**2.2. Principles of radiofrequency ablation**

174 Hyperthermia

Figure 2 shows tissue reaction to thermal injury at different temperatures [19]. For successful ablation, the tissue temperature should be maintained in the ideal range (50 – 100oC) to ablate tumour adequately and avoid carbonization around the tip of the electrode due to excessive heating. For adequate destruction of tumour tissue, the entire volume of a lesion must be subjected to cytotoxic temperatures. Hence effective heating throughout the target volume (i.e. the tumour and about 5mm thickness around normal tissue) is required as shown in figure 3. Thus, the main objective of radiofrequency ablation therapy is to reach and maintain a temperature range of 50o – 100oC throughout the entire target volume for at least 4 – 6 minutes. However, the relatively slow thermal conduction from the electrode

surface through the tissues increases the duration of application to 10 – 30 minutes.

**Figure 2.** Tissue reaction to thermal injury at different temperatures

Recommendations of heating for these extended durations are based on experimental and clinical data suggesting that thermal equilibrium and, hence, complete induction of coagulation are not achieved for a given radiofrequency application until these thresholds

**Figure 3.** Schematic diagram illustrating RF ablation

Radiofrequency ablation is physically based on radiofrequency current (about 460 kHz) that passes through the target tissue from the tip of an active electrode (RF thermal probe) towards a dispersive electrode which serves as the grounding pad. These two electrodes are connected to a radiofrequency generator. The active electrode has a very small crosssectional area (a few square millimetres) with respect to the passive electrode. The active electrode is usually fashioned into the form of a needle-like probe that is inserted into the tumour. The dispersive electrode has a much larger area than the active electrode, on the order of 100cm2 or larger, and is usually placed firmly behind the right shoulder or the thigh of the subject, depending on the location of the tumour in the body. Current flowing into the dispersive electrode is the same as the current flowing into the active electrode. But since the active electrode has a far smaller cross-sectional area than the dispersive electrode, the current density in amperes per square meter (A/m2) is far greater. As a result of the difference in current density between the two electrodes, the energy at the tip of the probe leads to ionic agitation with subsequent conversion of friction into heat. The tissue ions are agitated as they attempt to follow the changes in direction of alternating electric current as shown in figure 4 below.

**Figure 4.** Ionic agitation by alternating electric current

The agitation results in frictional heat around the electrode. The marked discrepancy between the surface area of the needle electrode and the dispersive electrode causes the generated heat to be tightly focused and concentrated around the needle electrode. The use of a large grounding pad ensures maximum surface area for dispersion of current from the needle electrode. The grounding pad also maximizes dispersion of equal amounts of energy and heat at the grounding pad sites, thereby minimizes the risk of burns. The tissue underneath the passive electrode heats up only slightly, while the tissue in contact with the active electrode is resistively heated to elevated temperatures sufficient for tumour ablation (coagulative necrosis). The strategy of RF ablation is to create a closed-loop circuit including the RF generator, the needle electrode, the patient (tissue) and the passive electrode (grounding pad) in series. The heating of tissue is due to the power dissipated in the tissue, which is found from the expression

$$P\_{\perp} = \rho V l\_{\perp}^{2} \tag{1}$$

Principles and Application of RF System for Hyperthermia Therapy 177

(2)

An essential objective of radiofrequency ablation therapy is to achieve and maintain a temperature range of 50 – 100oC throughout the entire target volume for at least 4 – 6 minutes [23-25]. From equation (1), power dissipated (P) is directly proportional to volume

> <sup>3</sup> *V r* (4 / 3)

where *r* is the radius of the sphere. It follows that, power dissipated is directly proportional to the cube of the radius. The temperature rise follows the accepted cube root heating function. This means that the outer limit of critical cell temperature where cell necrosis takes place is reasonably well-defined by the applied power and will be spherical around a point source if the impedance remains constant. In practice, we have a short cylindrical contact volume in the tumour with non linear impedances. This results in an egg shaped volume

To verify that the radiofrequency thermal probe developed is a device that is able to satisfy this minimum requirement for effective tumour ablation at the ideal cytotoxic temperature, experimental tests were done with different tissues types to determine how each tissue type responds to RF energy by observing and recording the temperature change at the probe tip. Liver, lung, brain, kidney and soft tissue were tested at different power settings to determine which power setting gives the best results with each tissue type in terms of the minimum time to reach the ideal temperature range, and the maximum time to remain within this range without charring or vapourizing. An RF generator (460 KHz) was connected in a closed circuit with the RF thermal probe, tissue sample, and dispersive electrode in series. Each tissue type was tested with different power settings, and each test

(V). Tumour is usually treated as a sphere, and volume of a sphere is given by,

being treated.

**Figure 5.** RF thermal probe

was done for about 15 minutes.

where *P* is the power in watts (W), is the resistivity of the tissue in Ohm-metres (-m), *V* is the tissue volume in cubic metres (m3), and *Id* is the current density in amperes per square metre (A/m2).

Appreciable advances have been made over the past decade to produce application devices for RFA. The *Radionics probe* is an internally cooled device that also uses pulsing sequences to improve heating. It is available in one size (17Ga) and 10, 15, and 25cm lengths. It comes with a single electrode with a tip exposure of 2-4cm, or cluster electrode [20]. The *RITA probe* is a 15Ga device that comes with various arrays. It has a thermocouple at the tip of the probe that registers the tissue temperature, and that is used to monitor its effect. The *LeVeen probe* has multiple (36, 37) tines. There are 2.0, 3.0, 3.5 and 4.0cm diameter needles from which the tines are deployed. The LeVeen needle electrode is designed to deliver a consistent pattern of heat throughout the lesion [21]. These and other application devices for RFA are available for use in the USA and some parts of Europe. In spite of technical progress in the development of various application devices for radiofrequency ablation therapy, most patients with malignant tumours, especially in Sub-Saharan Africa, have not yet benefitted from this technology due to their limited availability and exhorbitant cost. A typical RF generator costs \$25,000 and each single use probe costs approximately \$800 to \$1200 [22]. This paper presents the structure and experimental results of a low cost minimally invasive radiofrequency thermal probe developed for hyperthermia therapy. The probe developed is effective and economical, and represents more than 70% in cost reduction compared to commercially available reusable RF thermal probes reviewed.
