**3. Materials and methods**

The RF thermal probe developed was designed on a SolidWorks platform and manufactured according to design specifications. The device consists of an RF shielded insulated handle with a needle probe. The shaft of the needle is also insulated except for the tip which makes physical contact with the tumour or volume to be treated. A coaxial cable connects the device to the RF power unit. The RF thermal probe uses a stainless steel needle (size 14G x 3-1/4) with a diameter and length of 2.1 x 80 mm, connected to the conducting coaxial cable in one end, and housed in an epoxy resin holder (probe handle) that is 120 mm long and 15 mm in diameter. The stainless steel needle is insulated, except for the exposed 20 mm tip that makes direct contact with tissue. The insulation prevents normal tissue from being destroyed along with cancerous tissue during thermal ablation treatment. The probe (as shown in figure 5) is reusable and is made of epoxy-resin material that can be easily steam-cleaned.

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

$$W = \begin{pmatrix} 4/3 \end{pmatrix} \pi r^3 \tag{2}$$

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 being treated.

**Figure 5.** RF thermal probe

176 Hyperthermia

metre (A/m2).

which is found from the expression

where *P* is the power in watts (W),

**3. Materials and methods** 

steam-cleaned.

(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,

the tissue volume in cubic metres (m3), and *Id* is the current density in amperes per square

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

The RF thermal probe developed was designed on a SolidWorks platform and manufactured according to design specifications. The device consists of an RF shielded insulated handle with a needle probe. The shaft of the needle is also insulated except for the tip which makes physical contact with the tumour or volume to be treated. A coaxial cable connects the device to the RF power unit. The RF thermal probe uses a stainless steel needle (size 14G x 3-1/4) with a diameter and length of 2.1 x 80 mm, connected to the conducting coaxial cable in one end, and housed in an epoxy resin holder (probe handle) that is 120 mm long and 15 mm in diameter. The stainless steel needle is insulated, except for the exposed 20 mm tip that makes direct contact with tissue. The insulation prevents normal tissue from being destroyed along with cancerous tissue during thermal ablation treatment. The probe (as shown in figure 5) is reusable and is made of epoxy-resin material that can be easily

commercially available reusable RF thermal probes reviewed.

2 *<sup>d</sup> P VI* 

(1)

is the resistivity of the tissue in Ohm-metres (-m), *V* is

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 was done for about 15 minutes.
