**2.3 Local hyperthermia**

The term "Local hyperthermia" means superficial treatment, and typically the clinical range is up to 3–4 cm. It can be performed with so-called superficial

*Applications of Microwaves in Medicine and Biology DOI: http://dx.doi.org/10.5772/intechopen.105492*

**Figure 2.** *MW hyperthermia system schematics.*

applicators, e.g., based on EM waves in the lower part of the microwave frequency band (usually 434, 915, and 2450 MHz), ultrasound, and IR power. The technological base of EM wave applicators can be of different kinds: waveguides (water filled or with evanescent mode), microwave planar technology (e.g., patches and spirals), and according to results of this habilitation thesis, MTM applicators are very perspective as well.

From a physical point of view, we mostly want the superficial applicators to create the best possible approximation of a plane EM wave, which is the case of the deepest penetration of EM power into the area to be treated (at a given working frequency) and the best homogeneity of SAR distribution and thus, the best homogeneity of temperature distribution as well.

A system for local hyperthermia consists of a microwave (MW) power generator, an MW applicator for transfer of EM power into the treated area, see eq. (7) (tumor), a multichannel thermometer with several probes for temperature measurements in the tumor and its surroundings, and the main computer. See the schematics in **Figure 2**.

Invasive sensors then measure temperature, and according to it, MW power is being controlled in order to keep the temperature on a predetermined level.

The applicator is positioned upon the area to be treated and coupled to the tissue by a water bolus. The temperature and pressure of the water in the water bolus are possible to control, so it is possible to modify the temperature profile in the area to be treated.

In our discussion, it is essential to distinguish the following two important terms: "Depth of EM wave penetration" and "Depth of efficient treatment." The second one can have different definitions for different clinical applications of thermotherapy treatments (i.e., hyperthermia, physiotherapy, and ablations). Here, we will work with the definition for hyperthermia only.

#### **2.4 Depth of EM wave penetration and depth of efficient treatment**

For the initial estimation of EM wave penetration into biological tissue, we can take a model for the behavior of amplitude of the plane wave in a lossy media.

$$E(z) = E\_0 e^{-\alpha z} \tag{1}$$

where E is electrical field intensity, E0 is its value at the surface of biological tissue, z is the depth under the surface, and α means the attenuation constant of EM wave in lossy media.

The depth of the EM wave penetration *d* then has its definition in EM field theory based on the decrease of the amplitude of electrical field intensity to value *E*o/e (where *e* is a basis of natural logarithm) when a plane EM wave enters into a lossy media; see the following equation.

$$d = \frac{1}{\sqrt{\pi \sigma \mu\_0 f}}.\tag{2}$$

It can be seen that *d* is inversely proportional to the square root of σ and *μ*<sup>0</sup> of a given tissue and to the frequency *f* of the EM field. For the usual operating frequencies of the applicators 27, 70, 434, 915, and 2450 MHz (all of them except 70 MHz belong to ISM frequency bands reserved for industrial, scientific, and medical applications), it can be concluded that the lower the operating frequency, the deeper will be the penetration depth. The same conclusion can be made for the value of conductivity.

In the case of microwave hyperthermia, the depth of the efficient treatment is given by the distribution of temperature in the treated area—it is formulated as a 25% decrease of the SAR value with respect to the maximum value of SAR inside the treated area. That guarantees quick and quality heating of the treated area from 37°C to at least 41°C. Suppose the maximum temperature in the tumor will be at the level of 45°C, then the depth of the efficient treatment depends on the following factors:


There is a general rule for hyperthermia applicators optimization if we need to reach the maximal depth of efficient treatment and the best possible homogeneity of the temperature distribution inside the treated area. At least in the central part of their aperture, the distribution of the EM field should be very similar to plane wave; thus, it is possible to accomplish the deepest penetration depth for the particular frequency and aperture dimensions.
