**2.1. Hyperthermic (thermal) coagulation necrosis**

Coagulation necrosis denotes "irreversible thermal damage to cells even if the ultimate manifestations of cell death do not fulfill the strict histological criteria of coagulative necrosis" [18]. The nature of the thermal damage caused by radiofrequency heating is dependent on both the tissue temperature achieved and the duration of heating. Here is what happens at various temperatures:


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.

Principles and Application of RF System for Hyperthermia Therapy 175

are achieved. The development of a radiofrequency ablation system in this study is aimed at producing a device that is able to satisfy the minimum requirement for effective tumour

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

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

ablation at the ideal (cytotoxic) ablation temperature.

**2.2. Principles of radiofrequency ablation** 

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

shown in figure 4 below.

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

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

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 are achieved. The development of a radiofrequency ablation system in this study is aimed at producing a device that is able to satisfy the minimum requirement for effective tumour ablation at the ideal (cytotoxic) ablation temperature.
