converted into heat by friction. The process of cellular heating induces cellular damage. The amount of damage is a function of temperature and time. For example, a coagulation necrosis is achieved applying 70°C for less than 1 second or 50°C for about 200 seconds.

Several approaches have been made to increase the diameter of coagulation necrosis achieved by RF ablation techniques.11 These include: (1) the use of multiprobe, hooked, and bipolar needle arrays; (2) intraparenchymal injection/infusion of saline prior to and/or during RF application; (3) internally cooled RF electrodes; and (4) algorithms for current application which maximize energy deposition but avoid tissue boiling, charring or cavitation.