**1. Introduction**

170 Hyperthermia

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In recent times, different strategies for thermal ablation therapy have been in use. They include radiofrequency ablation, cryoablation therapy, laser ablation therapy, microwave ablation and high intensity focused ultrasound ablation, among others. *Radiofrequency ablation (RFA)* is used to destroy pathological tissue by inducing tissue necrosis through the heating of targeted tissue [1]. While ablation is currently used in the treatment of different diseases, tumour ablation is considered here, i.e. the treatment of cancerous tumours. Apart from RFA, thermal ablation therapy involves other strategies employed in the destruction of cancerous tumours. *Cryoablation therapy (or cryotherapy)* uses liquid nitrogen (or the expansion of argon gas) to freeze and kill abnormal tissue. After numbing the tissue around the mass, a cryoprobe, which is shaped like a large needle, is inserted into the middle of the lesion. An ice ball forms at the tip of the probe and continues to grow until the images confirm that the entire tumour has been engulfed, killing the tissue [2], [3]. The whole process involved in cryotherapy takes about 10 – 20 minutes to complete. The temperature and duration of freezing necessary to induce complete killing and necrosis are based on numerous in vivo and in vitro animal studies, some of which have been reviewed by Gage & Baust [4]. Generally, it has been accepted that a minimum freezing temperature of -40oC must be reached for at least 3 minutes for complete eradication of the tumour [5]. A rapid freeze followed by a slow thaw is the most damaging to cells, and a minimum of two freezethaw cycles (freeze-thaw-freeze-thaw) was necessary for effective cryonecrosis to take place than a single cycle [6]. The cost of a cryoablation unit ranges upwards from \$190,000, and each multi-use cryoprobe costs approximately \$3,750 [7]. *Laser Ablation (or interstitial laser photocoagulation)* uses a highly concentrated beam of light to penetrate the cancerous tissue. The laser energy is emitted from an optical fibre placed within a needle positioned at the centre of the tumour using either stereotactic guidance or Magnetic Resonance Imaging (MRI) [8], [9]. Two methods for delivery of light have been described to produce larger

© 2013 Okhai and Smith, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

volumes of necrosis: multiple bare fibres in an array and cooled-tip diffuser fibres. The major drawback to this technique is its cost, requiring \$30,000 to \$75,000 for a portable, solid-state laser and \$3,000 per set of multiple (50) user fibres [10]. *Microwave ablation (MWA) or microwave coagulation* uses microwave tissue coagulator for irradiation. Ultra-high frequency (2450 MHz) microwaves are emitted from a percutaneously placed microwave electrode inserted into the target tissue under ultrasonographic guidance. Microwave irradiation is carried out for about 60 seconds at a power setting of 60W per pulse. During irradiation, the ultrasonographic probe is placed adjacent to the microwave electrode to monitor the effectiveness of the tumour coagulation [11], [12]. A typical microwave generator costs approximately \$65,000 [13]. *High Intensity Focused Ultrasound (HIFU) ablation* is a non-invasive treatment modality that induces complete coagulative necrosis of a deep tumour through the intact skin. HIFU uses sound energy to produce heat [14]-[16]. HIFU treatments are usually carried out in a single session, often as a day case procedure in the doctor's office, with the patient either fully conscious, lightly sedated or under light general anaesthesia. One major advantage of HIFU over other thermal ablation techniques is that the transcutaneous insertion of probes into the target tissue is not necessary. The high powered focused beams employed in the procedure are generated from sources placed either outside the body (for treatment of tumours of the liver, kidney, breast, uterus, pancreas and bone) or in the rectum (for treatment of the prostate), and are designed to enable rapid heating of a target tissue volume, while leaving tissue in the ultrasound propagation path relatively unaffected [17]. Numerous extra-corporeal, transrectal and interstitial devices have been designed to optimise application-specific treatment delivery for HIFU procedures.

Principles and Application of RF System for Hyperthermia Therapy 173

ablation system is the RF signal generator. This is where the energy deposited by the needlelike active electrode is generated. The system comprises of a closed circuit consisting of a radiofrequency generator circuit, a power amplifier circuit, and the control circuit. A power supply circuit is also included to meet the power supply requirements of the system. The energy generated by the system is delivered to the tissue by the active electrode, whereas a dispersive electrode that acts as a patient plate provides a return part to complete the circuit.

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

 At 42oC, cells die but it may take a significant amount of time (approximately 60 min). Between 42oC and 45oC, cells are more susceptible to damage by other agents like

Between 50oC and 55oC, the duration necessary to shorten irreversible damage to cells is

 Between 50oC and 100oC there is near immediate coagulation of tissue, almost instantaneous protein denaturation, melting of lipid bilayers, irreversible damage to

mitochondrial and cytosolic (key cellular) enzymes of the cells, DNA and RNA. From 100oC to 110oC, tissue vaporizes and carbonizes, all of which decrease energy

Over 46oC irreversible damage occurs depending on the duration of heating.

A simplified block diagram of the whole system is shown in figure 1 below.

**Figure 1.** Block diagram of the RF ablation circuits

what happens at various temperatures:

chemotherapy and radiation.

shortened to 4 – 6 minutes.

transmission and impede ablation.

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

This chapter focuses on the discussion of principles and application of the radiofrequency ablation therapy system as a minimally invasive treatment modality for hyperthermia therapy. Detailed work completed in the use of radiofrequency (RF) energy in cancer management by developing and testing an economical and effective thermal probe that will effectively destroy volumes of pathological tumours by means of hyperthermia is presented.
